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
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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.
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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.
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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.
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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
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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
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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.
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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.
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Figure 9. Potential Well Treatment Locations
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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.
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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.
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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
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Figure 10. Treatment Well Capture Zone Example – Excerpt from Hydraulic Capture Zone Analysis
Report
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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
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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.
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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
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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.
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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.
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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. Office of Toxic Substances, U.S. Environmental Protection Agency: Washington,
DC, PB86112406,
Cleath-Harris Geologists (2014). Hydrogeologic Description and PCE Characterization,
Dalidio Laguna Ranch, San Luis Obispo County, California, prepared for Coastal
Community Builders, November 10, 2014.
Cleath-Harris Geologists (2019). Groundwater Flow Analysis, recycled Water Recharge
Project, San Lsui Valley Sub-Area, San Luis Obispo Groundwater Basin, prepared for City
of San Luis Obispo and Water Systenms Consulting, January 2019
Department of Water Resources (2020). Bulletin 118 - California's Groundwater Update.
Duffield, G. M. (2007). AQTESOLV for Windows Version 4.5 users guide. 2303 Horseferry
Court Reston, VA 20191 USA; HydroSOLVE, Inc.
Halford , K. J., and Kuniansky, E. L. (2002). Documentation of spreadsheets for the analysis of
aquifer-test and Slug-Test Data. Documentation of spreadsheets for the analysis of aquifer-
test and slug-test data | U.S. Geological Survey. 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%.