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HomeMy WebLinkAboutProp 1 PCE Plume Characterization Project - Feasibility Study Report_December 2022 Feasibility Study Report City of San Luis Obispo Tetrachloroethylene (PCE) Plume Characterization Project DECEMBER 2022 CITY OF SAN LUIS OBISPO CITY OF SAN LUIS OBISPO Feasibility Study Report DECEMBER 2022 Prepared by Water Systems Consulting, Inc ACKNOWLEDGEMENTS The Feasibility Study Report was prepared by Water Systems Consulting, Inc and in partnership with Cleath-Harris Geologists. The primary authors are listed below. Joshua Reynolds, PE Spencer Harris, PG, CHG, CEG Heather Freed, PE Cassandra Springer, GIT Water Systems Consulting, Inc. would like to acknowledge the significant contributions of the City of San Luis Obispo. The primary contributors are listed below. Nick Teague Jennifer Metz Bobby Browning Miguel Barcenas, PE Shawna Scott Mychal Boerman Aaron Floyd Table of Contents City of San Luis Obispo i DRAFT: Feasibility Study Report TABLE OF CONTENTS 1.0 Project Background ...................................................................................................... 1-1 1.1 Introduction ............................................................................................................... 1-2 1.2 Past Remedial Investigations .................................................................................... 1-4 1.3 Project Setting .......................................................................................................... 1-5 2.0 Constituents of Concern ............................................................................................... 2-1 2.1 Constituents of Concern in the Basin ........................................................................ 2-2 2.2 PCE Plume Within the Basin ..................................................................................... 2-2 3.0 Remedial Action Alternatives ........................................................................................ 3-1 3.1 Remedial Action Objective ........................................................................................ 3-2 3.2 Well Locations .......................................................................................................... 3-2 3.3 Hydraulic Capture Zone Analysis .............................................................................. 3-5 3.4 Treatment Alternatives .............................................................................................. 3-9 3.5 Treatment Location ................................................................................................. 3-13 3.6 Cost / Benefit Analysis ............................................................................................ 3-15 3.7 Recommendations .................................................................................................. 3-19 Appendix A Hydraulic Capture Zone Analysis ...................................................................... A Appendix B Cost Estimates .................................................................................................. B List of Figures City of San Luis Obispo ii DRAFT: Feasibility Study Report LIST OF FIGURES Figure 1. Project Area ............................................................................................................. 1-6 Figure 2. Regional Groundwater Elevation (Source, GSI, Fall 2021) ....................................... 1-9 Figure 3. Profiled Wells ........................................................................................................... 2-4 Figure 4. PCE Concentration Contours ................................................................................... 2-7 Figure 5. Cross Section Transect Lines Map ........................................................................... 2-8 Figure 6. Cross Section A-A .................................................................................................... 2-9 Figure 7. Cross Section B-B .................................................................................................. 2-10 Figure 8. Cross Section C-C ................................................................................................. 2-11 Figure 9. Potential Well Treatment Locations .......................................................................... 3-4 Figure 10. Treatment Well Capture Zone Example – Excerpt from Hydraulic Capture Zone Analysis Report ....................................................................................................................... 3-8 Figure 11. Estimated Alternative Implementation Schedule................................................... 3-18 Figure 12. Example Well Site Concept .................................................................................. 3-22 List of Tables City of San Luis Obispo iii DRAFT: Feasibility Study Report LIST OF TABLES Table 1. Particle Tracking Scenarios ....................................................................................... 3-6 Table 2. Hydraulic Capture Zone Results, Percent Capture of Particles Released in PCE Plume Area ........................................................................................................................................ 3-7 Table 3. PCE Treatment Type Cost Comparison .................................................................. 3-12 Table 4. Centralized and Decentralized Cost Comparison .................................................... 3-14 Table 5. Well Alternative Cost Comparison ........................................................................... 3-17 Table 6. Recommended Alternative Project Costs ................................................................ 3-21 Acronyms & Abbreviations City of San Luis Obispo iv DRAFT: Feasibility Study Report ACROYNMS & ABBREVIATIONS AFY acre-feet per year AMSL above mean sea level BASIN San Luis Obispo Valley Groundwater Basin CAL POLY California Polytechnic State University CITY City of San Luis Obispo CJAR Calle Joaquin Agricultural Reserve COC constituents of concern CR 6+ hexavalent chromium DNAPL Dense non-aqueous phase liquid DTSC Department of Toxic Substances Control DWR Department of Water Resources GAC granular activated carbon GPM gallons per minute GSP Groundwater Sustainability Plan MCL maximum contaminant limit O&M Operation and maintenance PCE tetrachloroethylene PFAS per- and polyfluoroalkyl substances QPS QORE Property Science RWQCB Regional Water Quality Control Board SLO San Luis Obispo SWRCB State Water Resources Control Board TPH total petroleum hydrocarbons URS URS Corporation VOC volatile organic compound WSC Water Systems Consulting, Inc. µG/L micrograms per liter City of San Luis Obispo 1-1 DRAFT: Feasibility Study Report 1.0 Project Background This chapter provides the project background and purpose, provides an overview of the project area and groundwater basin, and summarizes previous relevant remedial investigations in the project area. IN THIS SECTION  Introduction  Past Remedial Investigations  Project Setting Section 1.0 Project Background City of San Luis Obispo 1-2 DRAFT: Feasibility Study Report 1.1 Introduction The City of San Luis Obispo Tetrachloroethylene (PCE) Plume Characterization Project (Project) was initiated to characterize a PCE plume within the San Luis Valley Subarea of the San Luis Obispo Valley Groundwater Basin, DWR Bulletin 118 Basin No. 3-09 (Basin), in San Luis Obispo County, California. Funding for the planning phase of the Project came from the California State Water Resources Control Board (SWRCB) Proposition 1 Groundwater Grant Program Agreement No. SWRCB0000000000D1912530. Water Systems Consulting, Inc. (WSC), in coordination with the City of San Luis Obispo (City), has been responsible for the Project planning and management. This Feasibility Study Report evaluates potential projects and technologies to treat the PCE Plume while improving the City’s water supply resiliency and summarizes the results of the Hydraulic Capture Zone Analysis. The Hydraulic Capture Zone Analysis includes development and utilization of a groundwater model to determine preliminary locations for pump and treat wells using flow modeling and particle tracking. This report draws on data from previous remedial investigations, studies, and reports, including the Remedial Investigation Report (WSC, 2022) and the Remedial Investigation Feasibility Study Work Plan (WSC, 2021) prepared as part of this Proposition 1 Groundwater Grant Program. The next steps in this Project will be creating a fate and transport groundwater model to verify the initial treatment recommendations followed by implementation of the recommended pump and treat systems to remove PCE from the plume affecting the San Luis Valley Subarea of the Basin. The treated groundwater will ultimately be delivered to the City’s water distribution system, improving reliability and resiliency to the City’s drinking water system. 1.1.1 Project Benefits The Project is critical for the City to secure existing groundwater supplies and will provide the following benefits:  Provides the tools necessary to enhance local water supply reliability for the City, which otherwise relies on surface water supplies to meet demands;  Recommends cleanup alternatives (e.g., treatment) necessary to successfully improve the sustainability of the Basin, a High Priority Basin, which has limited groundwater as a source for drinking water; and  Benefits the City, a disadvantaged community, by improving its water supply resiliency and availability. 1.1.2 Project Goal and Objectives The primary purpose of the Project is to characterize existing PCE contamination in the Basin and evaluate and select remedial actions that will reduce plume migration and treat the PCE Section 1.0 Project Background City of San Luis Obispo 1-3 DRAFT: Feasibility Study Report contamination over time. From this purpose, the goal is to expand and diversify the City’s water supply portfolio to improve water supply resiliency. Specific objectives include: 1. Characterize and delineate the PCE plume through subsurface investigations and provide ongoing monitoring of the plume by installation of a groundwater monitoring well network. 2. Estimate plume migration and degradation over time – see the Remedial Investigation Report (WSC, 2022) for information on the plume characterization. Additional fate and transport modeling is recommended to better predict the PCE plume migration and degradation over time. 3. Identify influent water quality for well head treatment at existing non-operable production wells. 1.1.3 Project Scope The major Project scope milestones and activities are listed below:  Prepare a Remedial Investigation/ Feasibility Study Work Plan, finalized in 2021. The Remedial Investigation/Feasibility Study Work Plan summarizes historical data and background studies to understand the current PCE plume extents and data gaps. The report includes the proposed investigation work plan to further understand the full extent of the PCE plume. It also establishes the remedial action objective and identifies treatment alternatives for evaluating the feasibility of developing treatment wells.  2021-2022 Field Investigations o Well Profiling: Performed well profiling and water quality testing of one existing active and three inactive water supply wells. The tested wells were the Calle Joaquin Agricultural Reserve (CJAR) Agricultural Well, Highway 101 Well, San Luis Ranch No. 2 Well, and Pacific Beach No. 1 Well, see Section 2.2.2 for a map and discussion of the profiling. Generally, well profiling involved injecting a dye to determine where in the aquifer, vertically, the water is preferentially entering the well casing and if the tested wells were interacting with each other. The Highway 101 Well was also dynamically tested (under pumping conditions) using two aquifer pump tests to analyze how pumping would affect the water levels within the aquifer. Water quality testing was performed for volatile organic compounds (VOCs) including PCE and per- and polyfluoroalkyl substances (PFAS). Well profiling work was completed in September 2021. o Borings for Groundwater and Soil Sampling: The borings for groundwater and soil sampling field investigations consisted of drilling and collecting soil and groundwater samples from 30 locations throughout the previously known PCE plume delineation area to investigate the current physical extent of the PCE plume. This work is described in the Remedial Investigation/Feasibility Study Work Plan Section 1.0 Project Background City of San Luis Obispo 1-4 DRAFT: Feasibility Study Report and the Remedial Investigation Report and is also summarized in Section 2.2.3 of this report. The borings work was performed from March to July 2022.  Prepare a Remedial Investigation Report. The Remedial Investigation Report summarizes the field investigation work completed to improve the understanding of the plume. This Feasibility Study report relies on the current understanding of the Plume extents determined from the 2021-2022 field investigation work.  Prepare a Groundwater Modeling Report, included as Appendix A of this Feasibility Study Report. The data from the 2021-2022 field investigation was used to update and recalibrate the existing groundwater flow model. Once updated, the model was used to perform a hydraulic capture zone analysis to evaluate PCE capture with a range of treatment well alternatives. The hydraulic capture zone analysis is summarized in this Feasibility Study Report and the results of that analysis are used to understand the ability of the treatment alternatives to meet the remedial action objectives.  Prepare this Feasibility Study Report to evaluate treatment well alternatives and technologies to treat the PCE Plume and meet the remedial action objectives. The Feasibility Study Report builds upon information presented in the previous reports. 1.2 Past Remedial Investigations There have been multiple investigations within the San Luis Valley Subarea of the Basin that have focused on PCE contamination, both at the regional level and the site-specific level. One regional level investigation was conducted in 2005 by QORE Property Science (QPS) with the aim of evaluating the PCE plume. The study gathered and evaluated existing information regarding local area conditions to demonstrate concentrations of PCE present in groundwater. The 2005 QPS Report indicated evidence of PCE release at one or more locations near South and Higuera Streets as the source of the plume. In addition, data evaluation indicated the apparent presence of “baseline” concentrations of PCE in the San Luis Obispo Creek watershed upgradient of the plume point sources (QPS , 2005). Distribution of PCE concentrations indicate the length of the plume in a systematic pattern of exponentially decreasing concentrations with distance from the source area or areas. That pattern of distribution is consistent with prior plume depictions by the Regional Water Quality Control Board (RWQCB) and by others, as well as with that typical of degraded chlorinated VOC plume. Two other regional level investigations have been conducted. The 2013 Investigation Report – San Luis Obispo PCE Groundwater Plume, prepared by URS Corporation (URS, 2013) for the Department of Toxic Substances Control (DTSC), included the collection of passive (64 locations) and active (23 locations) soil vapor samples. The samples were analyzed for VOCs to assist in identifying the potential source(s) of the PCE plume in groundwater by evaluating PCE concentrations in soil vapor near select DTSC-identified properties. The passive and active soil vapor sample results were consistent and found the largest adsorbed PCE masses near three dry cleaner sites in the City. The 2015 Investigation Report – San Luis Obispo PCE Groundwater Plume (URS 2015) was a continuation of the 2013 Investigation Report. It included the collection Section 1.0 Project Background City of San Luis Obispo 1-5 DRAFT: Feasibility Study Report and analysis of soil vapor samples, indoor air samples, and grab groundwater samples near three dry cleaning properties where elevated PCE concentrations were detected during the previous soil vapor investigation conducted in November 2012 (URS, 2013). Sample results for each of the sites indicated that there had been releases of PCE into soil and/or groundwater. The lateral extent of elevated soil vapor concentrations had been reasonably well defined except for the southern end of the plumes. Many site-specific investigations into PCE have also been conducted within the San Luis Valley Subarea of the Basin. The DTSC and RWQCB have provided most of the regulatory oversight related to site investigations and clean-up efforts since the early 1990s, for both PCE and other contaminants of concern. Refer to the Remedial Investigation/Feasibility Study Work Plan (WSC, 2021) and Remedial Investigation Report (WSC, 2022) for more information on past remedial investigations. 1.3 Project Setting This section describes the project setting, including the Basin location, characterization and hydrogeology. 1.3.1 Project Area The project area, shown in Figure 1, is located in the San Luis Valley, south of California Polytechnic State University (Cal Poly) in the southern portion of the City, in San Luis Obispo County, California and within DWR Bulletin 118 Basin No. 3-09 and Region 3 of the SWRCB. The PCE plume lies between Los Osos Valley Road and S. Higuera Street, following alongside Highway 101 and San Luis Obispo Creek. The suspected source or sources of the PCE plume appear to be located south of Marsh Street and north of South Street, east of Highway 101. The source of the PCE has been attributed to several dry cleaners and power generation facilities that once existed in this area. These sites are currently undergoing or have undergone remedial clean- up efforts. See the Remedial Investigation Report (WSC, 2022) for additional information on plume location, presumed sources, prior studies, and prior remediation efforts by others. Figure 1 shows, in blue, the plume location as previously assumed based on the 2005 QPS plume delineation report and the current plume limits, in green, as delineated by the remedial investigation work performed as part of this Grant Agreement. Refer to the Remedial Investigation Report for details. Previous studies estimating the extent of the PCE plume completed by QPS and the RWQCB show that the plume has dispersed over time and passed through the narrow passageway between west and east South Hills, where San Luis Creek flows north to south, indicated as the 2005 QPS Plume Delineation on Figure 1. The 2022 results, shown as the 2022 Plume Delineation on Figure 1, indicate the PCE plume does not stretch as far south as the QPS Plume estimated and the northern tip of the plume lies further to the southeast. Additional information on the field work and Plume Delineation analysis is included in the Remedial Investigation Report (WSC, 2022) prepared for this Project. Section 1.0 Project Background City of San Luis Obispo 1-6 DRAFT: Feasibility Study Report Figure 1. Project Area The 2022 Plume Delineation is based on the site investigation as part of this Project, documented in the Remedial Investigation Report (WSC, 2022). The 2005 Plume Delineation is based on the 2005 QPS Investigation (QPS , 2005). Section 1.0 Project Background City of San Luis Obispo 1-7 DRAFT: Feasibility Study Report 1.3.2 San Luis Obispo Valley Groundwater Basin The Basin, shown in Figure 2, is oriented in a northwest-southeast direction and is composed of unconsolidated or loosely consolidated sedimentary deposits. It is approximately 14 miles long and 1.5 miles wide. It covers about 12,700 acres (19.9 square miles). The Basin is bounded on the northeast by relatively impermeable bedrock formations of the Santa Lucia Range, and on the southwest by the formations of the San Luis Range and the Edna fault system. The bottom of the Basin is defined by the contact of permeable sediments with the Miocene-aged impermeable bedrock and Franciscan Assemblage rocks (DWR, 2003). Ground surface elevations within the Basin area range from about 500 feet above mean sea level (amsl) in the southeastern extent of Edna Valley, to around 100 feet amsl where San Luis Obispo Creek flows out of the Basin (WSC et al., 2021). The Basin is commonly referenced as being composed of two distinct valleys, with the San Luis Valley in the northwest and the Edna Valley in the southeast. The San Luis Valley Subarea of the Basin is comprised of approximately the northwestern half of the Basin and is the area of the Basin drained by San Luis Obispo Creek and its tributaries (Prefumo Creek and Stenner Creek west of Highway 101, Davenport Creek, and smaller tributaries east of Highway 101). Surface drainage in San Luis Valley Subarea drains out of the Basin flowing to the south along the course of San Luis Obispo Creek toward the coast in the Avila Beach area, approximately along the course of Highway 101. The San Luis Valley Subarea includes part of Cal Poly jurisdictional boundaries, while the remainder of the valley is unincorporated land. Land use in the City is primarily municipal, residential, commercial, and industrial. The area in the northwest part of the Basin, along Los Osos Valley Road, has significant areas of irrigated agriculture, primarily row crops. 1.3.3 Hydrogeology Groundwater in the San Luis Valley Subarea of the Basin is primarily recharged by percolation of rainfall in the Santa Lucia Range contributing to mountain front recharge via subsurface inflow, percolation of streamflow from creeks and streams within the basin, and anthropogenic recharge from treated wastewater, agricultural irrigation, and domestic septic fields. Groundwater generally flows in a southwest direction and leaves the Basin by streamflow through San Luis Obispo Creek on the south side of the Los Osos Valley Fault (Figure 2). The San Luis Valley Subarea is comprised of three main water-bearing units: Recent Alluvium, Paso Robles Formation, and Pismo Formation; all three of which contain interbedded clayey aquitards. The primary non-water- bearing unit is the Franciscan Assemblage which is overlain by the three water-bearing units. There are no apparent laterally extensive impermeable strata acting as hydrogeologic barriers to groundwater flow separating the three water-bearing units in the project area. Water levels in the project area range between 90 and 150 feet amsl (approximately 20 to 40 feet below ground surface). Two faults exist in the area, the Madonna Fault and Los Osos Valley Fault, but only the Los Osos Valley Fault is known to act as a barrier to groundwater flow at depth (GSI Water Solutions, 2019). Section 1.0 Project Background City of San Luis Obispo 1-8 DRAFT: Feasibility Study Report Laguna Lake is the only lake in the Basin. It is a naturally occurring lake just north of Los Osos Valley Road and west of Highway 101. The downstream outlet of the lake flows into the Prefumo Creek culvert under Madonna Road. In the past, flashboards were used to maintain water elevation in the lake to support recreation and maintain wildlife habitat. However, these are no longer used. The water in the lake is partially supplied by seasonal flow in Prefumo Creek, which flows into Laguna Lake, and partially supplied by subsurface groundwater inflow. Laguna Lake is part of the Laguna Lake Park and Open Space and is used for recreation such as bird watching, swimming, boating (non-motorized), and fishing. According to the San Luis Obispo Valley Groundwater Sustainability Plan, groundwater interaction with streams and lakes in the Basin is not well quantified, but it is recognized as an important component of recharge in the water budget. (WSC et al., 2021) This space intentionally blank. Section 1.0 Project Background City of San Luis Obispo 1-9 DRAFT: Feasibility Study Report Figure 2. Regional Groundwater Elevation (Source, GSI, Fall 2021) Section 1.0 Project Background City of San Luis Obispo 1-10 DRAFT: Feasibility Study Report Hydraulic conductivities in the Recent Alluvium unit average 42 feet/day. The wells screened within the recent alluvium yield pumping rates from 10 to over 100 gallons per minute (GPM) and are subject to seasonal fluctuations in groundwater levels (GSI Water Solutions, 2019). The Paso Robles Formation consists of terrestrial deposits and the Pismo Formation of marine sedimentary beds. The Paso Robles Formation and the Pismo Formation have typical hydraulic conductivities in the intermediate to deep finer grained aquifer sediments averaging 13 feet/day (Cleath-Harris Geologists, 2019). Wells screened within the Paso Robles Formation and the Pismo Formation yield pumping rates from 100 to over 500 GPM and 100 to over 700 GPM, respectively. The non- water bearing Franciscan Assemblage is at its lowest point at the southwestern edge of the project area adjacent to the Los Osos Valley Fault. 1.3.4 Primary Users of Groundwater The primary groundwater users in the Basin include municipal, agricultural, and domestic (i.e., rural residential, small community water systems, and small commercial entities). Description of these entities are discussed in more detail in Chapter 2 of the San Luis Obispo Valley Basin Groundwater Sustainability Plan (WSC et al., 2021). In 2022, the City received its potable water supply from surface water sources including Whale Rock Reservoir, Santa Margarita Reservoir, and Nacimiento Reservoir, plus recycled water for non-potable irrigation. The City maintains its network of production wells for supplemental and emergency supply and intends to use groundwater as a resource to improve water supply resilience and to meet future water demand. 1.3.5 San Luis Obispo Valley Groundwater Basin Beneficial Use – City of San Luis Obispo The City’s General Plan Water and Wastewater Management Element (WWME) is the guiding policy document adopted by the City Council that outlines the goals, policies, and implementation measures required to provide adequate water and wastewater supply based on an assessment of current and future needs and available resources (City of San Luis Obispo, 2020). The City meets water supply demand as outlined in the WWME; a multi-source concept which includes groundwater. The City needs a better understanding of the groundwater movement and water quality characteristics in the Basin to plan for groundwater treatment and to reintegrate groundwater back into the City’s water supply portfolio. The City has two wells, Pacific Bell Well and Fire Station Well, currently approved as sources of supply. In 2022, the Pacific Beach well is in standby status and the Fire Station Well is listed as inactive. The Pacific Beach Well is non-operable because of previous detections of hexavalent chromium (Cr6+). The Fire Station Well is non-operable due to detections of methyl tert-butyl ether (MTBE) also above the Maximum Contaminant Level (MCL). In 2003, the City drilled the Highway 101 Well. The City decided not to equip the Highway 101 Well with a pump and conveyance pipeline due to concerns with overall reliability related to unknown water quality characteristics. The Highway 101 Well was capped for later use. Section 1.0 Project Background City of San Luis Obispo 1-11 DRAFT: Feasibility Study Report In response to the 2011 to 2017 statewide drought, the City reinitiated exploration of groundwater to diversify their water supply portfolio to improve resilience and sustainability. The City focused on placing the Highway 101 Well into service as a source of supply for the drinking water system. Water quality testing of the well in 2017 indicated the presence of PCE in the groundwater in exceedance of the Federal and State MCL1 of 5 μg/L. This Project is a critical first step in allowing the City to build water supply resiliency and to safely access groundwater. 1 MCLs are drinking water standards adopted as regulations by the State of California to protect public health. The MCL for PCE is 5 µg/L. This space intentionally blank. City of San Luis Obispo 2-1 DRAFT: Feasibility Study Report 2.0 Constituents of Concern This chapter discusses constituents of concern, and describes the PCE Plume properties, extents, and concentrations within the San Luis Valley Subarea of the Basin. IN THIS SECTION  Constituents of Concern  PCE Plume within the Basin Section 2.0 Constituents of Concern City of San Luis Obispo 2-2 DRAFT: Feasibility Study Report 2.1 Constituents of Concern in the Basin Overall, the San Luis Valley Subarea of the Basin has water quality issues that have been persistent for several decades. The constituents of concern (COCs) present in groundwater within the project area are metals, polychlorinated biphenyls, polycyclic aromatic hydrocarbons, nitrate, VOCs including PCE, hexavalent chromium (Cr6+), and total petroleum hydrocarbons (TPH) which are thought to be a result of industrial and agricultural activities within the San Luis Valley Subarea. Additionally, PFAS have been detected in multiple wells within the project area and are likely a concern in the San Luis Valley Subarea. As the City moves forward with the design and construction of additional monitoring and treatment wells, water samples in the proposed well locations will be tested for other constituents of concern to determine if treatment is needed for constituents in addition to PCE. PCE above California’s adopted drinking water MCL (5µg/L) was first detected in the San Luis Valley Subarea in the late 1980s, at which time it was detected in several well locations over a large area. These sites first identified with PCE exceeding the drinking water standards have been participating in ongoing PCE remediation efforts in the soil and groundwater for three decades. PCE is a pressing water quality issue that impacts the City’s ability to utilize its groundwater supply and is the focus of this report. 2.2 PCE Plume Within the Basin 2.2.1 PCE Properties within the Basin PCE is a dense non-aqueous phase liquid (DNAPL) which, in large quantities and substantial concentrations, can migrate down into the deeper parts of aquifers. Its mobility is described as moderate to high. Its estimated half-life in groundwater is typically one to two years but may be considerably longer under certain conditions (SWRCB, Division of Water Quality, 2017). Based on the age of the plume and current groundwater concentrations, it is likely that the half-life of PCE in the San Luis Valley Subarea is greater than two years. Degraded chlorinated VOC plumes generally decrease in concentration exponentially with distance from the source area, which has been observed in the PCE plume, as described in Section 2.2.3. Refer to the Remedial Investigation Report for additional information on the history, extent, and spread of the plume (WSC, 2022). 2.2.2 Remedial Investigation Work In 2021 and 2022, vertical profiling, sampling, and an exploratory boring investigation were completed within the San Luis Valley Subarea of the Basin as part of this Project. The fieldwork began with vertical flow profiling for Highway 101 Well, San Luis Ranch No. 2 Well, Pacific Beach No. 1 Well, and Calle Joaquin Agricultural Reserve (CJAR) Agricultural Well. See Figure 3 for well locations. The City owns the Pacific Beach No. 1 Well and the Highway 101 Well. The Pacific Beach No.1 is a standby well used for emergency supply and the Highway 101 Well is drilled but Section 2.0 Constituents of Concern City of San Luis Obispo 2-3 DRAFT: Feasibility Study Report not equipped for pumping. The San Luis Ranch No. 2 Well is an inactive agricultural well and CJAR Agricultural Well is an active agricultural well. Water samples were collected at various depths and analyzed for PFAS and PCE. PFAS testing was performed by the City outside of the Prop 1 Grant Agreement at the Pacific Beach Well, the Highway 101 Well, and the San Luis Ranch Well. PCE and PFAS were detected in the Highway 101 Well and San Luis Ranch Well and were non-detect2 in the Pacific Beach #1 Well. BESST, Inc., a groundwater services and technology company, also conducted downhole video surveys to assess the condition of the well structure. After the profiling was complete, BESST, Inc. performed an 8-hr step test and a 24-hr pump test on the Highway 101 Well to assess the pumping capacity and effects of pumping on water levels in the surrounding wells. The next phase of fieldwork was a subsurface investigation to understand the vertical and horizontal extent of the PCE plume and concentrations, summarized in the next sections. Refer to the Remedial Investigation Report (WSC, 2022) for more information on the work completed as part of this Grant Agreement. 2 The minimum concentration that could be detected depends on the laboratory and the sample. For PCE testing, FGL uses a practical quantitation limit of 0.5 μg/L for undiluted samples and 1.0 μg/L for diluted samples. Zalco Laboratories analyzed VOC samples from B-11R and B-18R on behalf of FGL. Zalco Laboratories uses a method detection limit of 0.45 μg/L for PCE. Eurofins Eaton Analytical, LLC performed the laboratory analysis of PFAS and uses a minimum reporting level for PFAS of 0.002 μg/L. This space intentionally blank. Section 2.0 Constituents of Concern City of San Luis Obispo 2-4 DRAFT: Feasibility Study Report Figure 3. Profiled Wells Section 2.0 Constituents of Concern City of San Luis Obispo 2-5 DRAFT: Feasibility Study Report 2.2.3 PCE Plume Extents Based on the past characterization of the PCE plume and recent remedial investigation work, the PCE plume has dispersed over time and passed through the narrow passageway between west and east South Hills where San Luis Creek flows north to south. The most recent characterization of the plume extents was completed as part of this Project, using data from the subsurface investigation, and is shown in Figure 4. This estimate refines the borders as drawn in 2005 (QPS, 2005), but due to the broad spacing between boring locations, some of the plume boundary can only be inferred, as shown as the dashed lines in the Figure 4. The two most southern borings during the 2021-2022 field investigation had no detections of PCE, meaning the current plume does not extend as far south as indicated in the 2005 PCE plume delineation. Overall, the current boundaries of the PCE contamination within the Basin are not as broad as the boundaries that were inferred from 1990 water quality data in the QPS report. Figure 5 shows the borehole locations and maximum PCE concentration measured during the 2021-2022 field investigation. As shown on the geologic cross sections (Figure 6 through Figure 8), PCE was detected throughout the vertical extent of the alluvium at multiple depths. However, concentrations tended to be higher at the deepest water samples collected in the borings. For detailed information on the plume extents and delineation prepared for this project refer to the Remedial Investigation Report (WSC, 2022). 2.2.4 PCE Concentrations Concentrations of PCE above the current MCL (5 µg/L) were first detected in the PCE plume in the San Luis Valley Subarea of the Basin in the late 1980s and early 1990s at wells located between Marsh Street and South Street on the east side of Highway 101 and near Los Osos Valley Road on the west side of Highway 101 (QPS , 2005). Sites identified in this specific area (which overlays the current PCE plume) have been participating in ongoing PCE remediation efforts in the soil and groundwater for three decades. The prior remediation efforts have been managed by the Regional Water Quality Control Board (RWQCB) and Department of Toxic Substances Control (DTSC), refer to the Remedial Investigation Report for additional information (WSC, 2022). The 2021-2022 field investigation performed as part of this Project included drilling and sampling for PCE and other constituents of concern at 30 locations in the groundwater and soils throughout the 2005 QPS plume delineation. Figure 4 shows the PCE concentration contours and groundwater sampling results from the field investigation. From this subsurface investigation, the highest PCE concentration was found along High Street, near Beebee Street, at 21 µg/L. For comparison, the highest PCE concentration documented in the QPS Report was 256 μg/L, which uses PCE contamination data collected from 1989-1992, and was located just north of the highest concentration found during the recent investigation. The concentration of PCE decreases as the plume extends southward. Currently, the central area of the PCE plume has an average concentration of approximately 5 μg/L, while the water quality Section 2.0 Constituents of Concern City of San Luis Obispo 2-6 DRAFT: Feasibility Study Report data collected in the 1990s and reported in the 2005 QPS Report showed this area had an average concentration of approximately 20 μg/L. The recent investigation also found the PCE concentrations in the plume are lower than the concentrations initially detected and reported in the 2005 QPS Report. Despite the reduction in PCE over these two decades, there are still multiple locations within the plume that contain PCE over the MCL. This space intentionally blank. Section 2.0 Constituents of Concern City of San Luis Obispo 2-7 DRAFT: Feasibility Study Report Figure 4. PCE Concentration Contours Section 2.0 Constituents of Concern City of San Luis Obispo 2-8 DRAFT: Feasibility Study Report Figure 5. Cross Section Transect Lines Map Section 2.0 Constituents of Concern City of San Luis Obispo 2-9 DRAFT: Feasibility Study Report Figure 6. Cross Section A-A See the Remedial Investigation Report for more information and the cross section transect map. (WSC, 2022) Section 2.0 Constituents of Concern City of San Luis Obispo 2-10 DRAFT: Feasibility Study Report Figure 7. Cross Section B-B See the Remedial Investigation Report for more information and the cross section transect map. (WSC, 2022) Section 2.0 Constituents of Concern City of San Luis Obispo 2-11 DRAFT: Feasibility Study Report Figure 8. Cross Section C-C See the Remedial Investigation Report for more information and the cross section transect map. (WSC, 2022) City of San Luis Obispo 3-1 DRAFT: Feasibility Study Report 3.0 Remedial Action Alternatives This chapter evaluates different treatment technologies, well locations, and centralized and decentralized treatment alternatives with the goal of pumping and treating the PCE Plume to provide groundwater as an additional drinking water supply source for the City of San Luis Obispo. IN THIS SECTION  Remedial Action Objective  Well Locations  Hydraulic Capture Zone Analysis  Treatment Alternatives  Treatment Location  Cost/Benefit Analysis  Recommendations Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-2 DRAFT: Feasibility Study Report 3.1 Remedial Action Objective The overall remedial action objective is to pump groundwater and treat it for PCE using City owned wells (either existing and/or future wells) to provide groundwater as an additional drinking water supply source for the City. The City plans to meet this objective by equipping groundwater wells with wellhead treatment to remove PCE to meet drinking water standards. Multiple treatment technologies and well locations were considered as remedial action alternatives. The remedial action alternatives are evaluated against multiple criteria, listed below:  Capital and lifecycle costs  The impact on the PCE plume based on a particle tracking analysis (i.e., does the alternative create a hydraulic capture zone, does it reduce remediation time, etc.)  Constructability (i.e., adequate sanitary seal depth, land space available for construction, offset from surface water sources, offset from other wells, permitting, environmental impacts, etc.) 3.2 Well Locations To meet the remedial action objective, the proposed Project includes two or more wells (which could be existing or future wells) that will pump and treat groundwater for distribution in the City’s drinking water system. This section describes the potential well locations considered for the Project, which are shown in Figure 9. The following rationale was used in selecting the treatment well locations:  Hydraulic capture closer to the historical sources of PCE will be more effective at source containment, compared to farther away.  Hydraulic capture closer to the greatest concentrations of PCE in groundwater will be more effective at plume containment and removal, compared to farther away.  Consideration for sanitary seal depth and setbacks from surface water (stream channels) is important to limit the treatment requirements for produced water. PCE removal is the objective, but there are other treatment considerations depending on the subsequent use of the produced water.  A minimum combined production of 300 acre feet per year (AFY) is assumed for low level pumping.  A maximum combined production of 600 AFY is assumed for optimal pumping. The San Luis Obispo Basin Groundwater Sustainability Plan (GSP) identified 700 AFY of surplus groundwater available in the San Luis Valley Subarea (WSC et al., 2021).  One of the treatment well locations is the existing City Highway 101 well.  Extractions within the plume area near Los Osos Valley Road, where significant land subsidence occurred historically, is not recommended. Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-3 DRAFT: Feasibility Study Report 3.2.1 Mid-Higuera Well (Treatment Well #1) This proposed well is located furthest north in the City and closest to the historical PCE source areas, near the former City Lawn Memorial and IOOF Cemetery Wells between Higuera Street and the Madonna Road overpass at Highway 101. This well is represented by Treatment Well #1 in the Hydraulic Capture Zone Analysis (see Appendix A). Based on the available information about the wells in the area, a new treatment well with a sanitary seal of 50 feet of depth and with a minimum 150-foot setback from San Luis Obispo Creek would be feasible in this location. Based on the groundwater model, this location would have sufficient pumping capacity to produce 150 AFY, but higher production would result in impacts to nearby private wells. A well pump test is needed to evaluate the pumping rate. For the purpose of evaluating treatment options, this report assumes the well can pump up to 300 gpm and would run for 10 to 12-hours per day. This would allow the well to produce up to 200 AFY3 which exceeds the 150 AFY capacity assumed in the modeling. The annual production of this well is assumed to be lower than the other wells to avoid impacting nearby wells. The proposed parcel for a new well in this area is undeveloped and appears to be privately owned. It is unknown if the private owner of this parcel would be willing to sell it to the City. The City would also likely need to acquire an easement for the well discharge piping through the existing Central Coast Brewing parking lot (6 Higuera Street). 3.2.2 Dalidio Drive Well (Treatment Well #2) This proposed well is located in the northeast corner of San Luis Ranch off Dalidio Drive in the City near the site of historically productive alluvial wells (Embassy Suites, San Luis Ranch #2, #3, and #7 Wells). The well is represented by Treatment Well #2 in the Hydraulic Capture Zone Analysis (see Appendix A). Based on available information, a new well with a 50-foot sanitary seal depth and 50-foot setback from existing wells that can produce a minimum of 200 AFY is feasible in this location, but a well pump test is needed to evaluate the pumping rate. This report assumes the well can pump up to 300 gpm and would run for 18 to 20 hours per day. This would allow the well to produce up to 350 AFY4 which exceeds the capacity of 200-350 AFY assumed in various modeling scenarios for this well. The City would need to coordinate with the private property owner for a future well site. The eastern portion of the San Luis Ranch neighborhood is a dedicated agricultural preserve that is protected from development, ,and there appears to be room for a new well and GAC system. 3 Treatment Well #1 production capacity of 200 AFY assumes a 300-gpm well pumped 12 hours per day 300 days per year. 4 Treatment Well #2 production capacity of up to 350 AFY assumes a 300-gpm well pumped 20 hours per day 320 days per year. Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-4 DRAFT: Feasibility Study Report Figure 9. Potential Well Treatment Locations Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-5 DRAFT: Feasibility Study Report 3.2.3 Highway 101 Well (Treatment Well #3) The Highway 101 Well is located on City property along the Highway 101 corridor near elevated PCE concentrations and downgradient of the PCE plume. The Highway 101 Well is represented by Treatment Well #3 in the Hydraulic Capture Zone Analysis (see Appendix A). The Highway 101 Well was drilled, constructed, and tested in February 2003, but has since been capped. The water quality testing results indicate that the well has had detectable PCE contamination ranging from 4 to 10 µg/L and has at times been above the 5 µg/L maximum contaminant limit. The well has a 12-inch casing and is 145 feet deep, with a 40-foot-deep sanitary seal and setback approximately 300 feet from San Luis Obispo Creek. Because the sanitary seal is less than the required 50-foot depth, a seal waiver stating that the well is not under surface water influence is needed to pump this well for drinking water use. The City has performed an initial consultation with the Division of Drinking Water to obtain a waiver for the well including discussing preliminary mitigation measures to permit use of the well in the drinking water system. Additional follow up work is required to obtain the waiver. If a wavier is not granted there is space available to drill an additional well nearby with the required 50-foot seal. Given the available information, the Highway 101 Well could produce sufficient water to pump 250 to 400 AFY5 at a maximum design pumping rate of 450 gpm based on well pump tests (Cleath-Harris Geologists, 2017). There is also adequate space at this well site for GAC treatment system and is the preferred well site for a centralized treatment location to treat multiple wells within the City. 3.3 Hydraulic Capture Zone Analysis As part of this Project, Cleath-Harris Geologists updated the City’s groundwater flow model to evaluate the feasibility of using treatment wells to control plume migration and expedite groundwater treatment. The work involved preparing an updated conceptual hydrogeologic model that incorporates the results of the Remedial Field Investigation and updating and recalibrating the numerical groundwater flow model for the project area. The calibrated groundwater flow model was used to simulate groundwater levels and particle capture over current wet and dry hydrologic conditions for the status quo and with treatment wells. The particle capture method relies on the groundwater flow model and simulates the PCE contamination as particle movement. While the hydraulic capture scenarios and results can be used to establish treatment locations and pumping schedules for controlling plume migration, it does not consider the PCE concentration gradient nor evaluate the full fate and transport of PCE within the basin. Fate and transport modeling of PCE is planned for a future phase of the Project. This section summarizes the results of the Hydraulic Capture Zone Analysis, and the complete report with details on the model update and recalibration is included in Appendix A. 5 Treatment Well #3 production capacity of up to 400 AFY assumes a 300-gpm well pumped 20 hours per day 320 days per year. Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-6 DRAFT: Feasibility Study Report A total of seven particle tracking scenarios were evaluated in the model, described below and summarized in Table 1: 1. Baseline Scenario: Simulates particle tracking with no new treatment wells. 2. Three Wells, Low Pumping Scenario: Assumes all three treatment wells described in this report are pumping 100 AFY each, for a total of 300 AFY pumped and treated. 3. Three Wells, High Pumping Scenario: Assumes all three treatment wells described in this report are pumping 200 AFY each, for a total of 600 AFY pumped and treated. 4. Two Wells Scenario A: High pumping scenario assumes 300 AFY extracted at the Dalidio Drive Well and 300 AFY extracted at the Highway 101 Well, for a total of 600 AFY pumped and treated. 5. Two Wells Scenario B: Moderate pumping scenario assumes 150 AFY extracted at the Mid-Higuera Well and 350 AFY extracted at the Dalidio Drive Well, for a total of 500 AFY pumped and treated. 6. Two Wells Scenario C: Moderate pumping scenario assumes 150 AFY extracted at the Mid-Higuera Well and 350 AFY extracted at the Highway 101 Well, for a total of 500 AFY pumped and treated. 7. Two Wells Scenario D: High pumping scenario assumes 200 AFY extracted at the Dalidio Drive well and 400 AFY extracted at the Highway 101 Well, for a total of 600 AFY pumped and treated. Table 1. Particle Tracking Scenarios Scenario Pumping Distribution by Well (AFY) Total Pumping (AFY) Mid- Higuera Dalidio Drive Highway 101 1 Baseline 0 0 0 0 2 3 Wells, Low Pumping 100 100 100 300 3 3 Wells, High Pumping 150 200 250 600 4 2 Wells, Scenario A 0 300 300 600 5 2 Wells, Scenario B 150 350 0 500 6 2 Wells, Scenario C 150 0 350 500 7 2 Wells, Scenario D 0 200 400 600 The groundwater model was used to simulate the water level response to pumping at the three potential well locations and identify well production restrictions to avoid impacting nearby private wells. Based on the water level response results and to limit impacts to private wells, the production at the Mid-Higuera Well is limited to 150 AFY in all particle tracking scenarios. For similar reasons, the production at either the Dalidio Drive Well or Highway 101 Well is limited to 350 AFY when pumped concurrently with the Mid-Higuera Well, which limits the total production to 500 AFY in particle tracking Scenario 5 and 6. Pumping impacts to surface water flow could further restrict pumping from these locations, but not enough information is available to gauge the risk at this point. This should be evaluated in future modeling efforts. Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-7 DRAFT: Feasibility Study Report Table 2 includes the results of the particle tracking scenarios, showing the percentage of particles captured in each of the ultimate particle destination locations. See Figure 10 for an example capture zone analysis. The particles shown on the figure represent the PCE Plume. Additional information on the capture zone analysis and the well areas of influence can be found in the Hydraulic Capture Zone Analysis Report, included as Appendix A. As shown by the results, about 35% of the simulated PCE particles are captured by pumping the three proposed treatment wells 300 AFY, 39% to 46% are captured pumping two wells a total of 500 AFY, and 54% are captured in all scenarios pumping 600 AFY. The pumping scenarios decrease the percentage of particles ultimately captured by surface streams (71% of particles captured by streams in the baseline scenario compared to 23% to 44% in the pumping scenarios). The capture by other wells is also reduced in the pumping scenarios compared to the baseline scenario. The total percentage of particles remaining in transit is 1% or less for all scenarios (Cleath-Harris Geologists, 2022). The total time for the particles to reach their capture location was also reduced by about half in the pumping scenarios compared to the baseline scenario, suggesting pump and treat will reduce the total plume remediation time. The actual remediation time is dependent on mass transport parameters not considered in this simplistic model and should be evaluated in future groundwater fate and transport modeling analyses. Future fate and transport modeling should also consider the known ongoing PCE source and the impacts of source control or clean up on the plume remediation and attenuation time frame. Table 2. Hydraulic Capture Zone Results, Percent Capture of Particles Released in PCE Plume Area Scenario Treatment Wells Other Wells Surface Streams Total Mid- Higuera Dalidio Drive Highway 101 Subtotal 1 Baseline 0 0 0 0 29 71 100 2 3 Wells, Low Pumping 9 9 17 35 20 44 99 3 3 Wells, High Pumping 11 20 23 54 17 29 100 4 2 Wells, Scenario A 0 28 26 54 20 26 100 5 2 Wells, Scenario B 11 28 0 39 14 47 100 6 2 Wells, Scenario C 11 0 35 46 25 28 99 7 2 Wells, Scenario D 0 20 34 54 24 23 100 Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-8 DRAFT: Feasibility Study Report Figure 10. Treatment Well Capture Zone Example – Excerpt from Hydraulic Capture Zone Analysis Report Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-9 DRAFT: Feasibility Study Report 3.3.1 Findings To maximize PCE capture at treatment wells, the City should pump up to 600 AFY as reflected in Scenario 3, 4, and 7. Two wells are recommended over three for capital costs saving and because the amount of PCE particles captured when pumping 600 AFY from two wells in Scenario 4 and 7 is equal to pumping the same volume from all three wells. This is less than the estimated 700 AFY of surplus groundwater available in the San Luis Valley Subarea documented in the San Luis Obispo Basin GSP (WSC et al., 2021). However, the actual pumping volumes may vary year to year depending on the available water supplies and groundwater and surface water monitoring needed to meet the Sustainable Groundwater Management Act requirements. Additionally, the impacts to surface water flows and nearby wells should be evaluated and minimized for the recommended treatment well production rates. As mentioned, this analysis does not consider PCE concentrations throughout the plume, and the total particle capture may not fully reflect the total PCE mass removed by treatment wells. Two scenarios that include the Mid-Higuera Well (Scenario 5 and 6) are limited to a total production of 500 AFY to minimize water level impacts to private wells, and the lower pumping results in less particle capture in the treatment wells. However, the Mid-Higuera Well is located closest to the PCE source and may capture higher PCE concentrations than the other treatment wells and should not yet be excluded from further analysis. Scenarios 3 through 7 are all considered in the cost-benefit analysis of this report. Subsequent fate and transport modeling is planned for the next phase of this Project to evaluate the PCE mass removed in each treatment alternative and inform the total remediation time. Depending on the project schedule and grant timeline, the City should use the data from the future monitoring wells to update and/or confirm the findings of the groundwater models. The City is considering a potential Indirect Potable Reuse (IPR) project through a Groundwater Recharge Reuse Project (GRRP). Potential locations for recharge are located along San Luis Obispo Creek between Elks Lane and Highway 101 (Cleath-Harris Geologists, 2019). If recharge along Elks Lane becomes the preferred recharge area for the GRRP, it could impact proposed Treatment Well #2 at Dalidio Drive. Further analysis of the GRRP project travel time and influence on siting of a treatment well is required as part of future modeling. 3.4 Treatment Alternatives Three wellhead treatment alternatives were considered for PCE treatment in the basin: (1) granular activated carbon (GAC); (2) Air Stripping; and (3) Advanced Oxidation. This section focuses on PCE treatment, but as the City moves forward with design of wells, the treatment system should consider other constituents of concern that may be in the groundwater in addition to PCE, (i.e., PFAS, Nitrates, Chromium-6, etc.). 3.4.1 Granular Activated Carbon The GAC treatment option involves pumping groundwater through two pressure vessels in series packed with GAC media, which removes PCE and other materials through adsorption to the GAC Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-10 DRAFT: Feasibility Study Report media. Adsorption by GAC is effective at reducing the concentration of PCE below the MCL (EPA, 2022). The GAC is “activated” with chemical or thermal treatment to give it a high ratio of surface area to volume, which increases adsorption per unit volume and decreases the volume of adsorptive material required in water treatment. As flow passes through the media, PCE and other material will accumulate on the GAC and the media will eventually become saturated and breakthrough will occur, leading to higher effluent concentrations of PCE. When the lead GAC vessel in the series reaches breakthrough, the GAC media is replaced with new GAC or reactivated-in-place, and the lead–lag vessels are switched. For the estimated volume of GAC required for one to two 300 gpm wells with an assumed average PCE concentration of 8 µg/L6, removing and replacing the GAC is likely to be more cost-effective than reactivating-in-place. However, additional constituents of concern, such as PFAS, may also adsorb to the GAC media and increase the frequency of replacement and make reactivating-in-place more cost effective. As the design of the treatment systems progresses, the City should sample for constituents of concern at the proposed wells and seek quotes from GAC vendors to compare cost of replacement with the cost of reactivating-in-place. The mass of GAC in the reactor vessels will be selected to balance capital costs, operational costs, and frequency of replacement. Both biological fouling and inorganic scaling can occur inside GAC vessels, which leads to higher head loss through the vessel and earlier breakthrough because the adsorptive surface area is covered. If needed, pre-treatment can include injection of sodium hypochlorite for biological fouling and acid to reduce scaling. Each is injected in the influent water to prevent fouling and scaling from occurring. On-site storage of sodium hypochlorite and acid are required for these options. In addition to changing out the activated carbon media, GAC treatment may require maintenance of the acid and sodium hypochlorite storage and injection systems if needed to control biological fouling. 3.4.2 Air Stripping Air Stripping involves passing an air stream through a stream of contaminated groundwater to transfer PCE from the aqueous phase to the gas phase. PCE has a relatively high Henry’s constant (0.02 atm-m3/mol), which enables efficient transfer of PCE out of the contaminated water. Air-stripping is commonly followed by GAC treatment of the air stream to remove PCE so it is not discharged to the environment; however, GAC treatment of air may not be required if the initial concentration of PCE in the groundwater is low enough. This report assumes GAC treatment of the air stream will be required since the intent of the treatment effort is to remove PCE from the area and the fate of airborne PCE could lead to contamination elsewhere. Similar to GAC treatment, pre-treatment for air-stripping may include injection of sodium hypochlorite and acid into the raw water prior to entering the stripping tower to prevent biological fouling and inorganic scaling in the tower, if needed. Operation and maintenance are limited to 6 This is an assumed concentration of PCE to be treated. The concentration of PCE will need to be refined when fate and transport modeling is completed. Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-11 DRAFT: Feasibility Study Report periodic cleaning and inspection of the stripping tower. On-site storage of sodium hypochlorite and acid may be required. Air-stripping is effective at reducing the concentration of PCE below the MCL, however, it transfers the PCE from the aqueous phase to the gas phase and does not capture it (EPA, 2022). The concentration of PCE in the groundwater may be low enough that waste air from the stripping tower could be discharged to the air without further treatment, but further analysis would be required to confirm this, and future air quality regulations for VOCs may eventually require treatment of the waste air. The costs for air stripping, included in Table 3, assume GAC on the airstream will be needed. However, removal of PCE from the air phase is much more efficient than the liquid phase, and the GAC usage and cost are much less than treating the water stream with GAC. If GAC treatment is not required on the waste air, air-stripping cost can be reduced. Air-stripping towers operate in open-to-atmosphere conditions which will sacrifice the hydraulic head delivered by the well pump. An additional pump station will be required on-site to boost treated water into the distribution system. 3.4.3 Advanced Oxidation Advanced Oxidation treatment removes PCE from groundwater using a strong oxidizing agent such as ozone or hydroxyl radicals formed from the combination of ozone and hydrogen peroxide. Treatment involves injecting an oxidizing agent into a continuous reactor at the well head to destroy PCE prior to distribution. Ozone and hydroxyl radicals are both short lived and will not remain as residuals in the water supply after treatment. On-site storage and continuous consumption of hydrogen peroxide and oxygen gas are required, as well as an on-site ozone generator. Advanced oxidation does not usually require pretreatment. However, pretreatment may be included if pilot testing suggests that pre-treatment using pre-filtration reduces the consumption of ozone and hydrogen peroxide by other constituents. Treating PCE with advanced oxidation is effective at reducing the concentration below the MCL, however, it is more suited for groundwater contaminated with much higher concentrations of PCE where the use of a strong oxidant is the only cost-effective means of meeting the MCL. There are no waste products to dispose of using advanced oxidation, but this is offset by the requirement to purchase and store oxygen and hydrogen peroxide on-site, and the operational energy cost associated with generating ozone on-site. The higher capital and operation and maintenance costs make advanced oxidation only a suitable option for contaminated areas with higher concentrations of PCE than were measured in the San Luis Valley Subarea of the Basin; therefore, it is not the preferred alternative. 3.4.4 Treatment Cost Comparison Table 3 presents the capital and lifecycle costs for each treatment type for comparative purposes in 2022 dollars. For each treatment alternative, the capital costs include the construction cost for a treatment facility to serve a single well operating at 300 gpm. The capital costs include the Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-12 DRAFT: Feasibility Study Report construction cost estimate, project development costs at 15% of construction cost, construction phase implementation support costs at 15% of construction cost, and a 30% construction cost contingency. The annual operation and maintenance (O&M) costs were estimated to include power consumption, carbon replacement, chemical use, and other maintenance costs. The net present costs include capital costs plus O&M costs over 30 years using an assumed inflation rate of 3% per year and a discount rate of 5.6% per year (US OMB, 2022). Table 3. PCE Treatment Type Cost Comparison TREATMENT TYPE CAPITAL COST ANNUAL O&M COST NET PRESENT COST GAC Treatment $693,000 $25,000 $1,163,000 Air Stripping (1) $2,037,000 $32,500 $2,442,000 Advanced Oxidation $1,437,000 $20,000 $1,862,000 Source: Adapted from City of San Luis Obispo PCE Plume Prop 1 Groundwater Grant Program Round 3 Application, Attachment 8: Cost Benefit Analysis Notes: Costs in 2022 dollars 1. Air Stripping assumes GAC treatment is needed for the airstream. 3.4.5 Recommended Treatment Method Based on the estimated net present costs (which may also be known as lifecycle costs), the recommended PCE treatment alternative is to use GAC treatment at the wells. Air-stripping will require an additional pump station to boost treated water into the distribution system and is significantly more complex and expensive to operate and maintain, making it more expensive to operate than GAC. Advanced oxidation is significantly more complex than the other treatment alternatives, typically used for higher PCE concentrations, and has higher lifecycle costs than GAC treatment. This space intentionally blank. Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-13 DRAFT: Feasibility Study Report 3.5 Treatment Location This section evaluates centralized versus decentralized GAC treatment for the wells. 3.5.1 Centralized GAC Treatment Centralized treatment includes a single GAC treatment system sized for all treatment wells. Because there is only one treatment system to operate and maintain, including GAC replacement, there are some capital and O&M savings for the treatment system. Because a GAC treatment system takes up a large footprint and can be up to 20 feet tall, depending on configuration, a treatment system may not be suitable for well locations where the footprint is limited or in residential neighborhoods where it may have increased visibility. A centralized system allows the City to construct wells in ideal locations for PCE capture and treatment that may not be suitable for co-location of GAC treatment systems. The preferred location for a centralized GAC treatment system is at the Highway 101 Well because there is adequate land, it is already owned by the City, and it is zoned for public facilities. While there will be some treatment cost savings, additional 6-inch system piping will be required to pump raw water from wells to the centralized GAC treatment system, which increases the overall capital costs. Due to the location of the Highway 101 Well, a pipeline will either need to be bored under Highway 101 or run through the City’s Water Resource Recovery Facility with dense utilities and potential utility conflicts. Although not included in the estimated centralized treatment costs shown in Table 4, the City could reduce the raw water pipeline construction costs for centralized treatment if these pipelines were combined with other planned City pipeline replacement projects. Additionally, the City saved an abandoned pipeline that runs underneath Highway 101 that could be used as a carrier casing for the 6-inch raw water pipe from the Dalidio Drive Well and reduce the estimated pipeline costs for centralized treatment in Table 4. The feasibility of using this pipeline as a carrier casing should be performed in the future once pipeline alignments are better defined. A centralized treatment system may also require a pump station after GAC treatment to pump water into the distribution system depending on the final selected well pumps and system operation. Costs for centralized treatment should be updated to incorporate these costs once treatment well locations and connecting pipeline alignments are finalized. 3.5.2 Decentralized GAC Treatment Decentralized treatment will consist of smaller GAC treatment systems located at each well site that can pump directly into the City’s distribution system. This means the City will have multiple GAC systems to construct and maintain, including regular carbon replacement. With decentralized treatment both well sites will need enough land to construct a well and the GAC treatment system. This may be infeasible if adequate property cannot be obtained. For aesthetic compatibility, the two wells located away from the Water Resource Recovery Facility may require a building to house the GAC system. The cost for a simple building is included in the cost estimate. Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-14 DRAFT: Feasibility Study Report 3.5.3 Treatment Location Cost Comparison Table 4 presents the capital and lifecycle costs in 2022 dollars for drilling one new well (assumed to be the Dalidio Drive Well), equipping the Dalidio Drive and Highway 101 Wells, and either constructing a centralized GAC treatment system sized for up to 600 gpm or constructing decentralized GAC treatment with two 300 gpm GAC treatment systems location at each well site. For the centralized treatment alternative, the GAC system is assumed to be located at Highway 101. This alternative includes approximately 4,000 feet of 6-inch pipeline needed to pump raw water from the Dalidio Drive Well to the centralized GAC system at the Highway 101 Well. The pipeline length increases to about 7,300 feet if the Mid-Higuera Well were constructed instead of the Dalidio Drive Well because it is a further distance from the Highway 101 Well and centralized treatment site. The centralized alternative also includes a pump station after GAC treatment to pump into to the distribution system, however, this component may not be needed after well pump selection and further system hydraulic analysis. The decentralized treatment alternative includes costs for a building to house the GAC system at the well located near existing homes and businesses (Dalidio Drive Well or Mid-Higuera Well). A building to house the GAC system is probably not needed at the Highway 101 Well and is not included in the cost estimates. The capital costs include the construction cost estimate, project development costs at 15% of construction cost, construction phase implementation support costs at 15% of construction cost, and a 30% construction cost contingency. The annual O&M costs were estimated including the power, carbon replacement, chemicals, and other maintenance costs. The net present costs include capital costs plus O&M costs over 30 years using an assumed inflation rate of 3% per year and a discount rate of 5.6% per year (US OMB, 2022). Table 4. Centralized and Decentralized Cost Comparison TREATMENT TYPE CAPITAL COST ANNUAL O&M COST NET PRESENT COST Decentralized Treatment (1) $6,564,000 $83,000 $7,903,000 Centralized Treatment (2) $7,700,000 $68,000 $8,672,000 Notes: Costs in 2022 dollars. 1. Decentralized treatment includes two wells, two 300 gpm GAC systems, and one GAC building. 2. Centralized treatment includes two wells, a single 600 gpm GAC system, 4,000 feet of 6-inch pipe, and a pump station. 3.5.4 Recommended Treatment Locations Overall, the costs are lower for the decentralized GAC treatment alternatives compared to the centralized GAC treatment alternative. This is because the wells with decentralized treatment can be connected to nearby distribution pipes while the centralized treatment system requires longer Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-15 DRAFT: Feasibility Study Report pipelines to connect to the centralized treatment system and may require a pump station after treatment to pump the treated water into the distribution system. If a pump station were not needed, centralized treatment becomes cost competitive with decentralized treatment when the pipeline needed to connect well sites is 4,000 feet or less. The pipeline length required to connect the Dalidio Well to the Highway 101 Well is about 4,000 feet and the length of pipeline required to connect the Mid-Higuera Well with the Highway 101 site is about 7,300 feet. While decentralized treatment is preferred due to overall lower costs, the City should continue to consider centralized treatment for alternatives that include the Dalidio Drive Well and Highway 101 Well. There are also potential barriers to obtaining the private property needed for a new well and well building. The City should conduct outreach to the existing property owners and begin discussions regarding obtaining land and easements for future wells with GAC treatment. 3.6 Cost / Benefit Analysis 3.6.1 Alternative Costs The proposed Project includes the development of at least two wells to pump and treat groundwater for distribution in the City’s water system. Particle tracking Scenarios 3 through 7 are all considered due to their PCE particle capture compared to the baseline (Appendix A). These five scenarios are represented as four alternatives, described in Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-16 DRAFT: Feasibility Study Report Table 5. Particle tracking Scenarios 4 and 7 were combined into a single alternative because they include the same two treatment wells, just with varying production between each well. Each alternative assumes decentralized GAC treatment. Centralized treatment and other treatment types were not considered due to their higher costs. Based on pumping rates from other City wells, the pumping rate from each well was assumed to be 300 gpm. The wells were assumed to operate 18 hours per day, and the number of days per year equal to the total annual production in AFY (i.e., a well pumping 300 gpm, 18 hours per day, for 200 days per year produces about 200 AFY). The total pumping in each alternative ranges from 500 to 600 AFY. Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-17 DRAFT: Feasibility Study Report Table 5 presents the capital and net present cost for each alternative in 2022 dollars. The capital costs include the construction cost estimate, project development costs at 15% of construction cost, construction phase implementation support costs at 15% of construction cost, and a 30% construction cost contingency. The annual O&M costs were estimated including power, carbon replacement, chemicals, and other maintenance costs. The cost per acre-foot of supply includes annual payments for 10% of the capital costs over 30-years (local match requirement only because the capital costs are assumed to be grant funded through Proposition 1) plus annual O&M costs. The net present costs include capital costs plus O&M costs over 30 years using an assumed inflation rate of 3% per year and a discount rate of 5.6% per year (US OMB, 2022). Detailed cost estimates for each alternative are provided in Appendix B. As shown in Table 5, Alternatives 2 and 4 have the lowest capital costs and are preferred as they include the Highway 101 Well. The Highway 101 Well is already drilled and does not require a building to house the GAC system. The second well can be either the proposed Dalidio Well or Mid-Higuera Well. Preference for the Dalidio or Mid-Higuera well will depend on subsequent fate and transport modeling and land availability. Alternative 4 has a lower O&M cost and net present cost because there is 100 AFY less pumping which results in lower electrical costs, but also in a slightly higher cost per acre-foot compared to Alternative 2. Alternative 3 has slightly higher costs than both Alternatives 2 and 4 because it includes two new wells to be drilled and both locations may require a building to house the GAC system. Alternative 1, which includes pumping from all three wells, has higher overall costs. This is due to higher capital and treatment costs for three wells with the same 600 AFY pumping volume as the other alternatives. This means the cost per acre-foot of water increases compared to the other options. Unless subsequent fate and transport modeling indicates three wells are required to control and capture the plume, this alternative is not preferred. 3.6.2 Project Schedule The estimated schedule for each cleanup alternative was also considered but was ultimately not a differentiator in determining the preferred alternative. As described in Section 3.3, the alternative remediation time is dependent on mass transport parameters not considered in the particle tracking model. Rather, the schedule for the planning, design, and construction for each alternative was estimated. At the current level of alternative development, the estimated schedule for each alternative was roughly the same and is provided in Figure 11. This space intentionally blank. Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-18 DRAFT: Feasibility Study Report Table 5. Well Alternative Cost Comparison Alternative Particle Tracking Scenario(s) Wells Total Production Project Cost Annual O&M Cost O&M Cost /AF Net Present Cost 1 3 (1) Mid-Higuera (2) Dalidio Drive (3) Highway 101 600 AFY $10,745,000 $108,300 $240 $12,369,000 2 4, 7 (1) Dalidio Drive (2) Highway 101 600 AFY $6,564,000 $83,300 $175 $7,903,000 3 5 (1) Mid-Higuera (2) Dalidio Drive 500 AFY $8,3662,000 $77,700 $211 $9,492,000 4 6 (1) Mid-Higuera (2) Highway 101 500 AFY $6,564,000 $77,700 $199 $7,790,000 Note: Costs in 2022 dollars. This space intentionally blank. Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-19 DRAFT: Feasibility Study Report Figure 11. Estimated Alternative Implementation Schedule PLAN PLAN ACTIVITY START DURATION MONTHS Jan Feb Mar AprMay Jun Jul Aug SepOct NovDec Jan Feb Mar AprMay Jun Jul Aug SepOct NovDec Jan Feb Mar Apr MayJun Jul Aug Sep Oct NovDec Jan Feb Mar AprMay Jun Jul Aug Sep Oct NovDec 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 Project Administration 4 36 Groundwater Model 4 4 Develop Extraction and Monitoring Plan 6 2 Design Monitoring Wells 8 8 Design Extraction and Treatment Wells (two locations) - Well Casing 7 5 Design Extraction and Treatment Wells (two locations) - Well Equipment 11 11 Permitting 7 27 Construction - Monitoring Well(s)18 8 Construction Administration - Monitoring Well(s)17 10 Construction - Extraction and Treatment Wells (two locations) - Well Casing 14 4 Construction Administration - Extraction and Treatment Wells (two locations) - Well Casing 13 6 Construction - Extraction and Treatment Wells (two locations) - Well Equipment 24 12 Construction Administration - Extraction and Treatment Wells (two locations) - Well Equipment 23 14 Monitoring and Performance 35 5 Outreach 4 36Planning, Design, Engineering, and EnvironmentalConstruction and ManagementTask Name 2026202320242025 Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-20 DRAFT: Feasibility Study Report 3.6.3 Benefits Analysis All alternatives provide environmental benefits by removing PCE contamination from an impaired groundwater basin. The project will reduce the ultimate capture of PCE in local streams and reduce potential for transmission of PCE contaminated groundwater to downstream groundwater basins and users. The mass of PCE that will be removed on an annual basis cannot be estimated until the future fate and transport modeling is completed. This Project also provides enhanced local water supply reliability. The City does not currently use their groundwater supply and this project will allow the City to produce from a largely underutilized source of water. By expanding the City’s water supply portfolio to include groundwater, it provides greater operational flexibility to conjunctively use its supplies depending on drought conditions and supply availability. Also, this Project will increase water supply resiliency against interruptions at the City’s Water Treatment Plant where all other supplies are treated. Additionally, as the City begins to utilize its groundwater as a potable supply, the capacity created in the groundwater basin may allow the City to recharge its recycled water to the Basin through GRRP. Although the City’s future IPR project is only in its early planning phases, it could include recharge of the Basin using the City’s recycled water. Likely areas for recharge are located upstream of proposed treatment wells TW#2, and TW#3 (Cleath-Harris Geologists, 2019). The percolated recycled water can then be pumped from the wells for potable use. This increases the volume of groundwater available to the community, makes beneficial use of recycled water, and provides a drought resilient supply (City of San Luis Obispo, 2022). The potential GRRP will require additional analysis to confirm travel time requirements between GRRP recharge sites and potential treatment wells. 3.7 Recommendations The recommendations to meet the remedial action objective based on the criteria described in Section 3.1 include:  GAC is the recommended well head treatment technology for PCE removal from the groundwater. The groundwater at the preferred well location should be tested for other constituents of concern prior to design so that the preferred treatment method(s) can be selected.  Once final treatment well locations are selected, the costs for centralized GAC treatment and decentralized GAC treatment should be updated and compared to make the final selection of where the GAC treatment should be located. The least expensive option for centralized treatment is expected to occur if the Dalidio Well (TW#2) and Highway 101 Well (TW#3) are selected. This combination of treatment wells requires the shortest length of pipe to connect the wells and was the basis of the cost comparison in Section 3.5. The cost for decentralized treatment is slightly less in this configuration; however, the final costs of a centralized system will depend on the length and alignment of the connecting pipeline and necessity for a booster station if required by well pump hydraulics. Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-21 DRAFT: Feasibility Study Report  Two wells are recommended for plume control and remediation, one of which should be the Highway 101 Well because it is already drilled and will not require a GAC building to mitigate visual impacts.  The City should conduct subsequent groundwater fate and transport modeling to incorporate plume concentrations and understand the mass of PCE removed with the preferred treatment alternative. The fate and transport modeling can be used to further verify the well locations, verify that the number of wells is acceptable for plume management, and establish a remediation time. If the fate and transport modeling shows remediation at a potential source location is the preferred project, the City will notify the RWQCB. The State and RWQCB may coordinate cleanup efforts with the Responsible Parties.  The City should also begin outreach with private property owners for purchase of land and obtaining an easement for future wells. Depending on these discussions, additional well locations may need to be considered and evaluated in future modeling efforts.  The potential for a GRRP should be considered when modeling and siting treatment wells. Table 6 presents the estimated Project costs for Alternative 2, which includes the drilling of 1 new well (Dalidio Drive Well), equipping two wells (Dalidio Drive and Highway 101 Well) and constructing GAC treatment at each site. At the current level of project development, the cost to drill and equip the Mid-Higuera Well (TW#1) is the same as the cost of the Dalidio Drive Well (TW#2). The total production for this alternative is 600 AFY. Also listed are the estimated O&M and net present costs. Figure 12 presents a conceptual site layout for a new well site with GAC treatment. To permit a new well, it must have an established 50-foot radius around the site, known as the control zone, to protect the source from vandalism, tampering, or other threats at the site. Only systems associated with the well can be located within this control zone and represents the minimum lot size for a new well. The example site layout shown in Figure 5 includes the GAC treatment within the wellhead building, although the building is not included in the cost estimates for the Highway 101 Well since that well may not require a building. This space intentionally blank. Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-22 DRAFT: Feasibility Study Report Table 6. Recommended Alternative Project Costs Component Quantity Unit Unit Cost Total Capital Costs Well Drilling 170 Ft $2,965 $504,000 Well Equipping 2 Each $750,000 $1,500,000 GAC System - 300 gpm 2 Each $410,000 $820,000 GAC Building 1600 Sq. Ft. $350 $560,000 Piping and Appurtenances 2 Each System $250,000 $500,000 Subtotal $3,884,000 Construction Contingency (30%) $1,165,000 Construction Total $5,049,000 Project Development & Implementation (30%) $1,515,000 Project Cost $6,564,000 Annual O&M Costs Power (Well 1 & 2) $33,300 GAC Replacement (2 Treatment Systems) $30,000 Equipment Maintenance, Repair, Replacement, and Testing $20,000 Annual O&M Cost $ 83,300 Lifecycle Costs Project Life, Years 30 AFY 600 Annual Operation $/AF(1) $175 Net Present Costs (2) $ 7,903,000 Note: Costs in 2022 Dollars. 1. The cost per acre-foot includes annual payments for 10% of the capital costs over 30-years (local match requirement because the capital costs are assumed to be grant funded through Proposition 1) plus annual O&M costs. 2. Includes capital costs, annual O&M costs over 30-years with 3% annual inflation rate, and a discount rate of 5.6%. Section 3.0 Remedial Action Alternatives City of San Luis Obispo 3-23 DRAFT: Feasibility Study Report Figure 12. Example Well Site Concept City of San Luis Obispo DRAFT: Feasibility Study Report REFERENCES City of San Luis Obispo. (2022). Retrieved from Recycled Water: https://www.slocity.org/government/department-directory/utilities- department/water/water-sources/recycled-water Cleath-Harris Geologists. (2014). Hydrogeologic Description and PCE Characterization Dalidio Laguna Ranch, San Luis Obispo County, California. Cleath-Harris Geologists. (2017). Results of Pumping Test at Highway 101 Well. Cleath-Harris Geologists. (2019). Groundwater Flow Analysis Recycled Water Recharge Project San Luis Valley Sub-Area San Luis Obispo Valley Groundwater Basin. Cleath-Harris Geologists. (2022). Hydraulic Capture Zone Analysis for the Remedial Investigation Feasibility Study; PCE Plume Characterization Project; San Luis Obispo Valley Groundwater Basin. DWR. (2003). California's Groundwater: Bulletin 118 - Update 2003, Groundwater Basin 28 Descriptions. GSI Water Solutions. (2019). San Luis Obispo Valley Basin Characterization and Monitoring Well Installation. QPS . (2005). DRAFT Background Study South San Luis Obispo Groundwater PCE Plume. QORE Property Sciences (QPS). SWRCB, Division of Water Quality. (2017). Groundwater Information Sheet, Tetrachloroethylene (PCE). URS. (2013). Final Investigation Report: San Luis Obispo PCE Groundwater Plume. URS. (Investigation Report: San Luis Obispo PCE Groundwater Plume). 2015. US OMB. (2022). Circular No. A-94: Guidelines and Discount Rates for Benefit-Cost Analysis of Federal Programs, Appendix C Discount Rates for Cost-Effectiveness, Lease Purchase, and Related Analyses. WSC. (2021). RI/FS Work Plan . WSC. (2022). Draft Remedial Investigation Report for the City of San Luis Obispo PCE Plume Characterization Project. WSC et al. (2021). San Luis Obispo Valley Groundwater Sustainability Plan . Water Systems Consulting, Inc.; GEI consultants, Cleath-Harris Geologists, Inc.; Stillwater Sciences, GSI Water Solutions Inc. Appendix A [Client] A [DRAFT: Feasibility Study Report] Appendix A Hydraulic Capture Zone Analysis HYDRAULIC CAPTURE ZONE ANALYSIS for the CITY OF SAN LUIS OBISPO TETRACHLOROETHYLENE (PCE) PLUME CHARACTERIZATION PROJECT SAN LUIS OBISPO VALLEY GROUNDWATER BASIN Prepared for CITY OF SAN LUIS OBISPO AND WATER SYSTEMS CONSULTING DECEMBER 2022 CLEATH-HARRIS GEOLOGISTS 75 Zaca Lane, Suite 110 San Luis Obispo, California 93401 (805) 543-1413 PCE Hydraulic Capture Zone Analysis i December 2022 TABLE OF CONTENTS SECTION PAGE EXECUTIVE SUMMARY ............................................................................................................ 1 1.0 INTRODUCTION ............................................................................................................... 2 2.0 SOURCES OF INFORMATION ........................................................................................ 2 2.1 Groundwater Flow Analysis, Recycled Water Recharge Project (January 2019) ........... 2 2.2 San Luis Obispo Valley Basin Groundwater Sustainability Plan (October 2021) ........... 3 2.3 San Luis Obispo PCE Plume Remedial Field Investigation (March-June 2022) ............ 3 2.4 Other Sources of Information ........................................................................................... 3 3.0 UPDATED HYDROGEOLOGIC CONCEPTUAL MODEL ............................................ 3 3.1 Expanded Project Area ..................................................................................................... 4 3.2 Updated Hydrostratigraphy .............................................................................................. 4 3.3 Updated Hydrologic Conditions....................................................................................... 6 4.0 FLOW MODEL UPDATE .................................................................................................. 7 4.1 Model Code Selection ...................................................................................................... 7 4.2 Model Domain.................................................................................................................. 7 4.3 Hydrologic Budget Items ............................................................................................... 11 5.0 MODEL CALIBRATION ................................................................................................. 13 5.1 Calibration Targets ......................................................................................................... 13 5.2 Calibration Statistics ...................................................................................................... 13 6.0 HYDRAULIC CAPTURE ZONE ANALYSIS ................................................................ 15 6.1 Particle Tracking ............................................................................................................ 15 6.2 Treatment Well Locations and Production..................................................................... 16 6.3 Particle Tracking Scenario Results ................................................................................ 18 6.4 Sensitivity Analysis ........................................................................................................ 20 7.0 REFERENCES .................................................................................................................. 22 APPENDIX ................................................................................................................................... 23 PCE Hydraulic Capture Zone Analysis ii December 2022 List of Tables Table 1 – Model Aquifer Parameters Table 2 – Model Calibration Statistics Table 3 – Treatment Well Pumping Distribution Table 4 – Management Scenario Particle Capture Distribution Table 5 – Management Scenario Percent Particle Capture Table 6 – Sensitivity Analysis List of Figures Figure 1 – Project Area Figure 2 – Model Domain Figure 3 – Base of Permeable Sediments Figure 4 – Base of Quaternary Alluvium Figure 5 – Model Boundary Conditions Figure 6 – Observed vs Computed Heads Plot Figure 7 – Conceptual Treatment Wells and Particle Release Area Figure 8 – Baseline Scenario Particle Capture Figure 9 – Treatment Scenario – Distributed High Pumping Figure 10 – Treatment Scenario – Two-well Scenario A Figure 11 – Treatment Scenario – Two-well Scenario B Figure 12 – Treatment Scenario – Two-well Scenario C List of Appendix Figures Figure A1 – K Layer 1 Figure A2 – K Layer 2 Figure A3 – K Layer 3 Figure A4 – K Layer 4 Figure A5 – Sy Layer 1 Figure A6 – Ss Layer 2 Figure A7 – Ss Layer 3 Figure A8 – Ss Layer 4 PCE Hydraulic Capture Zone Analysis 1 December 2022 EXECUTIVE SUMMARY Cleath-Harris Geologists was tasked with updating an existing groundwater flow model to evaluate the feasibility of using treatment wells to control Tetrachloroethylene (PCE) plume migration, and to meet City of San Luis Obispo water supply resiliency goals while expediting plume remediation. This report summarizes the results of the model update and hydraulic capture zone analysis. The work involved preparing an updated conceptual hydrogeologic model that incorporates the results of the Remedial Field Investigation, updating and recalibrating the numerical groundwater flow model for the project area, and use of the model to simulate groundwater levels and particle capture over current hydrologic conditions that include wet and dry years. Hydraulic capture zone analysis was evaluated with forward particle tracking. Flow lines (particle tracks) originating within the PCE plume area were processed to identify where each particle is captured. The particle tracks can terminate at treatment wells, other wells, streams, or they may remain in transit or escape from the model domain through the subsurface. The greater the quantity of particles that terminate at treatment wells, compared to other destinations, the greater the potential for PCE plume hydraulic capture by the remediation program. The number of particles captured by a treatment well, however, does not correlate with the amount of PCE mass removed, because particles originating from different areas of the plume represent different PCE concentrations. PCE concentration containment and remediation times are not evaluated in this phase of modeling. Analyzing the movement and removal of dissolved PCE mass would involve adding the MT3DMS mass transport package to the model. This package could be added during future phases of the project. The particle capture analysis indicates 35 percent hydraulic capture of particles by treatment wells at 300 AFY production, and 54 percent capture at 600 AFY production. There is a notable reduction in particle capture by streams between the baseline scenario (71 percent) and the 600 AFY treatment scenario (less than 30 percent capture). Percent hydraulic capture by other (non- treatment) wells decreases slightly, from 29 percent capture under baseline conditions, to 14-25 percent capture for treatment scenarios. Most of the particles captured by the treatment wells would otherwise have escaped from the model through stream flow. For the baseline scenario, most of the particles were captured within 20 years from their time of release into the model, whereas in the treatment scenarios, most of the particles were captured within 10 years of their time of release. Plume remediation times would be lowered by treatment well pumping, compared to baseline, although actual remediation times would depend on mass transport parameters that are not considered in the hydraulic capture zone analysis, but could be modeled during future phases of the project. The success of pump-and-treat for lowering plume remediation times is also contingent on PCE source containment. PCE Hydraulic Capture Zone Analysis 2 December 2022 1.0 INTRODUCTION The City of San Luis Obispo (City) has a long-term goal of utilizing a multi-source water supply to meet the community’s water demand, ease the impacts of drought, and to provide resiliency. The Tetrachloroethylene (PCE) Plume Characterization Project will provide the City with the tools to protect its groundwater supply and expand a reliable, locally controlled drinking water source. Cleath-Harris Geologists (CHG) was tasked with updating an existing groundwater flow model to evaluate the feasibility of using treatment wells to control PCE plume migration by hydraulic capture, and to meet City water supply resiliency goals while expediting plume remediation through pump-and-treat. This report summarizes the results of the model update and hydraulic capture analysis. The work involved preparing an updated conceptual hydrogeologic model that incorporates the results of the Remedial Field Investigation, updating and recalibrating the numerical groundwater flow model for the project area, and use of the model to simulate groundwater levels and hydraulic capture over current hydrologic conditions that include wet and dry years. The purpose of the current modeling effort was to identify optimal areas for treatment wells and to evaluate the effectiveness of pumping the wells to control PCE plume migration through hydraulic capture zones. The project area overlies the San Luis Obispo Valley Groundwater Basin, DWR Bulletin 118 Basin No. 3-09 (Basin), in San Luis Obispo County, California. Figure 1 shows the project area with respect to groundwater basin boundaries and City limits. 2.0 SOURCES OF INFORMATION The primary sources of information that were used as part of this model update and hydraulic capture zone analysis include the following: • Groundwater Flow Analysis, Recycled Water Recharge Project (CHG, 2019) • San Luis Obispo Valley Basin Groundwater Sustainability Plan (WSC, 2021) • Remedial Field Investigation (WSC, 2022) 2.1 Groundwater Flow Analysis, Recycled Water Recharge Project (January 2019) The Groundwater Flow Analysis performed for the City’s Recycled Water Recharge Project included development of the groundwater flow model used for the current model expansion and update. The 2019 project report provided an overview of basin characterization efforts, pumping tests, streamflow data, well production, and hydrogeology. The main purpose of the study was to evaluate options for groundwater recharge using recycled water in order to enhance groundwater availability for the City water supply. SLO Valley Groundwater Basin (B118) Edna Valley Subarea San Luis Valley Subarea SLO City Limits Model Domain Explanation PCE Hydraulic Capture Zone Analysis 3 December 2022 2.2 San Luis Obispo Valley Basin Groundwater Sustainability Plan (October 2021) The Groundwater Sustainability Plan (GSP) is a State-mandated plan under the Sustainable Groundwater Management Act (SGMA). Information from Chapter 6 of the GSP (Water Budget) was used to develop estimates for selected hydrologic budget items within the model domain under current conditions (such as stream flow, well production, subsurface inflow and bedrock inflow). The GSP datasets, including the first Annual Report, run through Fall 2021. Recent water level contour maps from the GSP and Annual Report were also used to help establish calibration targets. 2.3 San Luis Obispo PCE Plume Remedial Field Investigation (March-June 2022) An extensive field investigation, consisting of soil borings using sonic drilling and direct push methods, along with Cone Penetrometer Testing and discrete interval water sampling, was performed from March through June 2022. CHG and WSC staff participated in the field investigation, which provided lithologic data and water quality results used to characterize the PCE plume extent and establish updated aquifer parameters and hydrostratigraphic model layers. 2.4 Other Sources of Information Supplemental hydrologic data files (rainfall, stream flow, groundwater levels, groundwater quality, Well Completion Reports) were obtained from the City of San Luis Obispo, the County of San Luis Obispo, and the State of California. Geologic maps, geophysical logs, and LiDAR topographic data were also obtained to refine the conceptual model. 3.0 UPDATED HYDROGEOLOGIC CONCEPTUAL MODEL The hydrogeologic conceptual model is a compilation and interpretation of hydrogeologic conditions within the model domain. This section highlights the changes made to the conceptual model used for the 2019 Groundwater Flow Analysis. Reasons for updating the conceptual model include: • Expansion of the model domain to encompass the current (and historical) extent of the PCE plume. • Changing the main purpose of the flow model from a recycled water recharge analysis to a PCE plume hydraulic capture zone analysis. • Preparing model for mass transport (future phases) • Changing the model calibration period • Incorporating new information from the Remedial Field Investigation. PCE Hydraulic Capture Zone Analysis 4 December 2022 3.1 Expanded Project Area As mentioned above, the active model area was expanded to encompass the current extent of the PCE plume, based on the results of the Remedial Field Investigation. A description and the field investigation, including boring logs and analytical results of water quality samples, is provided in the Remedial Investigation report (WSC, 2022). The model domain, where groundwater flow is simulated and the PCE plume is present, is centered in the San Luis Valley subarea of the San Luis Obispo Valley groundwater basin, which is structurally controlled by Los Osos/Edna fault zone and associated regional tectonic blocks (Figure 2). The active model boundary follows the general shape of the GSP basin boundary but is not expected to match the GSP boundary, which is based on historical mapping at a state -wide scale for DWR Bulletin 118 (DWR, 2020). For example, the GSP basin boundary extends up to a few thousand feet south of mapped splays within the Los Osos/Edna Fault Zone, which is a barrier to groundwater flow and the effective basin boundary for the model. The model domain also truncates the basin to focus on the PCE plume within the San Luis Obispo Creek alluvial valley and Highway 101 corridor, which runs from northeast to southwest and intersects the broader northwest-southeast San Luis Valley. This intersection between the alluvial creek valley and the broader structural valley results in bedrock areas forming the “corners” of the model domain, and which generally extend out to include the Irish Hills to the west; Cerro San Luis to the north; Orcutt Knob to the east and the hills above Davenport Creek to the south. The active model domain runs from Prefumo Creek on the northwest to San Luis Obispo Airport on the southeast. The San Luis Obispo Creek valley within the model domain runs from just above the confluence of San Luis Obispo Creek and Brizzolari Creek on the northeast, to just downstream of the basin boundary on the southwest (Figure 2). 3.2 Updated Hydrostratigraphy Model hydrostratigraphy refers herein to the established model layers/zones that share similar characteristics with respect to groundwater flow. Recent to Older age alluvial and terrace deposits, along with the underlying Plio-Pleistocene age sedimentary beds comprise the groundwater basin aquifers and aquitards. The Plio-Pleistocene age sedimentary beds include the Paso Robles Formation terrestrial deposits and the Pismo Formation (Squire member) marine sedimentary beds. The non-water bearing bedrock beneath the basin is mainly Franciscan Assemblage metamorphic rock. Elevation contour on the base of the permeable sediments is shown on Figure 3. Four hydrostratigraphic layers were defined for the updated conceptual model: • Layer 1 – Alluvium (aquitard) • Layer 2 – Alluvium (aquifer) • Layer 3 – Paso Robles Formation (aquitard) • Layer 4 – Paso Robles and Pismo Formations (aquifer) Model Boundary Bedrock Elevation of base of permeable sediments (feet above mean sea level) Explanation Model Boundary 2019 Recharge Model Boundary Bedrock (No Flow) Los Osos/Edna Fault Zone Explanation PCE Hydraulic Capture Zone Analysis 5 December 2022 Layer 1 – Alluvium (aquitard) Model Layer 1 represents a shallow clay aquitard that simulates confining pressures common within the alluvium along the Highway 101 corridor south of Madonna Road. The shallow aquitard increases pressure in both Layer 1 and Layer 2, improving calibration, while limiting the flow from Layer 1 toward simulated PCE treatment wells in Layer 2, which is appropriate. Although Layer 1 has significantly lower hydraulic conductivity in most areas, compared to the underlying Layer 2, a buried alluvial channel with relatively permeable sediments is interpreted to extend beneath San Luis Obispo Creek in portions of Layer 1. The buried channel underlies the active creek channel in the Prado Road area, then moves southeast of the active creek along South Higuera Street, returning to the active San Luis Obispo Creek channel at the confluence with Prefumo Creek. This shallow buried stream channel alignment provides for stream recharge in the Elks Lane area and productive shallow wells along South Higuera, while allowing groundwater pressure to develop and create gaining stream conditions south of Prado Road. Gaining stream conditions were observed in the field as a transition from no-flow to flow in the creek bed, with visually increasing flow downstream. Layer 2 – Basal Alluvium (aquifer) Model Layer 2 represents basal alluvial sands and gravels that form a highly transmissive aquifer zone. This layer is also where most of the PCE detections were recorded during the Remedial Field Investigation. The greatest permeabilities are interpreted from lithology and pumping tests to generally follow the Highway 101 corridor beneath the area where Layer 1 is relatively confining and, in the vicinity of the City’s Highway 101 well, extends laterally from Auto Park Way on the west to South Higuera on the east. Beyond the lateral extent of the basal sand and gravel, the alluvium in Layer 2 is moderately permeable aquifer material and includes finer sediments. The alluvial deposits reach a thickness of about 60 feet. Elevation contours on the base of the Quaternary alluvium is shown on Figure 4. Layer 3 – Paso Robles Formation contact (aquitard) Beginning south of Prado Road and extending laterally across the active model area, the alluvium is underlain by older Plio-Pleistocene deposits associated with the Paso Robles Formation and underlying Pismo Formation Sediments within the Paso Robles Formation that directly underly the Recent alluvium are interpreted to form an aquitard, due to a much greater percentage of clay, compared to Layer 2. The buried formation contact may also have weathered into a lower permeability layer (such as a paleosol) prior to the Recent alluvial deposition, although this surface could have subsequently eroded. It may not be apparent in drillers logs where the formation contact is, and generally the base of the alluvium is either bedrock (in the northern model area) or a clay, logged below a sand and gravel, that matches elevations consistent with the anticipated base of the alluvium. In other words, Layer 3 is generally the top of a clay, and is therefore modeled as an aquitard. Model Boundary Bedrock Elevation of base of quaternary alluvium (feet above mean sea level) Explanation PCE Hydraulic Capture Zone Analysis 6 December 2022 Layer 4 – Paso Robles and Pismo Formations (aquifer) Layer 4 represents aquifer zones within the Plio-Pleistocene deposits, which deepen toward the southwest basin boundary, due to the relative motion of the Los Osos/Edna fault zone. The overall permeability of these sediments is much less than the Quaternary alluvium, although locally they can be very productive. The Paso Robles Formation has a wider range of hydraulic conductivity, commonly logged as interbeds of sand/gravel and clay, while the productive upper Pismo Formation member (locally referred to as the Squire) is a more uniformly sorted marine sand. 3.3 Updated Hydrologic Conditions Rainfall, runoff and stream flow contribute surface water inflow to the project area. Recharge to groundwater occurs through percolation of precipitation and seepage of surface waters in the creeks, lagoons and drainage basins. The simulated hydrologic conditions, and hydrologic budget, were updated from the historical 2007-2010 calibration period used for the Groundwater Flow Analysis, to current conditions from April 2018 to March 2022. The hydrologic budget is an accounting of the elements of inflow and outflow to the groundwater system. The various datasets compiled on hydrologic budget items for incorporation into the conceptual model include the following items, as available, that were updated for the time period from April 2018 to March 2022: Inflow • Precipitation and evapotranspiration records. • Stream flow estimates from SLO GSP water budget model Outflow • Production estimates for commercial and industrial supply wells using usage factors and population served • Estimates of total agricultural production from wells using crop acreages, crop coefficients, evapotranspiration and rainfall. • Static water levels for monitoring wells and from well completion reports. Methodologies developed for the water budget in the SLO GSP for hydrologic conditions were used to update estimates for various hydrologic budget items, such as surface water inflow and direct percolation of precipitation. Watershed runoff from precipitation were proportioned based on correlations with stream gage data, and the percolation of precipitation estimates were adjusted to account for both the updated rainfall records and the differences in area between the active model and the San Luis Valley subarea. Data sets used for the hydrologic budget are detailed in Section 4.3. PCE Hydraulic Capture Zone Analysis 7 December 2022 4.0 FLOW MODEL UPDATE 4.1 Model Code Selection The flow model was performed using the U.S. Geological Survey groundwater modeling code MODFLOW-2005 (Harbaugh, 2005). This model code is industry standard for simulating groundwater flow and predicting groundwater conditions. The graphical user interface used for the injection model is Advanced Groundwater Vistas (Version 8, Environmental Simulations, Inc). Hydraulic capture zone analysis was performed using the MODPATH particle tracking package. 4.2 Model Domain The model boundaries, grid orientation, and vertical and horizontal discretization are all based on elements of the conceptual model, such as aquifer zones and aquitards (discussed above), the lateral extent of the PCE plume, and groundwater flow direction. Model Area Model boundary limits were designed to encompass the area surrounding and interacting with the groundwater dynamics of the PCE plume. The model area is approximately 5,800 acres, or 9.2 mi2 (Figure 2). Model Grid The model was constructed using a regular 85 row by 75 column grid, rotated at N30°W, which generally aligns with the predominant groundwater flow directions and the groundwater basin orientation. The cell size was set to 200 feet x 200 feet, providing sufficient horizontal discretization for pumping analysis and plume migration analysis. The model grid aligns with the 2019 Groundwater Flow Analysis model grid, with extensions of 3,000 feet to the southwest and 5,800 feet to the northeast to incorporate a longer reach of San Luis Obispo Creek and associated alluvial channel. Vertical Discretization The vertical discretization of the aquifers/aquitards provide for the simulation of vertical hydraulic gradients, preferential pathways for flow, and three-dimensional heterogeneity in aquifer parameters, as well as confining conditions. A review of prior interpretation of hydrostratigraphic correlations was performed to assist in conceptualizing the appropriate vertical discretization for the model. Geologic borings performed as part of the Remedial Field Investigation provided high resolution stratigraphic information in the area of the PCE plume. PCE Hydraulic Capture Zone Analysis 8 December 2022 Based on existing lithologic logs of wells in the project area and recent exploratory borings conducted on and around the project area, hydrostratigraphic zones were designated and assigned to four model layers as previously described in Section 3.2. Hydrologic Boundary Conditions Model boundary conditions are shown in Figure 5. A brief description of the boundaries is provided below. No-Flow Boundaries The bottom of the model is the base of permeable sediments (top of bedrock), which is implicitly modeled as a no-flow boundary. Bedrock outcropping at ground surface, and the lateral extent of bedrock in the subsurface, were explicitly modeled by setting appropriate model layer cells to no- flow. For example, areas of the model where the alluvial deposits are directly underlain by bedrock, with no Paso Robles of Pismo Formations present (San Luis Obispo Creek valley northwest of Madonna Shopping center and southeast of Los Osos Valley Road) were modeled with Layer 3 and Layer 4 set to no-flow. One exception to the treatment of bedrock as a no-flow boundary is described below under general head boundaries. General Head Boundaries General head boundaries were used to control subsurface flow into the model from other basin areas, and to allow subsurface outflow through the creek alluvium southeast of Los Osos Valley Road. The eastern general head boundary was set to match calibration period water level contour elevations, and the subsurface inflow reasonably matches the annual inflow from the Edna subarea estimated in the GSP Water Budget. The northern and western general head boundaries were set to match calibration period contour elevations. A general head boundary was also added along the southeast bedrock contact to simulate mountain front recharge from fractured bedrock, with the amount of recharge proportioned in accordance with the GSP Water Budget. Constant Head Boundaries Laguna Lake was simulated as a constant head boundary. The lake elevation was set to generally match calibration period groundwater elevations. Inflow to the lake from upper Prefumo Creek and direct precipitation was reduced by the estimated local reservoir evaporation and simulated as discharge from the lake into the lower reach of Prefumo Creek. Stream Boundary Conditions The major streams in the model domain were simulated using the Stream Flow Routing package. Stormwater runoff and baseflows for each of the watershed areas that drain into the basin were estimated using the correlations with precipitation and stream flow gage data developed in the GSP Model Boundary Boundary Condition - No Flow Area (Layer 1 Shown) Boundary Condition - Stream Boundary Condition - Constant Head Boundary Boundary Condition - General Head (Layer 1) Boundary Condition - General Head (Layer 2) Boundary Condition - General Head (Layer 4) Production Well Model Head Target Explanation PCE Hydraulic Capture Zone Analysis 9 December 2022 Water Budget. The locations of gaining reaches of San Luis Obispo Creek and lower Prefumo Creek in the flow model matched the locations field checked by CHG staff in September 2022 (as the transition from no-flow to flow conditions with visually increasing flow downstream). Aquifer properties Aquifer properties, including hydraulic conductivity, specific yield, specific storage, and porosity are used to characterize the hydrostratigraphic layers within the model. The methodology used to estimate and distribute aquifer properties involved both lithologic correlation and pumping test analyses. As previously mentioned, the Remedial Field Investigation provided relatively high-resolution lithologic information across the PCE plume area. For lithologic correlation of aquifer properties, the individual borehole lithologies from the Remedial Field Investigation were first separated into the four model layers, and further divided into parameter zones. Aquifer parameter values were then assigned to each distinctive lithologic depth interval based on the interpreted ASTM soil group symbol. The parameter values associated with each soil type were sourced using published references from the USGS (Halford and Kuniansky, 2002), the California Department of Water Resources (Johnson, 1967) and others (Duffield, 2007; Batu, 1999; Bonazountas and Wagner, 1984). Weighted averages for lithologic correlations, along with the results of pumping tests, were combined to estimated aquifer parameter distribution in the model. Figures showing the parameter distributions are included in Appendix A (Figures A1 through A8). Hydraulic Conductivity Hydraulic conductivity (K) describes the permeability of basin sediments with respect to groundwater flow, expressed in model units of feet per day. Values for aquifer hydraulic conductivity were initially estimated as follows, and expressed in Equation (1) below. 𝐾𝑎𝑣𝑔=∑𝑆 ∑𝑑 (1) Using the depth interval (d) for each ASTM soil group symbol, K values were assigned. These values were provided as a range between maximum and minimum hydraulic conductivities. Most lithologic types were assigned the average range value for K, while some were interpreted be associated with either the minimum or maximum value. The thicknesses of each lithologic type were then multiplied by the corresponding hydraulic conductivity to produce transmissivity value (T) for each interval. The sum of transmissivities within the hydrostratigraphic model layers and zones were then divided by the sum of the depth intervals (equivalent to the total layer thickness) to produce an average hydraulic conductivity value (Kavg) for the model layer/zone. Specific Yield Specific yield (Sy) describes the volume of water as a percent (of total interstitial water) that is released from storage by an unconfined aquifer or aquitard, per unit decline of the water table. PCE Hydraulic Capture Zone Analysis 10 December 2022 Specific yield is related to the total porosity (n) and specific retention (Sr) of basin sediments, as presented in Equation (2) below. 𝑛=𝑆𝑦+𝑆𝑟 (2) The values for specific yield were sourced from the Geological Survey Water-Supply Paper 1662- D, Table 16, which contains the values for water-bearing sediments in San Luis Obispo (Johnson, 1967). Similar to Equation (1), the procedure for calculating average specific yield (Sy avg) is a weighted average method, where the specific yield values are multiplied by individual depth intervals, summed together, and divided by the sum of depth intervals as expressed in Equation (3). 𝑆𝑦 𝑎𝑣𝑔=∑(𝑆𝑦𝑑) ∑𝑑 (3) Specific Storage Specific storage (Ss) describes the volume of water that a unit volume of confined aquifer or aquitard releases from storage under a unit decline in head. Specific storage is related to confined aquifer thickness (b) and aquifer storativity (S) as expressed in Equation (4). 𝑆=𝑆𝑟𝑏 (4) Similar to Equations (1) and (3), the procedure for calculating average specific storage (Ss avg) is a weighted average using the sum of specific yield values multiplied by depth in the numerator, and the sum of the depth intervals in the denominator, as expressed in Equation (5). 𝑆𝑟 𝑎𝑣𝑔=∑𝑆 ∑𝑑 (5) Lithologic and Pumping Test Data Sets Multiple data sets were used to estimate model parameters. Lithologic data from 30 logs (sonic, direct push, and CPT) were used from the field investigation, along with close to two dozen logs from Well Completion Reports, and results of over a dozen pumping tests were incorporated into the distribution of model parameters using the above methodology. Calibrated model layers and associated zones were assigned model parameters as shown in Table 1 below PCE Hydraulic Capture Zone Analysis 11 December 2022 Table 1 – Model Aquifer Parameters Average Values Zone Description Model Layer K(xy) K(z) Ss Sy 1 Upstream alluvial channel 1 3 0.3 0.075 2 Downstream alluvial channel 1 25 2.5 0.087 3 Downstream alluvial clay 1 0.2 0.02 0.064 4 Alluvium - Northwest 1 2 0.2 0.091 5 Alluvium - Southeast 1 2 0.2 0.083 6 Upstream gravel channel 2 100 10 0.000172 7 Downstream gravel channel 2 200 20 0.000199 8 Off-channel gravel - Northwest 2 30 3 0.000509 9 Off-channel gravel - Southeast 2 30 3 0.000582 10-12 Aquitard 3 1 0.1 0.000650 13-14 Paso/Pismo Formations - Northwest 4 5 0.5 0.000530 15 Paso/Pismo Formations - Southeast 4 10 1 0.000530 Stress Periods The model described in this report is a transient model. There are eight, six-month stress periods (four years total) that are used for model calibration. The calibration time period is from April 2018 to March 2022, which represents current conditions and includes wet and dry years. This time period has a sufficient amount of water level measurement data to calibrate the model. For hydraulic capture zone scenarios, the model was expanded to 72 stress periods (36 years) by repeating the eight calibration stress periods over nine cycles. A 36-year simulation period allowed for 99 percent of particles released in the PCE plume are to reach their final (capture) destination. 4.3 Hydrologic Budget Items Inflow components of the hydrologic budget used in the model include percolation of precipitation, irrigation return flow, subsurface inflow, and stream infiltration. Outflow components in the model include groundwater production, lake evaporation, groundwater exfiltration to streams, and subsurface outflow. Inflow Percolation from stream infiltration represents the major inflow into the model, along with recharge from precipitation. As mentioned previously, there is also a lesser amount of subsurface inflow from other areas of the basin and from bedrock. PCE Hydraulic Capture Zone Analysis 12 December 2022 Streamflow The majority of flux in the model comes from infiltration and exfiltration from San Luis Obispo Creek, and to a lesser extent, Prefumo Creek. There are also two minor drainages in the east of the model area that allow flux in and out of the model domain, which include a drainage through the Tank Farm area and a portion of Acacia Creek. Values for streamflow were calculated using the methodology developed in the GSP Water Budget. These streamflow estimates were then input using the Stream Flow Routing Package to interact with groundwater. Percolation of Precipitation Deep infiltration of rainfall was estimated based on the methodology developed in the GSP Water Budget, which accounts for evapotranspiration and runoff from various types of land use. The volume of recharge from percolation of precipitation was proportioned based on the area of the active model domain, compared to the area of the San Luis Valley subarea in the GSP. Outflow Streamflow and Subsurface Outflow Outflow from the basin occurs as rising water in San Luis Obispo Creek and in Prefumo Creek and as subsurface outflow in the alluvium. This outflow is supplemented with recycled water discharge to San Luis Obispo Creek near the basin boundary. The semi-confining aquitard in portions of model Layer 1 creates an upward vertical hydraulic gradient that results in groundwater rising into Prefumo Creek and San Luis Obispo Creek. The stream reaches where rising water occurs in the model were field checked by CHG in September 2022, prior to seasonal rains, and found to be consistent with field observations. Groundwater Extraction Groundwater production from wells is not the primary component of outflow from the model, but can be significant locally. Wells in the model are specified flux boundaries with each well assigned a pumping value for each stress period, and a range of extraction layers based on screen interval depths. The methodology for assigning production to wells was based on the GSP Water Budget. Annual estimated production values for non-agricultural wells were calculated based upon duty factors that used the size of the population served for private water purveyors and the square footage of operations for commercial users. Agricultural operations were identified in the model area and annual production was estimated for each year from crop type, crop area, and irrigation demand factors. These annual estimates were then adjusted for irrigation efficiency and return flow and entered into the model for each well for each stress period. See Figure 3 for locations of pumping wells in the model area PCE Hydraulic Capture Zone Analysis 13 December 2022 5.0 MODEL CALIBRATION 5.1 Calibration Targets Calibration is a minimization of the sum of squared residuals in the objective function. The residuals are the difference between the model predictions for groundwater head (elevation) and the calibration targets. Calibration targets include 24 transient head target locations identified in the model area with 109 discrete head targets over the calibration period. There were 12 wells located in the model domain with water level data from 2018-2022 from the San Luis Obispo County water level monitoring database. Additional seasonal water level data came from well completion reports (5 wells) and monitoring program data in GeoTracker (7 wells). The data includes well coordinates, reference point elevation, and seasonal water depths. 5.2 Calibration Statistics Flow model error (residual) was evaluated using various calibration statistics, including: • Residual Mean • Residual Standard Deviation • Absolute Residual Mean • Ratio of Residual Standard Deviation to Range in Head Flow model calibration is generally considered successful when the standard deviation of the residual error is within 10 percent of the range in head (Rumbaugh, 2011). Statistics are listed in Table 2 and show a ratio of residual standard deviation to range in head of 6.1%. Table 2 – Model Calibration Statistics Statistic Model Value Residual Mean -0.53 Residual Standard Deviation 3.30 Absolute Residual Mean 2.30 Ratio of Residual Standard Deviation to Range in Head 0.061 A plot of model values versus head target values is shown in Figure 6 and shows the correlation between computed versus actual heads. Target locations are shown in Figure 5. Targets from County-monitored wells and from Well Completion Reports are distributed across the broad San Luis Valley with groundwater elevations between 100 and 125 feet above mean sea level, while the head targets from GeoTracker are in the upper San Luis Obispo Creek valley at elevations of 145 to 165 feet above mean sea level. PCE Hydraulic Capture Zone Analysis 14 December 2022 Figure 6 - Observed vs. Computed Model Heads Plot 80 90 100 110 120 130 140 150 160 170 180 80 90 100 110 120 130 140 150 160 170 180Model ValueObserved Value Observed vs Simulated Water Levels Group 1 - County Monitored Group 2 - GeoTracker Group 3 - WCR PCE Hydraulic Capture Zone Analysis 15 December 2022 6.0 HYDRAULIC CAPTURE ZONE ANALYSIS This section evaluates the feasibility of using groundwater extraction wells to create a hydraulic capture zone that would control and potentially expedite PCE plume movement and remediation. Hydraulic capture zone analysis was evaluated with particle tracking using MODPATH. Flow lines (particle tracks) originating within the PCE plume area were processed to identify where each particle is captured. The particle tracks can terminate at treatment wells, other wells, streams, or they may remain in transit or escape from the model domain through the subsurface. The greater the quantity of particles that terminate at treatment wells, compared to other destinations, the greater the potential for PCE plume hydraulic capture by the remediation program. The number of particles captured at a treatment well, hoverer, does not correlate with the amount of PCE mass removed, because particles originating from different areas of the plume represent different PCE concentrations. PCE concentration containment and remediation times are not evaluated in this phase of modeling. Analyzing the movement and removal of dissolved PCE mass would involve adding the MT3DMS mass transport package to the model. This package could be added during future project phases. 6.1 Particle Tracking The hydraulic capture zone analysis relies on forward particle tracking in the flow model. Particles are introduced into the model throughout the plume area and forward-tracked along their flow path to determine where capture occurs. Figure 7 shows conceptual locations for treatment wells, the approximate plume area with concentration of PCE above 1 microgram per liter based on the field investigation, and the particle release area used for the capture zone analysis. Minor revisions to the plume area in the Feasibility Report (WSC, 2022) may be updated in the model during future project phases and do not significantly change the hydraulic capture zone analysis. Potential destinations for particles in the model include future treatment (extraction) wells, other pumping wells, streams, and subsurface exit (escape) from the model. Particles may also be in transit or be effectively immobilized (trapped) and do not reach a final destination within the time period modeled. Simulated groundwater elevation contours are also shown in Figure 7. The use of particle tracking to interpret hydraulic capture zones allows direct comparison of a baseline scenario with alternative management scenarios. The methodology estimates the percent of hydraulic capture at each final destination location for particles released in a prescribed area, which in this case was the approximate limits of PCE. All the particles were released into model Layer 2, which is where PCE concentrations were primarily detected during the Remedial Field Investigation. Particle capture does not rely on PCE source containment. Hydraulic capture scenarios can be used to establish treatment well locations and pumping schedules for controlling plume migration, even if additional PCE mass is mobilized. Model Boundary Laguna Lake Major Creeks No Flow Area (Layer 2) Spring 2022 model hydraulic head contours (Layer 2 - basal alluvial gravel) Particle Release Area 2022 PCE Plume Area above 1 µg/L Explanation PCE Hydraulic Capture Zone Analysis 16 December 2022 6.2 Treatment Well Locations and Production Three conceptual treatment well locations were established based on initial flow model results and basin hydrogeology. The following rationale was used in selecting the treatment well locations: • Hydraulic capture closer to the historical sources of PCE will be more effective at source containment, compared to farther away. • Hydraulic capture closer to the greatest concentrations of PCE in groundwater will be more effective at plume containment, compared to farther away. • Consideration for sanitary seal depth and setbacks from surface water (stream channels) is important to limit the treatment requirements for produced water. PCE removal is the objective, but there are other treatment considerations depending on the subsequent use of the produced water. • A minimum combined production of 300 AFY is assumed for low level pumping. The PCE treatment facility is assumed to be located at the City Wastewater Treatment Plant. • A maximum combined production of 600 AFY is assumed for optimal pumping. The San Luis Obispo GSP identified 700 acre-feet of surplus groundwater available in the San Luis Valley subarea. • One of the treatment well locations is the existing City Highway 101 well. • Extractions within the plume area near Los Osos Valley Road, where significant land subsidence occurred historically, is not recommended. Three locations were selected for the capture zone analysis using the above rationale (Figure 7). These are locations that appear to be technically feasible, although there may be other constraints that the City would need to address, such as private property access/easements for two of the sites, environmental review with respect to surface water-groundwater interaction, and travel time considerations with respect to future indirect potable reuse projects. PCE Treatment Well #1 – Vicinity of former City Lawn Memorial/IOOF Cemetery Wells Treatment Well #1 is located at the farthest upstream site (closest to the historical PCE source areas) where there is documentation of sufficient alluvial aquifer depth and permeability for establishing a treatment well. The basal alluvial sand and gravel (Layer 2) was logged from 50- 59 feet depth during drilling and construction of the former Lawn Memorial Cemetery well for the City exploratory well program in 1993. This well also taps a shallower sand and gravel zone (interpreted to be the Layer 1 buried channel) logged from 32-36 feet depth, and is capable of 200 gpm. Setback from the creek is approximately 200 feet. The older IOOF Cemetery well was a few hundred feet south of the Lawn Memorial Well. The IOOF Cemetery well (Chorro Lodge #168) was completed in 1930 and logged the basal sand and gravel from 48-60 feet depth, with perforations from 54 feet to 64 feet depth. This well produced at 400-500 gpm. Setback from the creek was also approximately 200 feet. Geophysical exploration (passive seismic surveying) may be used to evaluate the relative depth of PCE Hydraulic Capture Zone Analysis 17 December 2022 bedrock in the vicinity of these wells to identify the optimal location for a treatment well. Given the available information, a treatment well with a sanitary seal of 50 feet depth, and with a minimum 150-foot setback from San Luis Obispo Creek, would be feasible at this location and would have sufficient pumping capacity water to pump a nominal 150 AFY or more. PCE Treatment Well #2 – Vicinity of Embassy Suites/San Luis Ranch Wells The northeast corner of San Luis Ranch (Dalidio Laguna Ranch) has historically been the site of productive alluvial wells, including the Embassy Suites (formerly Park Suite) well, and San Luis Ranch wells #2, 3, and 7. Drilling logs indicate the basal alluvial sand and gravel aquifer (Layer 2) extends from close to 46 feet depth to between 62 and 71 feet depth. Well capacities have been reported at 500+ gpm. In 2014, groundwater samples from San Luis Ranch area wells indicated the highest PCE concentrations in groundwater extended along the Highway 101 corridor, and included the northeast corner of the ranch (CHG, 2014) where PCE Treatment Well #2 is proposed. More recent water quality results from the Remedial Field Investigation indicate lower PCE plume concentrations overall, when compared to 2014, but the highest concentrations are still located along the Highway 101 corridor and include the location of proposed PCE Treatment Well #2 Geophysical soundings (passive seismic surveying) may be used to evaluate the relative depth of bedrock in the vicinity of these wells to identify the optimal location for a treatment well. Given the available information, a treatment well with a sanitary seal of 50-feet depth, and with a minimum 50-foot setback from existing wells would be feasible at this location and would produce sufficient water to pump a nominal 200 AFY or more. Surface water setbacks are not an issue in this area. PCE Treatment Well #3 – City Highway 101 well. The City’s Highway 101 well is the farthest hydraulically downgradient location for a proposed PCE treatment well. The well is located along the Highway 101 corridor where elevated PCE concentrations are documented, and is near the downgradient limits of the plume. This well taps the basal alluvial deposits and the Paso Robles Formation, with the upper screen interval from 45-85 feet depth, and the lower screened interval from 115-135 feet depth. There is a sanitary seal to 40 feet deep, which is less than the 50-foot seal currently required for a City supply well, although obtaining a waiver is possible (may involve testing for surface water influence). The setback from surface water (San Luis Obispo Creek) is an approximate 300 feet. Given the available information, if the Highway 101 was not granted a seal waiver, and/or any additional treatment requirements were not feasible to add to the PCE removal treatment, then a new treatment well with a sanitary seal of 50-feet depth would be feasible at this location, and would produce sufficient water to pump a nominal 250 AFY or more. Surface water setbacks are not an issue in this area. PCE Hydraulic Capture Zone Analysis 18 December 2022 6.3 Particle Tracking Scenario Results Seven particle tracking scenarios have been evaluated (Table 3): • Baseline Scenario: Hydraulic capture zone simulation with no treatment wells. • Low Production Scenario: Hydraulic capture zone with 100 AFY pumping at each of three treatment well locations (300 AFY total extraction). • High Production Scenario: Hydraulic capture zone simulation with 150 AFY, 200 AFY, and 250 AFY pumping at the three treatment well locations (600 AFY total extraction). • Two-Well Scenario A: Hydraulic capture zone simulation with 300 AFY at treatment well 2 and 300 AFY at treatment well 3 (600 AFY total extraction). • Two-Well Scenario B: Hydraulic capture zone simulation with 150 AFY at treatment well 1 and 350 AFY at treatment well 2 (500 AFY total extraction). • Two-Well Scenario C: Hydraulic capture zone simulation with 150 AFY at treatment well 1 and 350 AFY at treatment well 3 (500 AFY total extraction). • Two-Well Scenario D: Hydraulic capture zone simulation with 200 AFY at treatment well 2 and 400 AFY at treatment well 3 (600 AFY total extraction). Table 3 Treatment Well Pumping Distribution Scenario Pumping Distribution (AFY) TW #1 TW #2 TW #3 Baseline 0 0 0 Low Production (300 AFY) 100 100 100 High production (600 AFY) 150 200 250 2-Well Scenario A (600 AFY) 0 300 300 2-Well Scenario B (500 AFY) 150 350 0 2-Well Scenario C (500 AFY) 150 0 350 2-Well Scenario D (600 AFY) 0 200 400 The simulated water level response to pumping identified potential restrictions to treatment well production in order to avoid impacting nearby private wells. As a result, production at Treatment Well #1 is limited to 150 AFY, and production at either Treatment Well #2 or Treatment Well #3 is limited to 350 AFY when pumped concurrently with Treatment Well #1 (Two-Well Scenario B and Scenario C). Particles were released from each model cell within the PCE plume area, as interpreted from the Remedial Field Investigation. The particle travel times were extended to allow most of the particles to reach a capture destination point. The 4-year calibration interval of eight model stress periods was repeated over nine cycles to simulate 36 years of particle movement (72 stress periods). A total of 554 particles were released in Layer 2. Results of the particle capture analysis are shown in Table 4 and 5. Particle traces for the baseline scenario and four management scenarios are shown in Figures 8, 9, 10, 11 and 12. Model Boundary Treatment Well Commercial/Industrial Well Agricultural Well Particle Trace Particle Start Point Explanation 2 Particle Endpoint by Layer 3 4 1 Model Boundary Treatment Well Commercial/Industrial Well Agricultural Well Particle Trace Particle Start Point Explanation 2 Particle Endpoint by Layer 3 4 1 Model Boundary Treatment Well Commercial/Industrial Well Agricultural Well Particle Trace Particle Start Point Explanation 2 Particle Endpoint by Layer 3 4 1 Model Boundary Treatment Well Commercial/Industrial Well Agricultural Well Particle Trace Particle Start Point Explanation 2 Particle Endpoint by Layer 3 4 1 Model Boundary Treatment Well Commercial/Industrial Well Agricultural Well Particle Trace Particle Start Point Explanation 2 Particle Endpoint by Layer 3 4 1 PCE Hydraulic Capture Zone Analysis 19 December 2022 Table 4 Management Scenario Particle Capture Distribution Scenario Capture Location and Distribution of 554 Particles released in PCE Plume Area TW #1 TW #2 TW #3 Other Wells Streams Total Baseline 0 0 0 162 391 553 Low Production (300 AFY) 50 52 96 108 246 552 High production (600 AFY) 61 110 126 94 163 554 2-Well Scenario A (600 AFY) 0 157 145 111 141 554 2-Well Scenario B (500 AFY) 61 157 0 75 260 553 2-Well Scenario C (500 AFY) 63 0 196 139 153 551 2-Well Scenario D (600 AFY) 0 109 186 134 125 554 Table 4 shows the particle capture distribution for 554 particles released within the PCE plume in Layer 2. Note that the number of particles captured at a treatment well does not correlate with the amount of PCE mass removed from the basin, because particles originating from different areas of the plume would represent different PCE concentrations. Table 5 Management Scenario Percent Particle Capture Scenario Percent Capture Location and Distribution of Particles released in PCE Plume Area TW #1 TW #2 TW #3 Other Wells Streams Total Baseline 0 0 0 29 71 100 Low Production (300 AFY) 9 9 17 20 44 99 High production (600 AFY) 11 20 23 17 29 100 2-Well Scenario A (600 AFY) 0 28 26 20 26 100 2-Well Scenario B (500 AFY) 11 28 0 14 47 100 2-Well Scenario C (500 AFY) 11 0 35 25 28 99 2-Well Scenario D (600 AFY) 0 20 34 24 23 100 PCE Hydraulic Capture Zone Analysis 20 December 2022 The particle capture analysis summarized in Tables 5 indicates 35 percent hydraulic capture of particles by treatment wells at 300 AFY production, and 54 percent capture at 600 AFY production. There is a notable reduction in particle capture by streams between the baseline scenario (71 percent capture) and the treatment scenarios (23-47 percent capture). Percent hydraulic capture by other pumping wells is also reduced from 29 percent capture under baseline conditions (no treatment well pumping), to between 14-25 percent capture for various treatment scenarios. Most of the particles captured by the treatment wells would otherwise have escaped from the model through stream flow. The percent of particles in transit is minimal (one percent or less). For the baseline scenario, most of the particles were captured within 20 years from their time of release into the model, whereas in the treatment scenarios most of the particles were captured within 10 years of their time of release. Plume remediation times would be lowered by treatment well pumping, compared to baseline, although actual remediation times would depend on mass transport parameters that are not considered in the hydraulic capture zone analysis, but could be modeled during future phases of the project. The success of pump-and-treat for lowering plume remediation times is also contingent on PCE source containment. 6.4 Sensitivity Analysis A sensitivity analysis was conducted on the calibrated model to quantify the effects of changing model parameters on the simulated results. The analysis identified the most sensitive parameters with respect to both the calibration and the specific model objective, which is hydraulic capture zone evaluation. Parameters evaluated in the sensitivity analysis include: • Hydraulic conductivity • Specific Yield/Specific Storage/Porosity • Model Boundary Conductance Parameter values were reduced by one-half and doubled from the calibrated model values for the analysis. Results of the analysis are presented below in Table 6. PCE Hydraulic Capture Zone Analysis 21 December 2022 Table 6 Flow Calibration Sensitivity Analysis Flow Calibration Parameters Residual Mean Residual Std. Dev. Abs. Resid. Mean Scaled Res. Std. Dev. (feet) Calibrated Model (all wells) -0.53 3.30 2.30 6.1% Parameter Adjustment Sensitivity Analysis Values Hydraulic Conductivity x 0.5 -1.46 3.75 2.85 6.9% x 2 0.55 2.92 2.22 5.4% Storage/Porosity x 0.5 -0.49 3.38 2.30 6.3% x 2 -0.34 3.38 2.39 6.3% Boundary Conductance x 0.5 -0.51 3.29 2.29 6.1% x 2 -0.55 3.32 2.32 6.1% Flow calibration statistics are most sensitive to changes in aquifer hydraulic conductivity, with reductions to aquifer hydraulic conductivity resulting in slightly poorer model calibration, and increases in hydraulic conductivity resulting in slightly improved calibration, compared to the current model. Table 6 shows that the entire range of calibration results in the sensitivity analysis is acceptable (less than 10 percent scaled residual standard deviation). The slightly improved calibration statistics associated with doubling the model hydraulic conductivity are accompanied, however, by localized flooding and a reversed (northern) hydraulic gradient near the confluence of Prefumo and San Luis Creeks. These results support using the current model values for hydraulic conductivity, which were developed from multiple lithologic logs and pumping tests. Changes in hydraulic conductivity also have the greatest effect on hydraulic capture. A 50 percent reduction in hydraulic conductivity results in an average of 10 percent increased particle capture by project wells, while a 100 percent increase in hydraulic conductivity results in an average 10 percent decrease in particle capture by project wells. There is a corresponding change in the percent of particles captured by streams, while particle capture by other wells is fairly stable. In summary, the sensitivity analysis indicates that model calibration statistics and PCE plume hydraulic capture are most sensitive to changes in hydraulic conductivity. There is a 20 percent variation in particles captured by project wells over the range of hydraulic conductivity used in the sensitivity analysis: 10 percent more capture when model hydraulic conductivity is cut in half, and 10 percent less capture when hydraulic conductivity is doubled. PCE Hydraulic Capture Zone Analysis 22 December 2022 7.0 REFERENCES Batu, V. (1998) Aquifer Hydraulics: A Comprehensive Guide to Hydrogeologic Data Analysis. John Wiley & Sons, Inc., New York. Bonazountas, M. and Wagner, J.M., 1984. SESOIL: A Seasonal Soil Compartment Model, Draft Report. 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Retrieved August 24, 2022, from https://www.usgs.gov/publications/documentation-spreadsheets-analysis-aquifer-test-and- slug-test-data Harbaugh, A. (2005). MODFLOW-2005 , The U.S. Geological Survey Modular Ground-Water Model — the Ground-Water Flow Process. U.S. Geological Survey Techniques and Methods. Johnson, A.I.. (1967). Specific Yield-Compilation of Specific Yields for Various Materials, U.S. Geological Survey Water Supply Paper 1662-D Pollock, D. (2012). User Guide for MODPATH Version 6- A Particle-Tracking Model for MODFLOW. Section A, Groundwater Book 6, Modeling Techniques. Rumbaugh, J.O, and Rumbaugh, D.B. (2011). Groundwater Vistas ‐ Version 6 Users Guide, Environmental Simulations Inc. Water Systems Consulting (2021). San Luis Obispo Valley Basin Groundwater Sustainability Plan, prepared for County of San Luis Obispo and City of San Luis Obispo, October 2021.. Water Systems Consulting (2022), Feasibility Study Report - City of San Luis Obispo Tetrachloroethylene (PCE) Plume Characterization Project. PCE Hydraulic Capture Zone Analysis 23 December 2022 APPENDIX Figure A1 – K Layer 1 Figure A2 – K Layer 2 Figure A3 – K Layer 3 Figure A4 – K Layer 4 Figure A5 – Sy Layer 1 Figure A6 – Ss Layer 2 Figure A7 – Ss Layer 3 Figure A8 – Ss Layer 4 Model Boundary No Flow Area Explanation 100 200 Horizontal Hydraulic Conductivity (ft/day) 0.2 1 2 3 25 30 5 10 Model Boundary No Flow Area Explanation 100 200 Horizontal Hydraulic Conductivity (ft/day) 0.2 1 2 3 25 30 5 10 Model Boundary No Flow Area Explanation 100 200 Horizontal Hydraulic Conductivity (ft/day) 0.2 1 2 3 25 30 5 10 Model Boundary No Flow Area Explanation 100 200 Horizontal Hydraulic Conductivity (ft/day) 0.2 1 2 3 25 30 5 10 Model Boundary No Flow Area Explanation Specific Yield 0.064 0.075 0.083 0.087 0.091 Model Boundary No Flow Area Explanation 0.000172 0.000199 Specific Storage (1/ft) 0.000582 0.000650 0.000509 0.000530 Model Boundary No Flow Area Explanation 0.000172 0.000199 Specific Storage (1/ft) 0.000582 0.000650 0.000509 0.000530 Model Boundary No Flow Area Explanation 0.000172 0.000199 Specific Storage (1/ft) 0.000582 0.000650 0.000509 0.000530 Appendix B Appendix B Cost Estimates Appendix B Cost Opinion Basis and Assumptions The cost opinions (estimates) for each alternative have been prepared in conformance with industry practices as planning level cost opinions and are classified as Class 5 Conceptual Report Classification of Opinion of Probable Construction Costs as developed by AACE International. The purpose of a Class 5 Estimate is to provide a conceptual level of effort that is expected to range in accuracy from ‐50% to +100%. A Class 5 Estimate also includes an appropriate level of contingency so that it can be used in future planning and feasibility studies. The design concepts and associated costs presented in this capital improvement plan are conceptual in nature due to the limited design information that is available at this stage of project planning. These cost estimates have been developed using a combination of data from RS Means CostWorks® and recent bids, experience with similar projects, current and foreseeable regulatory requirements, and an understanding of necessary project components. As the projects progress, the designs and associated costs could vary significantly from the project components identified in this capital improvement plan. These cost opinions are based on the following assumptions:  Projects cost opinions are generally derived from bid prices from similar projects, vendor quotes, material prices, and labor estimates, with adjustments for inflation, size, complexity, and location.  Cost opinions are in 2022 dollars (Engineering News Record Construction Cost Index of 13171 for August 2022). When budgeting for future years, appropriate escalation factors should be applied. The past 5‐year average increase of the Engineering News Record Construction Cost Index 20 City Average is considered a reasonable factor to use for escalation.  Cost opinions are “planning‐level” and may not fully account for site‐specific conditions that will affect actual costs, such as soil conditions and utility conflicts.  Construction costs include the following mark‐up items: a. 30 percent construction contingency based on construction sub‐total.  Total project costs include the following allowances: a. 30 percent of construction total for project development, including administration, alternatives analysis, planning, engineering, surveying, and construction phase support services, including administration, inspection, materials testing, office engineering, construction administration, etc.  Power Costs assume: a. $0.12 rate per kWh b. 80% pump efficiency and 90% motor efficiency City of San Luis Obispo PCE Prop 1 Project, Feasibilty Study  Alternative 1, Mid‐Higuera, Dalidio Drive, and Highway 101 Well, Decentralized GAC Treatment  Component Quantity Unit Unit Cost Total Well Drilling 340 Ft $2,965 $1,008,050 Well Equipping 3 Ea $750,000 $2,250,000 GAC System ‐ 300 gpm 3 Ea $410,000 $1,230,000 GAC Building 3200 Sq. Ft. $350 $1,120,000 Piping and Appurtenances 3Ea System $250,000 $750,000 6,358,050$             1,907,000$             8,265,050$             2,480,000$             10,745,000$           Power (Well 1, 2, 3) 33,300.00$             GAC Replacement (3 Treatment Systems) $45,000 Equipment Maintenance, Repair, and Replacement, and Testing $30,000 Annual O&M Cost 108,300$                 Project Life, Years 30 AFY 600 $/AF $240 Net Present Cost 12,369,000$           Annual O&M Costs Lifecycle Costs Capital Costs Subtotal Construction Contingency (30%) Construction Total Project Development & Implementation (30%) Project Cost Note: Costs in 2022 Dollars.  The cost per acre‐foot includes 10% of the capital costs as the local match because the project is assumed to  be grant funded through Proposition 1, plus O&M costs. Includes capital costs, annual O&M costs over 30‐years with 3% annual inflation rate, and a discount rate of  City of San Luis Obispo PCE Prop 1 Project, Feasibility Study  Alternative 2, Dalidio Drive and Highway 101 Well, Decentralized GAC Treatment  Component Quantity Unit Unit Cost Total Well Drilling 170 Ft $2,965 $504,000 Well Equipping 2 Ea $750,000 $1,500,000 GAC System ‐ 300 gpm 2 Ea $410,000 $820,000 GAC Building 1600 Sq. Ft. $350 $560,000 Piping and Appurtenances 2Ea System $250,000 $500,000 3,884,000$     1,165,000$      5,049,000$     1,515,000$      6,564,000$      Power (Well 1 & 2) $33,300 GAC Replacement (2 Treatment Systems) $30,000 Equipment Maintenance, Repair, and Replacement, and Testing $20,000 Annual O&M Cost 83,300$            Project Life, Years 30 AFY 600 $/AF $175 7,903,000$     Net Present Cost Capital Costs Annual O&M Costs Lifecycle Costs Subtotal Construction Contingency (30%) Construction Total Project Development & Implementation (30%) Project Cost Note: Costs in 2022 Dollars.  The cost per acre‐foot includes 10% of the capital costs as the local match because the project is  assumed to be grant funded through Proposition 1, plus O&M costs. Includes capital costs, annual O&M costs over 30‐years with 3% annual inflation rate, and a  discount rate of 5.6%.   City of San Luis Obispo PCE Prop 1 Project, Feasibility Study  Alternative 3, Mid‐Higuera and Dalidio Drive Well, Decentralized GAC Treatment  Component Quantity Unit Unit Cost Total Well Drilling 340 Ft $2,965 $1,008,050 Well Equipping 2 Ea $750,000 $1,500,000 GAC System ‐ 300 gpm 2 Ea $410,000 $820,000 GAC Building 3200 Sq. Ft. $350 $1,120,000 Piping and Appurtenances 2Ea System $250,000 $500,000 4,948,050$     1,484,000$      6,432,050$     1,930,000$      8,362,000$      Power (Well 1 & 2) 27,700.00$      GAC Replacement (2 Treatment Systems) $30,000 Equipment Maintenance, Repair, and Replacement, and Testing $20,000 Annual O&M Cost 77,700$            Project Life, Years 30 AFY 500 $/AF $211 Net Present Cost 9,492,000$      Annual O&M Costs Lifecycle Costs Capital Costs Subtotal Construction Contingency (30%) Construction Total Project Development & Implementation (30%) Project Cost Note: Costs in 2022 Dollars.  The cost per acre‐foot includes 10% of the capital costs as the local match because the  project is assumed to be grant funded through Proposition 1, plus O&M costs. Includes capital costs, annual O&M costs over 30‐years with 3% annual inflation rate, and a  discount rate of 5.6%.   City of San Luis Obispo PCE Prop 1 Project, Feasibility Study  Alternative 4, Mid‐Higuera and Highway 101 Well, Decentralized GAC Treatment  Component Quantity Unit Unit Cost Total Well Drilling 170 Ft $2,965 $504,000 Well Equipping 2 Ea $750,000 $1,500,000 GAC System ‐ 300 gpm 2 Ea $410,000 $820,000 GAC Building 1600 Sq. Ft. $350 $560,000 Piping and Appurtenances 2Ea System $250,000 $500,000 3,884,000$     1,165,000$      5,049,000$     1,515,000$      6,564,000$      Power (Well 1 & 2) $27,700 GAC Replacement (2 Treatment Systems) $30,000 Equipment Maintenance, Repair, and Replacement, and Testing $20,000 Annual O&M Cost 77,700$            Project Life, Years 30 AFY 500 $/AF $199 7,790,000$      Annual O&M Costs Lifecycle Costs Net Present Cost Capital Costs Subtotal Construction Contingency (30%) Construction Total Project Development & Implementation (30%) Project Cost Note: Costs in 2022 Dollars.  The cost per acre‐foot includes 10% of the capital costs as the local match because the project is assumed  to be grant funded through Proposition 1, plus O&M costs. Includes capital costs, annual O&M costs over 30‐years with 3% annual inflation rate, and a discount rate of  5.6%.   City of San Luis Obispo PCE Prop 1 Project, Feasibility Study  Treatment Location Cost Comparison: Centralized GAC Treatment  Component Quantity Unit Unit Cost Total Well Drilling 170 Ft $2,965 $504,025 Well Equipping 2 Ea $750,000 $1,500,000 GAC System ‐ 600 gpm 1 Ea $724,000 $724,000 Piping and Appurtenances 1 Ea System $250,000 $250,000 Pipeline (6‐in)4,100 ft $225 $922,500 Freeway Crossing 350 ft $450 $157,500 Pump Station 1 Ea $500,000 $500,000 4,558,025$               1,367,000$                5,925,025$               1,778,000$                7,700,000$                Power (Well 1 & 2) $33,300 Power (Booster Pump Station) $9,400 GAC Replacement (1 Treatment System) $15,000 Equipment Maintenance, Repair, and Replacement, and Testing $10,000 Annual O&M Cost $68,000 Project Life, Years 30 AFY 600 $/AF $113 Net Present Cost 8,672,000$                Capital Costs Annual O&M Costs Lifecycle Costs Subtotal Construction Contingency (30%) Construction Total Project Development & Implementation (30%) Project Cost Note: Costs in 2022 Dollars.  The cost per acre‐foot includes 10% of the capital costs as the local match because the project is  assumed to be grant funded through Proposition 1, plus O&M costs. Includes capital costs, annual O&M costs over 30‐years with 3% annual inflation rate, and a  discount rate of 5.6%.   City of San Luis Obispo PCE Prop 1 Project, Feasibility Study  Treatment Location Cost Comparison: Decentralized GAC Treatment  Component Quantity Unit Unit Cost Total Well Drilling 170 Ft $2,965 $504,000 Well Equipping 2 Ea $750,000 $1,500,000 GAC System ‐ 300 gpm 2 Ea $410,000 $820,000 GAC Building 1600 Sq. Ft. $350 $560,000 Piping and Appurtenances 2Ea System $250,000 $500,000 3,884,000$     1,165,000$      5,049,000$     1,515,000$      6,564,000$      Power (Well 1 & 2) $33,300 GAC Replacement (2 Treatment Systems) $30,000 Equipment Maintenance, Repair, and Replacement, and Testing $20,000 Annual O&M Cost 83,300$            Project Life, Years 30 AFY 600 $/AF $175 7,903,000$     Net Present Cost Capital Costs Annual O&M Costs Lifecycle Costs Subtotal Construction Contingency (30%) Construction Total Project Development & Implementation (30%) Project Cost Note: Costs in 2022 Dollars.  The cost per acre‐foot includes 10% of the capital costs as the local match because the project is  assumed to be grant funded through Proposition 1, plus O&M costs. Includes capital costs, annual O&M costs over 30‐years with 3% annual inflation rate, and a  discount rate of 5.6%.   City of San Luis Obispo PCE Prop 1 Project, Feasibility Study  Treatment Type Cost Comparison, GAC Treatment Component Quantity Unit Unit Cost Total GAC System ‐ 300 gpm 1 Ea $410,000 $410,000 410,000$         123,000$          533,000$         160,000$          693,000$         Power (Well 1 & 2) $0 GAC Replacement (2 Treatment Systems) $15,000 Equipment Maintenance, Repair, and Replacement, and Testing $10,000 Annual O&M Cost 25,000$            Project Life, Years 30 AFY 600 $/AF $46 Net Present Cost 1,163,000$      Capital Costs Subtotal Construction Contingency (30%) Construction Total Project Development & Implementation (30%) Project Cost Annual O&M Costs Lifecycle Costs Note: Costs in 2022 Dollars.  The cost per acre‐foot includes 10% of the capital costs as the local match because the project is  assumed to be grant funded through Proposition 1, plus O&M costs. Includes capital costs, annual O&M costs over 30‐years with 3% annual inflation rate, and a  discount rate of 5.6%.   City of San Luis Obispo PCE Prop 1 Project, Feasibility Study  Treatment Type Cost Comparison, Air Stripping Treatment Component Quantity Unit Unit Cost Total Air Stripping ‐ 300 gpm 1 Ea $500,000 $500,000 GAC System for Air  Stream 1 Ea $205,000 $205,000 Pump Station 1 Ea $500,000 $500,000 1,205,000$               362,000$  1,567,000$               470,000$  2,037,000$                Power (Pump Station) $10,000 GAC Replacement (2 Treatment Systems)$7,500 Chemicals $2,500 Equipment Maintenance, Repair, and Replacement, and Testing $12,500 Annual O&M Cost 32,500$  Project Life, Years 30 AFY 600 $/AF $65 Net Present Cost 2,442,000$                Lifecycle Costs Capital Costs Subtotal Construction Contingency (30%) Construction Total Project Development & Implementation (30%) Project Cost Annual O&M Costs Note: Costs in 2022 Dollars.  The cost per acre‐foot includes 10% of the capital costs as the local match because the project is  assumed to be grant funded through Proposition 1, plus O&M costs. Includes capital costs, annual O&M costs over 30‐years with 3% annual inflation rate, and a  discount rate of 5.6%.   City of San Luis Obispo PCE Prop 1 Project, Feasibility Study  Treatment Type Cost Comparison, Advanced Oxidation Component Quantity Unit Unit Cost Total Advanced Oxidation ‐  300 gpm 1 Ea $350,000 $350,000 Pump Station 1 Ea $500,000 $500,000 850,000$ 255,000$  1,105,000$              332,000$  1,437,000$               Power (Pump Station) $10,000 GAC Replacement (2 Treatment Systems)$0 Chemicals $0 Equipment Maintenance, Repair, and Replacement, and Testing $10,000 Annual O&M Cost 20,000$  Project Life, Years 30 AFY 600 $/AF $41 Net Present Cost 1,862,000$               Annual O&M Costs Lifecycle Costs Capital Costs Subtotal Construction Contingency (30%) Construction Total Project Development & Implementation (30%) Project Cost Note: Costs in 2022 Dollars.  The cost per acre‐foot includes 10% of the capital costs as the local match because the project is  assumed to be grant funded through Proposition 1, plus O&M costs. Includes capital costs, annual O&M costs over 30‐years with 3% annual inflation rate, and a  discount rate of 5.6%.