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Home My WebLink AboutItem 06 - COUNCIL READING FILE_e_Draft GSP Chapter 5 Draft Groundwater Sustainability Plan Chapter 5 – Ground Water Conditions for the San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies Prepared by 3/1/2020 SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO i TABLE OF CONTENTS Table of Contents List of Figures ................................................................................................................................................. i Tables ........................................................................................................................................................... iv Appendices .................................................................................................................................................... v List of Terms Used ........................................................................................................................................ vi Executive Summary ....................................................................................................................................... 1 1 Introduction to the SLO Basin GSP ......................................................................................................... 1.1 Purpose of the Groundwater Sustainability Plan ............................................................................ 1.2 Description of SLO Basin ................................................................................................................. 1.3 Basin Prioritization .......................................................................................................................... 2 Agency Information (§ 354.6) ................................................................................................................ 2.1 Agencies Names and Mailing Addresses ......................................................................................... 2.2 Agencies Organization and Management Structures ..................................................................... 2.2.1 County of San Luis Obispo ....................................................................................................... 2.2.2 City of San Luis Obispo ............................................................................................................ 2.2.3 Other Participating Parties in the MOA .................................................................................. 2.2.3.1 Edna Valley Growers Mutual Water Company ............................................................... 2.2.3.2 Varian Ranch Mutual Water Company ........................................................................... 2.2.3.3 Edna Ranch Mutual Water Company .............................................................................. 2.2.3.4 Golden State Water Company ........................................................................................ 2.3 Authority of Agencies ...................................................................................................................... 2.3.1 Groundwater Sustainability Agencies ..................................................................................... 2.3.1.1 County of San Luis Obispo ............................................................................................... 2.3.1.2 City of San Luis Obispo .................................................................................................... 2.3.2 Memorandum of Agreement .................................................................................................. 2.3.3 Coordination Agreements ....................................................................................................... 2.4 Contact information for Plan Manager ........................................................................................... 3 Description of Plan Area (§ 354.8) ......................................................................................................... 3.1 SLO Basin Introduction .................................................................................................................... 3.2 Adjudicated Areas ........................................................................................................................... 3.3 Jurisdictional Areas ......................................................................................................................... 3.3.1 Federal Jurisdictions ................................................................................................................ 3.3.2 Tribal Jurisdiction .................................................................................................................... SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO ii 3.3.3 State Jurisdictions ................................................................................................................... 3.3.4 County Jurisdictions ................................................................................................................ 3.3.5 City and Local Jurisdictions ..................................................................................................... 3.3.6 Special Districts ....................................................................................................................... 3.4 Land Use .......................................................................................................................................... 3.4.1 Water Source Types ................................................................................................................ 3.4.2 Water Use Sectors ................................................................................................................... 3.5 Density of Wells .............................................................................................................................. 3.6 Existing Monitoring and Management Programs ........................................................................... 3.6.1 Groundwater Monitoring ........................................................................................................ 3.6.1.1 Groundwater Level Monitoring ..................................................................................... 3.6.1.2 Groundwater Quality Monitoring .................................................................................. 3.6.1.3 Surface Water Monitoring .............................................................................................. 3.6.1.4 Climate Monitoring ......................................................................................................... 3.6.2 Existing Management Plans .................................................................................................... 3.6.2.1 SLO Basin Characterization and Monitoring Well Installation ........................................ 3.6.2.2 San Luis Obispo County Master Water Report (2012) .................................................... 3.6.2.3 San Luis Obispo County Integrated Regional Water Management Plan (2014) ............. 3.6.2.4 City of San Luis Obispo 2015 Urban Water Management Plan (2016) ........................... 3.6.3 Existing Groundwater Regulatory Programs ............................................................................ 3.6.3.1 Groundwater Export Ordinance (2015) .......................................................................... 3.6.3.2 Well Ordinances, County and City .................................................................................. 3.6.3.3 Countywide Water Conservation Program Resolution 2015-288 (2015) ....................... 3.6.3.4 Agricultural Order R3-2017-002 (2017) .......................................................................... 3.6.3.5 Water Quality Control Plan for the Central Coast Basins (2017) .................................... 3.6.3.6 California DWR Well Standards (1991) ........................................................................... 3.6.3.7 Requirements for New Wells (2017) ............................................................................... 3.6.3.8 Title 22 Drinking Water Program (2018) ......................................................................... 3.6.3.9 Waterway Management Plan – San Luis Obispo Creek Watershed (2003) .................... 3.6.3.10 Incorporation Into GSP .................................................................................................... 3.6.3.11 Limits to Operational Flexibility ...................................................................................... 3.7 Conjunctive Use Programs .............................................................................................................. 3.8 Land Use Plans ................................................................................................................................ 3.8.1 City of San Luis Obispo General Plan ...................................................................................... 3.8.2 County of San Luis Obispo General Plan ................................................................................. SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO iii 3.8.3 Los Ranchos/Edna Village Plan ................................................................................................ 3.8.4 Plan Implementation Effects on Existing Land Use ................................................................. 3.8.5 Plan Implementation Effects on Water Supply ....................................................................... 3.8.6 Well Permitting ....................................................................................................................... 3.8.7 Land Use Plans Outside of Basin ............................................................................................. 3.9 Management Areas ......................................................................................................................... 3.9.1 Reason for Creation ................................................................................................................ 3.10 Additional GSP Elements, if Applicable ........................................................................................... 4 Basin Setting (§ 354.14) ......................................................................................................................... 4.1 Basin Topography and Boundaries ................................................................................................. 4.2 Primary Users of Groundwater ....................................................................................................... 4.3 Soils Infiltration Potential................................................................................................................ 4.4 Regional Geology ............................................................................................................................ 4.4.1 Regional Geologic Structures .................................................................................................. 4.4.2 Geologic Formations within the Basin .................................................................................... 4.4.2.1 Alluvium .......................................................................................................................... 4.4.2.2 Paso Robles Formation ................................................................................................... 4.4.2.3 Pismo Formation ............................................................................................................. 4.4.3 Geologic Formations Surrounding the Basin .......................................................................... 4.4.3.1 Monterey Formation ....................................................................................................... 4.4.3.2 Obispo Formation ........................................................................................................... 4.4.3.3 Franciscan Assemblage ................................................................................................... 4.5 Principal Aquifers and Aquitards .................................................................................................... 4.5.1 Cross Sections ......................................................................................................................... 4.5.2 Aquifer Characteristics ............................................................................................................ 4.5.3 Aquitards ................................................................................................................................. 4.6 Surface Water Bodies ...................................................................................................................... 4.7 Subsidence Potential ....................................................................................................................... 5 Groundwater Conditions (§ 354.16) .................................................................................................... 2 5.1 Groundwater Elevations and Intepretation .................................................................................. 2 5.1.1 Fall 1954 Groundwater Elevations ........................................................................................ 2 5.1.2 Spring 1990 Groundwater Elevations ................................................................................... 5 5.1.3 Modeled 1990s Groundwater Elevations ............................................................................. 7 5.1.4 Spring 1997 Groundwater Elevations ................................................................................... 7 5.1.5 Spring 2011 Groundwater Elevations ................................................................................... 9 SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO iv 5.1.6 Spring 2015 Groundwater Elevations ................................................................................. 11 5.1.7 Spring 2019 Groundwater Elevations ................................................................................. 13 5.1.8 Fall 2019 Groundwater Elevations ...................................................................................... 15 5.1.9 Changes in Groundwater Elevation .................................................................................... 17 5.1.10 Vertical Groundwater Gradients ......................................................................................... 21 5.2 Groundwater Elevation Hydrographs ......................................................................................... 21 5.3 Groundwater Recharge and Discharge Areas ............................................................................. 24 5.3.1 Groundwater Recharge Areas ............................................................................................. 24 5.3.1.1 Infiltration of Precipitation ......................................................................................... 24 5.3.1.2 Subsurface Inflow ........................................................................................................ 28 5.3.1.3 Percolation of Streamflow .......................................................................................... 28 5.3.1.4 Anthropogenic Recharge ............................................................................................ 28 5.3.2 Groundwater Discharge Areas ............................................................................................ 29 5.4 Change in Groundwater Storage................................................................................................. 29 5.5 Seawater Intrusion ...................................................................................................................... 29 5.6 Subsidence .................................................................................................................................. 29 5.7 Interconnected Surface Water .................................................................................................... 30 5.7.1 Depletion of Interconnected Surface Water ....................................................................... 30 5.8 Potential groundwater dependent ecosystems .......................................................................... 30 5.8.1 Hydrology ............................................................................................................................ 31 5.8.1.1 Overview of GDE Relevant Surface and Groundwater Hydrology .............................. 31 5.8.1.2 Losing and Gaining Reaches ........................................................................................ 32 5.8.2 Vegetation and Wetland Groundwater Dependent Ecosystem Identification ................... 34 5.8.3 Identification of Special-Status Species and Sensitive Natural Communities Associates with GDE’s .......................................................................................................................................... 36 5.9 Groundwater Quality Distribution and Trends ........................................................................... 37 5.9.1 Groundwater Quality Suitability for Drinking Water .......................................................... 37 5.9.2 Distribution and Concentrations of Point Sources of Groundwater Constituents ............. 37 5.9.3 Distribution and Concentrations of Diffuse or Natural Groundwater Constituents ........... 41 5.9.3.1 Total Dissolved Solids .................................................................................................. 41 5.9.3.2 Nitrate ......................................................................................................................... 43 5.9.3.3 Arsenic ......................................................................................................................... 45 5.9.3.4 Boron ........................................................................................................................... 47 5.9.3.5 Other Constituents ...................................................................................................... 47 6 Water Budget (§ 354.18) ........................................................................................................................ SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO v 6.1 Climate ............................................................................................................................................ 6.1.1 Historical Climate .................................................................................................................... 6.1.2 Projected Climate .................................................................................................................... 6.2 Water Budget Data Sources and Groundwater Model ................................................................... 6.3 Historical Water Budget .................................................................................................................. 6.3.1 Historical Time Period ............................................................................................................. 6.3.2 Inflows ..................................................................................................................................... 6.3.3 Outflows .................................................................................................................................. 6.3.4 Change in Storage ................................................................................................................... 6.3.5 Sustainable Yield ..................................................................................................................... 6.3.6 Quantification of Overdraft .................................................................................................... 6.4 Current Water Budget ..................................................................................................................... 6.4.1 Inflows ..................................................................................................................................... 6.4.2 Outflows .................................................................................................................................. 6.4.3 Change In Storage ................................................................................................................... 6.4.4 Sustainable Yield ..................................................................................................................... 6.4.5 Quantification of Overdraft .................................................................................................... 6.5 Projected Water Budget ................................................................................................................. 6.5.1 Assumptions ............................................................................................................................ 6.5.2 Inflows ..................................................................................................................................... 6.5.3 Outflows .................................................................................................................................. 6.5.4 Change In Storage ................................................................................................................... 7 Sustainable Management Criteria (§ 354.22-30) ................................................................................... 7.1 Sustainability Goal........................................................................................................................... 7.2 Process for Establishing Sustainable Management Criteria ........................................................... 7.2.1 Minimum Thresholds .............................................................................................................. 7.2.2 Measurable Objectives ........................................................................................................... 7.2.3 Undesirable Results................................................................................................................. 7.3 Chronic Lowering of Groundwater Levels Sustainability Indicator ................................................. 7.3.1 Locally Defined Undesirable Results ....................................................................................... 7.3.2 Minimum Thresholds and Measurable Objectives ................................................................. 7.3.3 Relation to Other Sustainability Indicators ............................................................................. 7.4 Change in Storage Sustainability Indicator ..................................................................................... 7.4.1 Locally Defined Undesirable Results ....................................................................................... 7.4.2 Minimum Thresholds .............................................................................................................. SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO vi 7.4.3 Measurable Objectives ........................................................................................................... 7.4.4 Relation to Other Sustainability Indicators ............................................................................. 7.5 Seawater Intrusion Sustainability Indicator .................................................................................... 7.5.1 Locally Defined Undesirable Results ....................................................................................... 7.5.2 Minimum Thresholds .............................................................................................................. 7.5.3 Measurable Objectives ........................................................................................................... 7.5.4 Relation to Other Sustainability Indicators ............................................................................. 7.6 Degraded Water Quality Sustainability Indicator ........................................................................... 7.6.1 Locally Defined Undesirable Results ....................................................................................... 7.6.2 Minimum Thresholds .............................................................................................................. 7.6.3 Measurable Objectives ........................................................................................................... 7.6.4 Relation to Other Sustainability Indicators ............................................................................. 7.7 Subsidence Sustainability Indicator ................................................................................................ 7.7.1 Locally Defined Undesirable Results ....................................................................................... 7.7.2 Minimum Thresholds .............................................................................................................. 7.7.3 Measurable Objectives ........................................................................................................... 7.7.4 Relation to Other Sustainability Indicators ............................................................................. 7.8 Depletion of Interconnected Surface Water Sustainability Indicator ............................................. 7.8.1 Locally Defined Undesirable Results ....................................................................................... 7.8.2 Minimum Thresholds .............................................................................................................. 7.8.3 Measurable Objectives ........................................................................................................... 7.8.4 Relation to Other Sustainability Indicators ............................................................................. 7.9 Management Areas ......................................................................................................................... 7.9.1 Minimum Thresholds and Measurable Objectives ................................................................. 7.9.2 Monitoring and Analysis ......................................................................................................... 7.9.3 Explanation of How Operation of Management Area Will Avoid Undesirable Results .......... 8 Monitoring Networks (§ 354.34) ............................................................................................................ 8.1 Monitoring Objectives .................................................................................................................... 8.2 Monitoring Network ....................................................................................................................... 8.2.1 Chronic Lowering of Groundwater Levels ............................................................................... 8.2.2 Reduction of Groundwater Storage ........................................................................................ 8.2.3 Seawater Intrusion .................................................................................................................. 8.2.4 Groundwater Quality .............................................................................................................. 8.2.5 Land Subsidence...................................................................................................................... 8.2.6 Depletion of Interconnected Surface Water ........................................................................... SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO vii 8.3 Groundwater Monitoring Protocol ................................................................................................. 8.4 Data Management System .............................................................................................................. 8.5 Assessment and Improvement of Monitoring Network ................................................................. 8.6 Annual Reports ................................................................................................................................ 8.7 Periodic Evaluation by Agency ........................................................................................................ 9 Projects and Management Actions (§ 354.44) ....................................................................................... 9.1 Projects ........................................................................................................................................... 9.1.1 Project A .................................................................................................................................. 9.2 Management Actions ...................................................................................................................... 9.2.1 Management Action A ............................................................................................................ 9.3 Projects Needed to Mitigate Overdraft .......................................................................................... 10 Implementation Plan .............................................................................................................................. 10.1 Cost of Implementation .................................................................................................................. 10.2 Funding Alternatives ....................................................................................................................... 10.3 Implementation Schedule ............................................................................................................... 10.4 GSP Annual Reporting ..................................................................................................................... 10.5 Periodic Evaluations of GSP ............................................................................................................ 11 Notice and Communications (§ 354.10) ................................................................................................. 11.1 Communications and Engagement Plan ......................................................................................... 11.2 Nature of Consultations .................................................................................................................. 11.3 Public Meetings ............................................................................................................................... 11.4 Incorporation of Feedback in Decision-Making Process ................................................................. 11.5 Comments Received ....................................................................................................................... 11.6 Responses to Comments ................................................................................................................. 12 Interagency Agreements (§ 357.2-4) ..................................................................................................... 12.1 Coordination Agreements ............................................................................................................... 13 References .............................................................................................................................................. 14 Appendices The grey highlighted sections in the Table of Contents (TOC) indicate that the section has been previously released (Chapters 1 through 4) or will be released in the future (Chapters 6 through 14). The complete list of the anticipated TOC is presented to give the reader context as to how Chapter 5 – Groundwater Conditions, connects with the complete Groundwater Sustainability Plan. SLO Basin Groundwater Sustainability Plan List of Figures County of SLO and City of SLO viii LIST OF FIGURES Figure 5-1: Groundwater Elevation Surface Fall 1954. ................................................................................. 4 Figure 5-2: Groundwater Elevation Surface Spring 1990. ............................................................................ 6 Figure 5-3: Groundwater Elevation Surface Spring 1997. ............................................................................ 8 Figure 5-4: Groundwater Elevation Surface Spring 2011. .......................................................................... 10 Figure 5-5: Groundwater Elevation Surface Spring 2015. .......................................................................... 12 Figure 5-6: Groundwater Elevation Surface Spring 2019. .......................................................................... 14 Figure 5-7: Groundwater Elevation Surface Fall 2019. ............................................................................... 16 Figure 5-8: Change in Groundwater Elevation Spring 1997 to Spring 2011. .............................................. 18 Figure 5-9: Change in Groundwater Elevation Spring 2011 to Spring 2015. .............................................. 19 Figure 5-10: Change in Groundwater Elevation Spring 2015 to Spring 2019. ............................................ 20 Figure 5-11: Selected Hydrographs. ............................................................................................................ 23 Figure 5-12: Stillwater Percolation Zone Study Results. ............................................................................. 26 Figure 5-13: Soil Agricultural Groundwater Banking Index Study Results. ................................................. 27 Figure 5-14: Losing and Gaining Reaches Within the Basin. ....................................................................... 33 Figure 5-15: Potential Groundwater-Dependent Ecosystems (GDEs). ....................................................... 35 Figure 5-16: Location of Potential Point Sources of Groundwater Conditions. .......................................... 39 Figure 5-17: Distribution of TDS in Basin. ................................................................................................... 42 Figure 5-18: Distribution of Nitrate in Basin. .............................................................................................. 44 Figure 5-19: Distribution of Arsenic in Basin. ............................................................................................. 46 SLO Basin Groundwater Sustainability Plan Tables County of SLO and City of SLO ix TABLES Table 5-1: Potential Point Sources of Groundwater Contamination .......................................................... 40 SLO Basin Groundwater Sustainability Plan Appendices County of SLO and City of SLO x APPENDICES SLO Basin Groundwater Sustainability Plan List of Terms Used County of SLO and City of SLO xi LIST OF TERMS USED Abbreviation Definition AB Assembly Bill ADD Average Day Demand AF Acre Feet AFY Acre Feet per Year AMSL Above Mean Sea Level Basin Plan Water Quality Control Plan for the Central Coast Basin Cal Poly California Polytechnic State University CASGEM California State Groundwater Elevation Monitoring program CCR California Code of Regulations CCRWQCB Central Coast Regional Water Quality Control Board CCGC Central Coast Groundwater Coalition CDFM Cumulative departure from the mean CDPH California Department of Public Health CIMIS California Irrigation Management Information System City City of San Luis Obispo County County of San Luis Obispo CPUC California Public Utilities Commission CPWS-52 Cal Poly Weather Station 52 CRWQCB California Regional Water Quality Control Board CWC California Water Code DDW Division of Drinking Water Du/ac Dwelling Units per Acre DWR Department of Water Resources EPA Environmental Protection Agency ERMWC Edna Ranch Mutual Water Company ET0 Evapotranspiration EVGMWC Edna Valley Growers Ranch Mutual Water Company °F Degrees Fahrenheit FAR Floor Area Ratio FY Fiscal Year GAMA Groundwater Ambient Monitoring and Assessment program GHG Greenhouse Gas GMP Groundwater Management Plan GPM Gallons per Minute GSA Groundwater Sustainability Agency GSC Groundwater Sustainability Commission GSP Groundwater Sustainability Plan GSWC Golden State Water Company IRWMP San Luis Obispo County Integrated Regional Water Management Plan kWh Kilowatt-Hour LUCE Land Use and Circulation Element LUFTs Leaky Underground Fuel Tanks MAF Million Acre Feet MCL Maximum Contaminant Level SLO Basin Groundwater Sustainability Plan List of Terms Used County of SLO and City of SLO xii Abbreviation Definition MG Million Gallons MGD Million Gallons per Day Mg/L Milligrams per Liter MOA Memorandum of Agreement MOU Memorandum of Understanding MWR Master Water Report NCDC National Climate Data Center NOAA National Oceanic and Atmospheric Administration NWIS National Water Information System RW Recycled Water RWQCB Regional Water Quality Control Board SB Senate Bill SGMA Sustainable Groundwater Management Act SGMP Sustainable Groundwater Management Planning SGWP Sustainable Groundwater Planning SLO Basin San Luis Obispo Valley Groundwater Basin SLOFCWCD San Luis Obispo Flood Control and Water Conservation District SCML Secondary Maximum Contaminant Level SOI Sphere of Influence SNMP Salt and Nutrient Management Plan SWRCB California State Water Resources Control Board TDS Total Dissolved Solids TMDL Total Maximum Daily Load USGS United States Geological Survey USFW United States Fish and Wildlife Service USTs Underground Storage Tanks UWMP Urban Water Management Plan UWMP Act Urban Water Management Planning Act UWMP Guidebook Department of Water Resources 2015 Urban Water Management Plan Guidebook VRMWC Varian Ranch Mutual Water Company WCS Water Code Section WMP Water Master Plan WPA Water Planning Areas WRF Water Reclamation Facility WRCC Western Regional Climate Center WRRF Water Resource Recovery Facility WSA Water Supply Assessment WTP Water Treatment Plant WWTP Wastewater Treatment Plant SLO Basin Groundwater Sustainability Plan Executive Summary County of SLO and City of SLO 1 EXECUTIVE SUMMARY This section to be completed after GSP is complete. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 2 5 GROUNDWATER CONDITIONS (§ 354.16) This section describes the current and historical groundwater conditions in the Alluvial Aquifer, the Paso Robles Formation Aquifer, and the Pismo Formation Aquifer in the San Luis Obispo Valley Groundwater Basin. In accordance with the SGMA Emergency Regulations §354.16, current conditions are any conditions occurring after January 1, 2015. By implication, historical conditions are any conditions occurring prior to January 1, 2015. This Chapter focuses on information required by the GSP regulations and information that is important for developing an effective plan to achieve sustainability. The organization of Chapter 5 aligns with the six sustainability indicators specified in the GSP regulations, including: 1. Chronic lowering of groundwater elevations; 2. Groundwater storage reductions; 3. Seawater intrusion; 4. Land Subsidence; 5. Depletion of interconnected surface waters, and; 6. Degradation of groundwater quality. 5.1 GROUNDWATER ELEVATIONS AND INTEPRETATION As discussed in Chapter 4, information from available boring logs indicates that there is no regional or laterally extensive aquitard separating the Alluvial Aquifer, Paso Robles Formation aquifer, and Pismo Formation aquifer in the Basin. In the San Luis Valley, a physical distinction between Alluvium and Paso Robles Formation is often not apparent, and information from well completion reports in the Basin indicate that wells are regularly screened across productive strata in both formations, which effectively function as a single hydrogeologic unit. Likewise, in the Edna Valley, information from well completion reports indicates that wells are routinely screened across productive strata in both the Paso Robles Formation Aquifer and the Pismo Formation Aquifer, which effectively function as a single hydrogeologic unit. Boyle (1991) states that there is no strict boundary between the Alluvial Aquifer and the Paso Robles Formation Aquifer in the Buckley Road area. DWR (1997) states that all the sediments in the Subbasin are in hydraulic continuity. Because there is no available groundwater elevation data specific to the three individual aquifers, and because these formations appear to function as combined hydrogeologic units, groundwater elevation data are combined and presented as a single groundwater elevation map for each time period presented. In general, the primary direction of groundwater flow in the Basin is from the area of highest groundwater elevations in the Edna Valley northwestward toward San Luis Creek, where the flow leaves the Basin along the stream course. Groundwater in the northwestern areas of the Basin near the City of San Luis Obispo boundary and Los Osos Valley Road flows southeastward toward the San Luis Creek alluvium. In the southeastern portion of the Basin there are also local areas of flow discharging from the Basin along Pismo Creek tributaries of East and West Corral de Piedras Creek, and alluvium of other smaller tributaries further to the south. Groundwater Elevation maps for various recent and historical time periods are presented and discussed in the following sections. 5.1.1 Fall 1954 Groundwater Elevations DWR (1958) published a series of maps depicting groundwater elevations for various basins in the County, including groundwater elevations in the San Luis Obispo Valley Groundwater Basin for fall 1954 (Figure 5-1), SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 3 with contours based on field measurements of over 40 control points in the Basin. Groundwater flow direction arrows were added to Figure 5-1 to illustrate the primary direction of flow in the Basin. This is the oldest Basin-wide groundwater elevation data available. In the Los Osos Valley portion of the Basin, this map displays dominant groundwater flow direction from higher elevations in the in the northwestern extent of the Basin southeastward toward the discharge area where San Luis Creek leaves the Basin. The hydraulic gradient (the ratio of horizontal distance along the groundwater flow path to the change in elevation) in this area is approximately 0.004 feet/feet (ft/ft). In the Edna Valley portion of the Basin, the dominant groundwater flow direction is northwestward from the higher groundwater elevations in the southeastern part of the Basin (over 280 ft AMSL) to lower elevations (less than 110 feet AMSL) where San Luis Creek exits the Basin. The gradient across this area is steeper than in Los Osos Valley, approximately 0.009 ft/ft. This map also displays local areas of discharge coincident with the areas where San Luis Creek and Pismo Creek tributaries leave the Basin. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 4 Figure 5-1: Groundwater Elevation Surface Fall 1954. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 5 5.1.2 Spring 1990 Groundwater Elevations Boyle (1991) presents water level elevation contour maps for the spring of 1986 and 1990, based on water level data collected from 18 control points in the field. A digitized recreation of the Boyle groundwater elevation contours for spring of 1990 is presented in Figure 5-2 and displays patterns of groundwater flow direction in the Basin similar to those exhibited in the DWR 1954 map, although the flow gradient does not appear to be as steep as it is in the 1954 map. The year 1990 was in the midst of a significant period of drought in the Basin. The northwestward gradient across the central area of the Basin is approximately 0.006 ft/ft. Contours for the spring of 1986 are not re-presented in this report, but 1986 represents wetter conditions than the 1990 map, and it is noted in Boyle (1991) that there is a difference of approximately 10 feet of elevation between the two maps, representing the variation in water levels observed between wet and dry weather cycles in this time period. The contours in Figure 5-2 do not display an area of discharge where Corral de Piedras Creeks leave the Basin, but this is likely due to a lack of control points in this area. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 6 Figure 5-2: Groundwater Elevation Surface Spring 1990. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 7 5.1.3 Modeled 1990s Groundwater Elevations In its draft report, DWR (1997) used a computer groundwater model to generate a series of modeled water level maps representing wet, dry, and average weather conditions. The model results are not re-presented in this GSP, but a review of the draft report indicates the maps display the same general flow direction patterns as the DWR (1958) and Boyle (1991) maps, which were based on data collected in the field. Water level elevations in the San Luis Valley in wet years were approximately 10 to 20 feet higher than in dry years. In the Edna Valley, the difference in groundwater elevations between wet and dry years was greater, approximately 20 to 30 feet. 5.1.4 Spring 1997 Groundwater Elevations More recent groundwater level data collected as a part of San Luis Obispo County’s groundwater monitoring program were obtained and used to generate groundwater elevation maps to evaluate more recent conditions. The following assessment of groundwater elevation conditions is based primarily on data from the San Luis Obispo County Flood Control and Water Conservation District’s (SLOFCWCD) groundwater monitoring program. Groundwater levels are measured through a network of public and private wells in the Basin. Figure 5-3 through Figure 5-7 presents the contours generated from the data for the Spring 1997, Spring 2011, Spring 2015, Spring 2019, and Fall 2019 monitoring events. The set of wells used in the groundwater elevation assessment were selected based on the following criteria: • The wells have groundwater elevation data for the periods of record of interest; • Groundwater elevation data were deemed representative of static conditions. Additional information on the monitoring network is provided in Chapter 8 – Monitoring Networks. Based on available data, the following information is presented in subsequent subsections. • Groundwater elevation contour maps for spring 1997, 2011, 2015, 2019, and Fall 2019; • A map depicting the change in groundwater elevation between 1997 and 2011; • A map depicting the change in groundwater elevation between 2011 and 2015; • A map depicting the change in groundwater elevation between 2015 and 2019; • Hydrographs for select wells with publicly available data. Figure 5-3 presents a groundwater surface map for Spring 1997 based on field data collected by the County (control points are not displayed to maintain confidentiality agreements negotiated with well owners). The southeast (near Lopez Lake) and northwest (Los Osos Valley) areas of the Basin had no wells monitored during these events to calculate water levels, so contours are not presented for those areas. Several features on this map are apparent. First, a pronounced groundwater mound is evident at the location where West Corral de Piedras Creek enters the Basin in Edna Valley, near the corner of Biddle Ranch Road and Orcutt Road; three control points are present in this area, providing reliable documentation for water levels in this vicinity. This indicates that this is a groundwater recharge area. The regional northwesterly flow direction apparent in the previously discussed water level maps is still evident here; the groundwater flow gradient is about 0.011 ft/ft, somewhat steeper than the Spring 1990 gradient presented by Boyle. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 8 Figure 5-3: Groundwater Elevation Surface Spring 1997. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 9 5.1.5 Spring 2011 Groundwater Elevations Spring 2011 represents a time period just prior to the recent drought, but after the expansion of agricultural pumping in Edna Valley (discussed further in Chapter 6, Water Budget). As such, effects of the recent drought should not yet be apparent, but reduced groundwater levels due to expanded agricultural pumping should be evident. Figure 5-4 displays groundwater elevation contours for Spring 2011. The groundwater mound near Biddle Ranch Road and Orcutt Road is again evident, with a maximum groundwater elevation of over 320 feet. Groundwater flow direction appears to indicate areas of discharge from the Basin in Edna Valley along Corral de Piedras Creeks and Canada Verde Creek, and along San Luis Creek in San Luis Valley. The area near Edna Road and Biddle Ranch Road indicates a steep local gradient, likely associated with local pumping. The contour near the exit of Corral de Piedras Creeks is 180 feet. The gradient across the central Basin is almost identical to the Spring 1997 map, about 0.011 ft/ft. The gradient is much shallower in the San Luis Valley part of the Basin. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 10 Figure 5-4: Groundwater Elevation Surface Spring 2011. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 11 5.1.6 Spring 2015 Groundwater Elevations Figure 5-5 presents groundwater elevation contours for Spring 2015. Spring 2015 represents a time period in the midst of the recent drought, and after the expansion of agricultural pumping in Edna Valley. The effects of the drought are apparent upon close inspection of the contours in Figure 5-5. In the Edna Valley, the maximum contour of the recharge area near Orcutt Road and Biddle Ranch Road is 280 feet, about 40 feet lower than in the Spring 2011 map. The contours immediately west of the mound are still steep, but flatten out significantly along Davenport Creek, resulting in a much shallower gradient in this area than in the Spring 2011 map. Contours east of the mound along Orcutt Road are 20 to 40 feet lower than in the Spring 2011 map. In the San Luis Valley, a 100-foot contour is evident near the exit of San Luis Creek from the Basin, which is about 10 feet lower than the contour in the Spring 2011 map. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 12 Figure 5-5: Groundwater Elevation Surface Spring 2015. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 13 5.1.7 Spring 2019 Groundwater Elevations Figure 5-6 presents a groundwater surface elevation map for Spring 2019. Spring 2019 represents a time period at the end of seasonal winter rains, and after the end of the recent drought. Rebounds of groundwater elevations from the drought are apparent upon inspection of the contours. In the Edna Valley, the maximum contour of the recharge area near Orcutt Road and Biddle Ranch Road is 300 feet, about 20 feet higher than in the Spring 2015 map. Contours east of the mound are about 20 feet higher than in the Spring 2015 map. Contours along Davenport Creek are about 20 feet higher than in the Spring 2015 map. The elevation at Edna Road and Biddle Ranch Road is about 230 feet, over 50 feet higher than in the Spring 2015 map. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 14 Figure 5-6: Groundwater Elevation Surface Spring 2019. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 15 5.1.8 Fall 2019 Groundwater Elevations Figure 5-7 presents a groundwater surface elevation map for Fall of 2019. This time period represents recent conditions at the end of the summer dry season for comparison against the spring conditions. Overall, the contours indicate lower groundwater levels than those displayed in the Spring 2019 map. Groundwater contours east of the recharge mound at West Corral de Piedras are about 20 feet lower than the Spring 2019 map. The groundwater elevation at Edna Road and Biddle Ranch Road is about 220 feet, approximately 10-20 feet lower than in the Spring 2018 map. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 16 Figure 5-7: Groundwater Elevation Surface Fall 2019. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 17 5.1.9 Changes in Groundwater Elevation In order to demonstrate how groundwater elevations have varied over the recent history of the Basin, a series of maps were generated that display changes in groundwater elevation. These maps were developed by comparing groundwater elevations from one year to the next and calculating the differences in elevation over the specified time period. It should be noted that the results of this analysis are largely dependent on the density of data points, and should be viewed as indicative of general trends, not necessarily as accurate in specific areas where little data is available. The first time period selected compares changes in groundwater elevation from 1997 through 2011. The year 1997 was selected as a starting point because it is assumed to represent conditions prior to the significant expansion of agricultural groundwater pumping in the Basin. The year 2011 was selected as the end point because it represents conditions prior the start of the recent drought. Calculated changes in groundwater elevation over this 14-year period are presented in Figure 5-8. This figure indicates a maximum decline in groundwater elevation of over 60 feet in the Edna Valley, southeast of East Corral de Piedras Creek between Orcutt Road and Corbett Canyon Road. The calculated groundwater elevation shows declining groundwater levels to the northwest of this location. No significant declines are indicated northwest of Biddle Ranch Road over this time period. The next time period selected compares changes in groundwater elevation from 2011 through 2015. This time period was selected to capture the start of the drought to a point four years into the drought, thereby capturing the period of greatest groundwater elevation change. Calculated changes in groundwater elevation over this 4-year period are presented in Figure 5-9. This figure indicates a maximum decline in groundwater elevation of over 80 feet located in the Edna Valley, near the intersection of Edna Road and Biddle Ranch Road. The calculated reductions in groundwater elevation decline in all directions from this location. No significant declines are indicated in the San Luis Creek Valley portion of the Basin over this time period. The next time period selected compares changes in groundwater elevation from 2015 through 2019. This time period was selected to capture the potential recovery of the Basin following the drought. Calculated changes in groundwater elevation over this 3-year period are presented in Figure 5-10. Groundwater elevations are shown to have rebounded throughout the entire area in which data was available. The greatest increase in groundwater elevation is coincident with the area of greatest declines from 2011-2015, near the intersection of Edna Road and Biddle Ranch Road. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 18 Figure 5-8: Change in Groundwater Elevation Spring 1997 to Spring 2011. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 19 Figure 5-9: Change in Groundwater Elevation Spring 2011 to Spring 2015. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 20 Figure 5-10: Change in Groundwater Elevation Spring 2015 to Spring 2019. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 21 5.1.10 Vertical Groundwater Gradients Vertical groundwater gradients are calculated by measuring the difference in head at a single location between specific and distinct strata or aquifers. The characterization of vertical gradients may have implications with respect to characterization of flow between aquifers, migration of contaminant plumes, and other technical details describing groundwater flow in specific areas. In order to accurately characterize vertical groundwater gradient, it is necessary to have two (or more) piezometers sited at the same location, with each piezometer screened across a unique interval that does not overlap with the screened interval of the other piezometers(s). If heads at one such piezometer are higher than the other(s), the vertical flow direction can be established, since groundwater flows from areas of higher heads to areas of lower heads. However, because such a “well cluster” must be specifically designed and installed as part of a broader investigation, limited data exists to assess vertical groundwater gradients. Previous hydrologic studies of the Basin (Boyle 1991, DWR 1997) indicate that groundwater elevations are generally higher in the Alluvial Aquifer than the underlying Paso Robles Formation Aquifer, resulting in groundwater flow from the Alluvial Aquifer to the underlying Paso Robles Formation aquifer (although this may change seasonally). The lack of nested or clustered piezometers to assess vertical gradients in the Basin is a data gap that will be discussed further in Chapter 8. There are no paired wells that provide specific data comparing water levels in wells screening the bedrock and the Basin sediments. However, from a conceptual standpoint, the Monterey Formation is assumed to receive rainfall recharge in the surrounding mountains at higher elevations than the Basin sediments. For this reason, it is assumed that an upward vertical flow gradient exists between the bedrock and the overlying Basin sediments. Because the bedrock formations are significantly less productive than the Basin sediments, the rate of this flux is not expected to be significant. 5.2 GROUNDWATER ELEVATION HYDROGRAPHS The San Luis Valley and the Edna Valley are characterized by different patterns of groundwater use. In the San Luis Valley, groundwater use has been dominated by municipal and industrial use, with total groundwater use decreasing since the 1990s, as the City has diversified its surface water supplies, and placed most of its wells on standby status. During this time several in-City agricultural operations have also been developed into housing and commercial districts and now rely on the City’s surface water supplies in place of groundwater pumping. In the Edna Valley, groundwater use is dominated by agricultural use, with total use increasing since the 1990s. During the past 15 to 20 years, wine grapes have supplanted other crop types (such as pasture grass and row crops) as the dominant agricultural use within the Edna Valley. Available water level data was reviewed, and data from wells with the longest period of record are presented in Figure 5-11, and discussed in this section. Most of the data was obtained from the County’s groundwater monitoring network database. Figure 5-11 presents groundwater elevation hydrographs for the ten wells throughout the Basin with the longest period of record. State well identification numbers are not displayed for reasons of owner confidentiality. Appendix 5A presents depth to water hydrographs for all wells for which the county had water level data. Three distinct patterns are evident in different areas of the Basin and are discussed below. The hydrographs for the wells in the San Luis Valley indicate that water levels in these wells, although somewhat variable in response to seasonal weather patterns, water use fluctuations, and longer-term dry weather periods, are essentially stable. There are no long-term trends indicating steadily declining or SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 22 increasing water levels in this area. The wells along Los Osos Valley Road (hydrographs 1 and 2 on Figure 5-11) display fluctuations within a range of less than 20 feet over a period of record from the late 1950s to the mid-1990s. This period includes the drought of the late 1980s to early 1990s. The well just west of the intersection of Tank Farm Road and Orcutt Road (hydrograph 4 in Figure 5-11) displays a similar pattern, with water level variations within a range of about 10 feet from 1965 to 2013. The wells in the vicinity of Highway 101 and Los Osos Valley Road (hydrograph 3 in Figure 5-11) also display water levels in relative equilibrium, with the exception of the early 1990s, when drought-related pumping and weather patterns resulted in noticeable declines in the water level in this well. These water levels recovered to their pre- drought levels by the mid-1990s. The long-term stability of groundwater elevations in these hydrographs indicates that groundwater extractions and natural discharge in the areas of these wells are in approximate equilibrium with natural recharge and subsurface capture, and that no trends of decreasing groundwater storage are evident. A second distinct pattern is evident in hydrographs from wells in the area immediately east of the intersection of Biddle Ranch Road and Orcutt Road, where West Corral de Piedras Creek enters the Basin (hydrographs 5 and 6 in Figure 5-11). The hydrographs of the two wells in this area display much greater volatility in response to seasonal and drought cycle fluctuations than the wells in San Luis Valley, with water levels fluctuating within a range of over 40 feet, as opposed to the range of 10 to 20 feet in the San Luis Valley wells. However, water levels appear to rebound to pre-drought levels when each drought cycle ends. Groundwater elevations displayed in these two hydrographs do not display a long-term decline of water levels. This pattern is likely associated with local recharge of the aquifer derived from percolation of stream water in West Corral de Piedras Creek as it leaves the mountains and enters the Basin. By contrast, several wells in the Edna Valley display steadily declining water levels during the past 15 to 20 years. Hydrographs for four wells (hydrographs 7, 8, 9, and 10 on Figure 5-11) in the Edna Valley display groundwater elevation declines of about 60 to 100 feet since the year 2000. Groundwater elevations in the Edna Valley displayed the largest historical declines in the Basin. This hydrograph pattern indicates that a reduction of groundwater storage has occurred over this period of record in the area defined by these well locations. It is understood, and will be discussed in greater detail in Chapter 6 (Water Budget), that agricultural pumping has increased in Edna Valley during this time period, likely explaining the patterns of declining groundwater elevations in these hydrographs. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 23 Figure 5-11: Selected Hydrographs. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 24 5.3 GROUNDWATER RECHARGE AND DISCHARGE AREAS Areas of significant areal recharge and discharge within the Basin are discussed below. Quantitative information about all natural and anthropogenic recharge and discharge is provided in Chapter 6: Water Budgets. 5.3.1 Groundwater Recharge Areas In general, natural areal recharge occurs via the following processes: 1. Distributed areal infiltration of precipitation, 2. Subsurface inflow from adjacent “non-water bearing bedrock”, and 3. Infiltration of surface water from streams and creeks. 4. Anthropogenic recharge The following sections discuss each of these components. 5.3.1.1 Infiltration of Precipitation Areal infiltration of precipitation is a significant component of recharge in the Basin. Water that does not run off to stream or get taken up via evapotranspiration migrates vertically downward through the unsaturated zone until it reaches the water table. By leveraging available GIS data that defines key factors such as topography and soil type, locations with higher likelihood of recharge from precipitation have been identified. These examinations are desktop studies and therefore are conceptual in nature, and any recharge project would need a site-specific field characterization and feasibility study before implementation. Still, although they differ in scope and approach, the results of these studies provide an initial effort at identifying areas that may have the intrinsic physical characteristics to allow greater amounts of precipitation-based recharge in the Basin. Stillwater Sciences (Stillwater), in cooperation with the Upper Salinas-Las Tablas Resource Conservation District (USLTRCD), published a grant funded study (Stillwater 2015) designed to improve data gaps in the County’s Integrated Regional Water Management (IRWM) plan. The Percolation Zone Study of Pilot-Study Groundwater Basins in San Luis Obispo County, California identified areas with relatively high natural percolation potential that, through management actions, could enhance local groundwater supplies for human and ecological benefits to the aquatic environment for steelhead habitat. The study used existing data in a GIS analysis to identify potentially favorable areas for enhanced recharge projects in the combined San Luis Creek and Pismo Creek Watershed. The results of the Stillwater-USLTRCD study are presented in Figure 5-12. The analysis indicates that approximately 2,220 acres in the Basin are categorized with high potential for intrinsic percolation, and 6,583 acres have medium potential. Conceptually, areas with higher potential for intrinsic percolation would transmit a higher percentage of rainfall to aquifer recharge. The largest area in the Basin that is classified with high recharge potential is the alluvium along East and West Corral de Piedras Creeks in the Edna Valley. The University of California (UC) at Davis and the UC Cooperative Extension published a study in 2015 that also uses existing GIS data to identify areas potentially favorable for enhanced groundwater recharge projects (UC Davis Cooperative Extension, 2015). While the Stillwater study focused on local San Luis SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 25 Obispo stream corridors and emphasized fish habitat conditions, the UC study is statewide in scope includes more than 17.5 million acres, is scientifically peer reviewed, and focuses on the possibilities of using fallow agricultural land as temporary percolation basins during periods when excess surface water is available. The UC study developed a methodology to determine a Soil Agricultural Groundwater Banking Index (SAGBI) to assign an index value to agricultural lands through the state. The SAGBI analysis incorporates deep percolation, root zone residence time, topography, chemical limitations (salinity), and soil surface conditions into its analysis. The results of the SAGBI analysis in the Basin are presented in Figure 5-13. Areas with excellent recharge properties are shown in green. Areas with poor recharge properties are shown in red. Not all land is classified, but similar to the Stillwater map in Figure 5-12, this map provides guidance on where natural recharge likely occurs. The two studies discussed herein yield similar results in the Basin, particularly in Edna Valley. The Stillwater study identifies much of the drainage area of East and West Corral de Piedras Creeks, as well as area of alluvium of smaller streams t the southeast, as having high recharge potential. The SAGBI study identifies very similar areas in Edna Valley as having a moderately to good index value. These two studies, with differing methodologies, study areas, and objectives, converge on the characterization of the same portions of Edna Valley as having high natural recharge potential. By extension, areas with high natural recharge potential would be favorable locations to investigate the feasibility of enhanced recharge projects. If source water is available, water in these areas would have a higher likelihood of percolating to the underlying aquifers. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 26 Figure 5-12: Stillwater Percolation Zone Study Results. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 27 Figure 5-13: Soil Agricultural Groundwater Banking Index Study Results. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 28 5.3.1.2 Subsurface Inflow Subsurface inflow is the flow of groundwater from the surrounding bedrock into the basin sediments. This process is sometimes referred to as mountain front recharge. Groundwater flows from areas of high head to areas of lower head, and water levels in the mountains are at a higher elevation than the Basin. Flow across the basin boundary is predominantly via highly conductive, but random and discontinuous fracture systems. The rate of subsurface inflow to the Basin from the surrounding hill and mountain area varies considerably from year to year depending upon precipitation (intensity, frequency and duration, seasonal totals, etc.) and groundwater level gradients. There are no available published or unpublished inflow data for the hill and mountain areas surrounding the Basin. An estimate of this component of recharge is presented in Chapter 6 (Water Budget). 5.3.1.3 Percolation of Streamflow Percolation of streamflow is a locally significant source of recharge in areas where the local creeks enter the Basin. Water levels in wells monitored by the County in the area where Corral de Piedras Creeks enter the Basin reflect this phenomenon, as discussed in the previous discussion of water level elevations in the Basin. Groundwater recharge from percolation of streamflow is thought to occur in the area along Davenport Creek, near Buckley Road as well. Most wells in this vicinity are on the order of 100 feet deep, which is too deep to be screened only in the local alluvium; these wells are assumed to screen the Paso Robles Formation Aquifer. During the seasonal winter rains when the creeks are flowing, groundwater levels are at approximately the same level as the water in the creek. During the dry season, water levels decrease to about 15 to 20 feet below land surface. Therefore, the alluvium appears to recharge the underlying Paso Robles Formation in this area. It is likely that similar processes contribute to recharge via percolation of streamflow along the San Luis Creek corridor, as well. Specific isolated monitoring of alluvial wells compared to the underlying aquifers’ water levels could clarify this recharge component. 5.3.1.4 Anthropogenic Recharge Significant anthropogenic recharge occurs via the three processes discussed below: 1. Percolation of treated wastewater treatment plant (WWTP) effluent, 2. Percolation of return flow from agricultural irrigation, and 3. Percolation of return flow from domestic septic fields. A wastewater treatment plant serving the City of San Luis Obispo operates within the Basin on Prado Road along San Luis Creek. Treated wastewater effluent from this plant is discharged to San Luis Creek and used in the City’s recycled water system for irrigation and construction-related uses. The County operates a small WWTP near the golf course in the service area of Golden State Water Company, and uses the effluent largely to irrigate the golf course. Residences in Edna Valley beyond the city or county WWTP service area dispose of wastewater via septic tanks. Water from septic fields can percolate into the underlying aquifers. Irrigated agriculture is prevalent in the Basin, especially along Los Osos Valley Road and in Edna Valley. Return flows from irrigated agriculture occur when water is supplied to the irrigated crops in excess of the crop’s water demand. This is done to avoid excess build-up of salts in the soil and overcome non-uniformity in the irrigation distribution system. These are all general standard practices. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 29 5.3.2 Groundwater Discharge Areas Natural groundwater discharge occurs as discharge to springs, seeps and wetlands, subsurface outflows, and evapotranspiration (ET) by phreatophytes. Figure 5-14 includes the locations of significant active springs, seeps, and wetlands within or adjacent to the Basin identified from previous studies or included on USGS topographic maps covering the watershed area. There are no mapped springs or seeps located within the Basin boundaries; most are located at higher elevations in the surrounding mountain areas. Natural groundwater discharge can also occur as discharge from the aquifer directly to streams. Groundwater discharge to streams and potential groundwater dependent ecosystems (GDEs) are discussed in Section 5.8. In contrast to mapped springs and seeps, whose source water generally comes from bedrock formations in the mountains, groundwater discharge to streams is derived from the alluvium. Discharge to springs or streams can vary seasonally as precipitation and stream conditions change throughout the year. Groundwater discharge to the Corral de Piedras Creeks occur seasonally at the location where the creeks leave the basin, where relatively impermeable bedrock rises to the surface along the Edna Fault, causing groundwater to daylight at this location, at least in the wet season. Subsurface outflow and ET by phreatophytes are discussed in Chapter 6 (Water Budget). 5.4 CHANGE IN GROUNDWATER STORAGE Changes in groundwater storage for the Alluvial Aquifer and Paso Robles Formation Aquifer are correlated with changes in groundwater elevation, previously discussed, and are addressed in Chapter 6 (Water Budget). 5.5 SEAWATER INTRUSION Seawater intrusion is not an applicable sustainability indicator for the Basin. The Basin is not adjacent to the Pacific Ocean, a bay, or inlet. 5.6 SUBSIDENCE Land subsidence is the lowering of the land surface. While several human-induced and natural causes of subsidence exist, the only process applicable to the GSP is subsidence due to lowered groundwater elevations caused by groundwater pumping. Historical incidence of subsidence within the Basin was discussed in Chapter 4 (Basin Setting). Direct measurements of subsidence have not been made in the Basin using extensometers or repeat benchmark calibration; however, interferometric synthetic aperture radar (InSAR) has been used in the County to remotely map subsidence and DWR is expected to continue to collect InSAR data. This technology uses radar images taken from satellites that are used to map changes in land surface elevation. One study done in the area, which evaluates the time period between spring 1997 and fall 1997 (Valentine, D. W. et al., 2001), did not report any measurable subsidence within the Basin. Subsidence as a sustainability indicator will be addressed further in Chapter 8. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 30 5.7 INTERCONNECTED SURFACE WATER Surface water/groundwater interactions may represent a significant, portion of the water budget of an aquifer system. Where the water table is above the streambed and slopes toward the stream, the stream receives groundwater from the aquifer; that is called a gaining reach (i.e., it gains flow as it moves through the reach). Where the water table is beneath the streambed and slopes away from the stream, the stream loses water to the aquifer; that is called a losing reach. In addition, a stream may be disconnected from the regional aquifer system if the elevation of streamflow and alluvium is significantly higher than the elevation of the water table in the underlying aquifer. The spatial extent of interconnected surface water in the Basin was evaluated using water level data from Alluvial Aquifer and Paso Robles Formation Aquifer wells adjacent to the Basin creeks and streams. In accordance with the SGMA Emergency Regulations §351 (o), “Interconnected surface water refers to surface water that is hydraulically connected at any point by a continuous saturated zone to the underlying aquifer and the overlying surface water is not completely depleted”. The interconnected surface water analysis for the Basin consisted of comparing average springtime water level elevations in wells adjacent to the San Luis Creek with the elevation of the adjacent San Luis Creek channel. In cases where average springtime water levels were greater than the elevation of the adjacent San Luis Creek channel, the stream reach was considered as potentially ‘gaining’. In cases where average springtime water levels were below the adjacent channel elevation, the stream reach was considered ‘losing’ and potentially ‘disconnected’. It is important to recognize that the results of these analyses may reflect conditions that occur occasionally, in response to precipitation events. They may not be representative of long-term average conditions. The analysis outlined above resulted in identification of two areas of San Luis Creek that occasionally ‘gain’ water from the Alluvial Aquifer; the confluence of Stenner Creek and San Luis Creek, and the reach of San Luis Creek downstream from the Wastewater Treatment Plant to the confluence with Prefumo Creek. These are displayed in Figure 5-14. Several reaches of San Luis Creek are identified that occasionally ‘lose’ water to the Alluvial Aquifer. Groundwater levels in the San Luis Valley part of the Basin are generally high enough that the creek is connected to the underlying aquifer. Along most of Corral de Piedras Creeks, by contrast, surface water levels are generally greater than 30 feet above the groundwater level, and the streams are considered disconnected from the underlying Alluvial Aquifer in this area. 5.7.1 Depletion of Interconnected Surface Water Groundwater withdrawals are balanced by a combination of reductions in groundwater storage and changes in the rate of exchange across hydrologic boundaries. In the case of surface water depletion, this rate change could be due to reductions in rates of groundwater discharge to surface water, and increased rates of surface water percolation to groundwater. Seasonal variation in rates of groundwater discharge to surface water or surface water percolation to groundwater occur naturally throughout any given year, as driven by the natural hydrologic cycle. However, they can also be affected by anthropogenic actions. Since, as presented in the discussion of hydrographs in the San Luis Valley in Section 5.2, there has been no long- term water level declines in this area, there is no evidence of long-term depletion of interconnected surface water in this area. 5.8 POTENTIAL GROUNDWATER DEPENDENT ECOSYSTEMS The SGMA Emergency Regulations §351.16 require identification of groundwater dependent ecosystems within the Basin. Several datasets were utilized to identify the spatial extent of potential groundwater SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 31 dependent ecosystems (GDEs) in the Basin, as discussed in the following sections. In accordance with the SGMA Emergency Regulations §351 (o), “groundwater dependent ecosystems refers to ecological communities or species that depend on groundwater emerging from aquifers or on groundwater occurring near the ground surface”. In areas where the water table is sufficiently high, groundwater discharge may occur as evapotranspiration (ET) from phreatophyte vegetation within these GDEs. The overall distribution of potential GDEs within the Basin has been initially estimated in the Natural Communities Commonly Associated with Groundwater (NCCAG) dataset (DWR, 2018). This dataset was reviewed by Stillwater Sciences, and the resulting distribution of potential GDEs is shown in Figure 5-15. There has been no verification that the locations shown on this map constitute GDEs. Additional field reconnaissance is necessary to verify the existence and extent of these potential GDEs, and may be considered as part of the monitoring network for future planning efforts. 5.8.1 Hydrology 5.8.1.1 Overview of GDE Relevant Surface and Groundwater Hydrology Instream flows in San Luis and Pismo Creeks can be divided into wet season flows, typically occurring from January to April, and dry season flows, typically from June to October. Short transitional periods occur between the wet and dry seasons. Wet season instream flows originate from a range of sources including precipitation-driven surface runoff events, water draining from surface depressions or wetlands, shallow subsurface flows (e.g., soil), and groundwater discharge. Dry season instream flows, however, are likely fed primarily by groundwater discharge. As groundwater levels fall over the dry season, so do the corresponding instream flows. If groundwater elevations remain above instream water elevations, groundwater discharges into the stream and surface flows continue through the dry season (creating perennial streams). If groundwater elevations fall below the streambed elevation, the stream can go dry. Streams that typically flow in the wet season and dry up in the dry season are termed intermittent. Over time, streams can transition from historically perennial to intermittent conditions due to climactic changes or groundwater pumping (Barlow and Leake 2012). Dry season flows supported by groundwater are critical for the survival of various special status species, including the federally threatened California red-legged frog (Rana draytonii) and Steelhead (Oncorhynchus mykiss). San Luis Creek and Pismo Creek are underlain by the Alluvial Aquifer, the Paso Robles Formation Aquifer, and the Pismo Formation Aquifer, as previously discussed. These aquifers have hydraulic connection to one another, and to surface waters, but the degree of connection varies spatially. Aquifers can include confined aquifers, unconfined aquifers, and perched aquifers (Chapter 4). Aquifers can discharge into ponds, lakes or creeks or vice versa. In the San Luis Obispo Valley Groundwater Basin, little data exists to characterize the connection between surface water and groundwater. While the groundwater in the San Luis Valley and Edna Valley is hydraulically connected, a shallow subsurface bedrock divide between the two sub-areas partially isolates the deeper portions of the two aquifers (Figure 5-10 and Figure 5-11). Groundwater in the Edna Valley flows both towards the San Luis Valley in the northwest portion of the basin and towards Price Canyon in the southwest portion of the basin. Groundwater flowing towards Price Canyon rises to the surface as it approaches the bedrock constriction of Price Canyon and the Edna fault system. The 1954 DWR groundwater elevation map (Figure 5-1) best illustrates the pre-development groundwater flow from the Edna Valley both towards San Luis Obispo and into Price Canyon. Observations of stream conditions indicate a perennial reach of Pismo Creek that flows through Price Canyon and supports year-round critical habitat for threatened steelhead just south of the Basin Boundary. A conceptual explanation for this is that groundwater from the Edna sub-area flows towards the discharge area at Price Canyon, and rises to the surface (daylights) as the groundwater SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 32 flow encounters the impermeable zone of the Edna Fault and the bedrock outside of the Basin. Piezometers in this area could confirm this interpretation of observed stream conditions. 5.8.1.2 Losing and Gaining Reaches Streams are often subdivided into losing and gaining reaches to describe their interaction of surface water in the stream with groundwater in the underlying aquifer. In a losing reach water flows from the stream to the groundwater, while in a gaining reach water flows from the groundwater into the stream. The connection between losing reaches to the regional aquifer may be unclear as water can be trapped in perched aquifers above the regional water table. Figure 5-14 shows the likely extent of known gaining and losing reaches in San Luis and Pismo Creeks during typical dry season conditions. This map is compiled from various data sources, including: • A field survey of wet and dry reaches of San Luis Creek (Bennett 2015), • Field surveys and flow measurements of Pismo Creek (Balance Hydrologics 2008), • An instream flow study of Pismo Creek (Stillwater Sciences 2012), • A regional instream flow assessment that included San Luis and Pismo Creeks (Stillwater Sciences 2014), • Spring and summer low flow measurements in San Luis and Pismo Creeks (2015–2018) (Creek Lands Conservation 2019), and • Consideration of the effects of local geologic features such as bedrock outcrops and faults, both of which can force deeper groundwater to the surface. The effect of faults and bedrock outcrops can be localized or extend for some distance downstream. Portions of the San Luis and Pismo Creeks and their tributaries for which no data exist are left unhighlighted in Figure 5-14. In general, the extent of losing or gaining reaches can vary by water year type or pumping conditions. East and West Corral de Piedras Creeks on the north-east side of the basin can be dry in the spring and summer during drier years but be flowing, losing reaches in wetter years (Creek Lands Conservation 2019). (To be clear, a stream segment can be a losing reach even if it is not hydraulically connected to the aquifer, since the stream will be losing surface flow to the subsurface via percolation.) In contrast, gaining reaches shown on San Luis Obispo Creek are fairly consistent across water year types (Bennett 2015, Creek Lands Conservation 2019). Figure 5-14 is based on limited data sources. Improved surface flow monitoring is recommended to refine and update the extent of losing and gaining reaches, as well as to provide data for unhighlighted reaches. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 33 Figure 5-14: Losing and Gaining Reaches Within the Basin. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 34 5.8.2 Vegetation and Wetland Groundwater Dependent Ecosystem Identification DWR has compiled a statewide Natural Communities Commonly Associated with Groundwater (NCCAG) database (DWR 2019). This database identifies potentially groundwater dependent ecosystems based on the best available vegetation and wetland data (Klausmeyer et al. 2018). DWR identifies potentially groundwater dependent wetland areas using National Wetland Inventory (NWI) wetland data (USFWS 2018). These data were evaluated and assessed to accurately capture wetland and riverine features. In the Basin, the best available vegetation mapping data set was from the California Fire and Resource Assessment Program Vegetation (FVEG, California Department of Forestry and Fire Protection 2015). FVEG is a remotely sensed dataset that classifies vegetation to coarse types (i.e., the California Wildlife Habitat Relationship System). Given the limitations of this dataset to accurately capture and identify vegetation using a precise classification system, it was deemed inappropriate for use in determining potential GDEs. Instead, a manual assessment of vegetation with potential groundwater dependence was conducted using National Agricultural Imagery Program 2018 color aerial imagery (NAIP 2018). Vegetation communities identified as potentially groundwater dependent included riparian trees and shrubs, and oak woodlands. Oak woodlands were considered potentially groundwater dependent due to their deep rooting depths (up to 70 feet [Lewis and Burgy 1964]). Potential vegetation and wetland GDEs were retained if the underlying depth to water in 2019 was inferred to be 30 feet or shallower based on the existing well network (Figure 5-15). Depth to groundwater was interpolated from seventeen wells for which groundwater level data was available in the spring of 2019 (Figure 5-6). The depth to groundwater estimated in Figure 5-15 is assumed to represent regional groundwater levels; however, the screening depth is known for only 6 of the 17 of the wells. Wells where the screened depth is unknown may be measuring groundwater levels for deeper aquifers that are unconnected to the shallow groundwater system, and thus groundwater deeper than 30 ft for a given well may not reflect the absence of shallow groundwater, but instead reflects the absence of data. To determine the hydraulic connectivity between potential perched aquifers to the regional aquifer, additional monitoring with nested piezometers could be utilized. For the purposes of differentiating between potential and unlikely GDE’s, different assumptions were made for the San Luis Valley versus Edna Valley in areas of no groundwater data. In the San Luis Valley, underlying San Luis Creek, it was assumed that the depth to regional groundwater was less than 30 feet because the limited available data indicate that groundwater in this sub-area is generally relatively shallow. In the Edna Valley (underlying Pismo Creek), it was assumed that the depth to regional groundwater was more than 30 feet because the limited available data indicate that the groundwater in this sub-area is generally deeper; therefore, much of the area of the lower reaches of East and West Corral de Piedras Creeks is unlikely to have GDEs. One exception to this assumption was made on upper East Corral de Piedra where the conditions were assumed to be similar to those on upper West Corral de Piedra where early dry season wet conditions have been observed by Stillwater Sciences and Balance Hydrologics (2008). The 30-foot depth criterion is consistent with guidance provided by The Nature Conservancy (Rohde et al. 2019) for identifying GDEs. Additionally, the area where East and West Corral de Piedras Creeks leave the Basin near Price Canyon has groundwater elevation data within 30 feet of the streams, and so are presented as having potential GDEs. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 35 Figure 5-15: Potential Groundwater-Dependent Ecosystems (GDEs). SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 36 5.8.3 Identification of Special-Status Species and Sensitive Natural Communities Associates with GDE’s For the purposes of this GSP, special-status species are defined as those: • Listed, proposed, or under review as endangered or threatened under the federal Endangered Species Act (ESA) or the California Endangered Species Act (CESA); • Designated by California Department of Fish and Wildlife (CDFW) as a Species of Special Concern; • Designated by CDFW as Fully Protected under the California Fish and Game Code (Sections 3511, 4700, 5050, and 5515); • Protected under the federal Bald and Golden Eagle Protection Act; • Designated as rare under the California Native Plant Protection Act (CNPPA); and/or • Included on CDFW’s most recent Special Vascular Plants, Bryophytes, and Lichens List (CDFW 2019a) with a California Rare Plant Rank (CRPR) of 1, 2, 3, or 4. In addition, sensitive natural communities are defined as: • Vegetation communities identified as critically imperiled (S1), imperiled (S2), or vulnerable (S3) on the most recent California Sensitive Natural Communities List (CDFW 2019b). To determine the terrestrial and aquatic special-status species that may utilize potential GDE units overlying the Basin, Stillwater ecologists queried existing databases on regional and local occurrences and distributions of special-status species. Databases accessed include the California Natural Diversity Database (CNDDB) (CDFW 2019c), eBird (2019), and TNC freshwater species list (TNC 2019). Spatial database queries were centered on the potential GDEs plus a 1-mile buffer. Stillwater’s ecologists reviewed the database query results and identified special-status species and sensitive natural communities with the potential to occur within or to be associated with the vegetation and aquatic communities in or immediately adjacent to the potential GDEs. The table in Appendix 5B lists these special-status species and sensitive natural communities, describes their habitat preferences and potential dependence on GDEs, and identifies known nearby occurrences (Appendix B - Table 1). Wildlife species were evaluated for potential groundwater dependence using the Critical Species Lookbook (Rohde et al. 2019). The San Luis Obispo Valley Groundwater Basin supports steelhead belonging to the South-Central California Coast Distinct Population Segment (DPS) which is federally listed as “threatened.” Within this DPS, the population of steelhead within the San Luis Creek, and Pismo Creek portions of the groundwater basin have both been identified as Core 1 populations which means they have the highest priority for recovery actions, have a known ability or potential to support viable populations, and have the capacity to respond to recovery actions (NMFS 2013). One critical recovery action listed by the National Marine Fisheries Service (NMFS) includes the management of groundwater extractions for protection and restoration of natural surface flow patterns to ensure surface flows allow for essential steelhead habitat functions (NMFS 2013). Based on criteria promulgated by The Nature Conservancy (TNC), the San Luis Obispo Valley Groundwater Basin was determined to have high ecological value because: (1) the known occurrence and presence of suitable habitat for several special-status species including the Core 1 population status of South-Central California Coast Steelhead DPS and several special-status plants and animals that are directly or indirectly dependent on groundwater (Appendix B - Table 1); and (2) the vulnerability of these species and their habitat to changes in groundwater levels (Rohde et al. 2018). SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 37 5.9 GROUNDWATER QUALITY DISTRIBUTION AND TRENDS Groundwater quality samples have been collected and analyzed throughout the Basin for various studies and programs and are collected on a regular basis for compliance with regulatory programs. Water quality data surveyed for this GSP were collected from: • The California Safe Drinking Water Information System (SDWIS), a repository for public water system water quality data, • The National Water Quality Monitoring Council water quality portal (this includes data from the recently decommissioned EPA STORET database, the USGS, and other federal and state entities [Note: in the Basin the agencies include USGS, California Environmental Data Exchange Network (CEDEN), and Central Coast Ambient Monitoring Program {CCAMP}]), and • The California State Water Resources Control Board (SWRCB) GeoTracker GAMA database. In general, the quality of groundwater in the Basin is good. Water quality trends in the Basin are stable, with no significant trends of ongoing deterioration of water quality based on the Regional Water Quality Control Board’s Basin Objectives, outlined in the Water Quality Control Plan for the Central Coast Basin (Basin Plan, June 2019). The Basin Plan takes all beneficial uses into account and establishes measurable goals to ensure healthy aquatic habitat, sustainable land management, and clean groundwater. The distribution, concentrations, and trends of some of the most commonly cited major water quality constituents are presented in the following sections. 5.9.1 Groundwater Quality Suitability for Drinking Water Groundwater in the Basin is generally suitable for drinking water purposes. Groundwater quality data was evaluated from the SDWIS and GeoTracker GAMA datasets. The data reviewed includes 2,885 sampling events from 403 supply wells and monitoring wells in the Basin, collected between June 1953 and September 2019. Primary drinking water standards Maximum Contaminant Levels (MCLs) and Secondary MCLs (SMCLs) are established by Federal and State agencies. MCLs are legally enforceable standards, while SMCLs are guidelines established for nonhazardous aesthetic considerations such as taste, odor, and color. Primary water quality standard exceedances in the Basin include exceedance of the MCL for nitrate, which equaled or exceeded the standard in 269 samples out of 2,605 samples (or 10% of samples, with 190 of the exceedances occurring in four wells), and exceedance of the MCL for arsenic, which exceeded the MCL in 30 out of 771 samples (or 4% of samples collected). The SMCL for total dissolved solids (TDS) was equaled or exceeded in 126 out of 843 samples (or 15% of total samples). In the case of public water supply systems, these water quality exceedances are effectively mitigated with seasonal well use, treatment, or water blending practices to reduce the constituent concentrations to below their respective water quality standard. In general, these statistics meet the Central Coast Water Board Basin Plan measurable goals that by 2025, 80% of groundwater will be clean, and the remaining 20% will exhibit positive trends in key parameters. 5.9.2 Distribution and Concentrations of Point Sources of Groundwater Constituents Potential point sources of groundwater quality degradation due to release of anthropogenic contaminants were identified using the State Water Resources Control Board (SWRCB) Geotracker website. Waste Discharge permits were also reviewed from on-line regional SWRCB websites. Table 5-1 summarizes information from these websites for open/active sites. Figure 5-16 shows the locations of these open SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 38 groundwater contaminant point source cases, and the locations of completed/case closed sites. Based on available information there are no mapped ground-water contamination plumes at these sites. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 39 Figure 5-16: Location of Potential Point Sources of Groudnwater Conditions SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 40 Table 5-1: Potential Point Sources of Groundwater Contamination Site ID Site Name Case Type Status Constituent(s) of Concern (COCs) Potentially Affected Media T0607900100 American Gas and Tire LUST Cleanup Site Open - Verification Monitoring Benzene, Gasoline, MTBE / TBA / Other Fuel Oxygenates Aquifer used for drinking water supply SL203011375 Chevron (Former UNOCAL) - Tank Farm Road Bulk Storage Cleanup Program Site Open - Remediation Arsenic, Lead, Asphalt, Crude Oil, Other Petroleum Contaminated Surface / Structure, Other Groundwater (uses other than drinking water), Soil, Surface water T10000002287 Conoco Phillips site # 5143 Cleanup Program Site Open - Site Assessment Crude Oil, Diesel, Gasoline Soil SL0607944973 COP Pipeline at San Luis Drive Cleanup Program Site Open - Assessment & Interim Remedial Action Crude Oil Other Groundwater (uses other than drinking water), Well used for drinking water supply T10000001025 KIMBALL MOTORS Cleanup Program Site Open - Verification Monitoring Other Chlorinated Hydrocarbons, Tetrachloroethylene (PCE), Trichloroethylene (TCE), Vinyl chloride Aquifer used for drinking water supply, Soil SLT3S0851312 MODEL INDUSTRIAL SUPPLY Cleanup Program Site Open - Site Assessment Aquifer used for drinking water supply SLT3S0161285 PG&E-FORMER MANUFACTURED GAS PLANT-SAN LUIS OBISPO Cleanup Program Site Open - Remediation Aquifer used for drinking water supply SL0607937854 PISMO ST. AND MORRO ST. PIPELINE RELEASE Cleanup Program Site Open - Site Assessment Crude Oil Aquifer used for drinking water supply T10000012768 SAN LUIS COUNTY RGNL Non-Case Information Pending Review Per- and Polyfluoroalkyl Substances (PFAS) T10000002286 South Higuera St & Pismo St Pipeline (Chevron Site 351317) Cleanup Program Site Open - Site Assessment Crude Oil, Diesel, Gasoline Aquifer used for drinking water supply, Soil T10000010079 Thread Lane Properties, LLC Cleanup Program Site Open - Site Assessment SL0607965995 TRACT 1259 Cleanup Program Site Open - Assessment & Interim Remedial Action Crude Oil Aquifer used for drinking water supply T10000000060 Union Pacific Railroad - Round House/Pond Site Cleanup Program Site Open - Inactive Waste Oil / Motor / Hydraulic / Lubricating Other Groundwater (uses other than drinking water), Soil T10000012125 UPRR Tie Fire Non-Case Information Pending Review T10000010082 Volny Investment Company Cleanup Program Site Open - Site Assessment SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 41 5.9.3 Distribution and Concentrations of Diffuse or Natural Groundwater Constituents The distribution and concentration of several constituents of concern are discussed in the following subsections. Groundwater quality data was evaluated from the SDWIS and GeoTracker GAMA datasets. The data reviewed includes 2,884 sampling events from 403 wells in the Basin, collected between June 1953 and June 2019. Each of the constituents are compared to their drinking water standard, if applicable, or their Basin Plan Median Groundwater Quality Objective (RWQCB Objective) (CCRWQCB, 2017). This GSP focuses only on constituents that might be impacted by groundwater management activities. The constituents discussed below are chosen because they have either a drinking water standard, a known effect on crops, or concentrations have been observed above either the drinking water standard or the level that affects crops. 5.9.3.1 Total Dissolved Solids TDS is defined as the total amount of mobile charged ions, including minerals, salts or metals, dissolved in a given volume of water and is commonly expressed in terms of milligrams per liter (mg/L). Specific ions of salts such as chloride, sulfate, and sodium may be evaluated independently, but all are included in the TDS analysis, so TDS concentrations are correlated to concentrations of these specific ions. Therefore, TDS is selected as a general indicator of groundwater quality in the Basin. TDS is a constituent of concern in groundwater because it has been detected at concentrations greater than its RWQCB Basin Objective of 900 mg/l in the Basin. The TDS Secondary MCL has been established for color, odor and taste, rather than human health effects. This Secondary MCL includes a recommended standard of 500 mg/L, an upper limit of 1,000 mg/L and a short-term limit of 1,500 mg/l. TDS water quality results ranged from 180 to 3,100 mg/l with an average of 727 mg/l and a median of 613 mg/l. The distribution and trends of TDS concentrations in the Basin groundwater are presented on Figure 5-16. TDS concentrations are color coded and represent the average result if multiple samples are documented. Most of the samples with the highest values (dark red in the figure) are outside or on the edge of the Basin. This is consistent with observations that groundwater from the Basin sediments generally has better water quality than groundwater from bedrock wells. Eleven wells with the greatest amount of data over time were selected. Graphs displaying TDS concentration with time are included on Figure 5-17. Most of these graphs do not display any upward trends in TDS concentrations with time. The sustainability projects and management actions implemented as part of this GSP are not anticipated to increase groundwater TDS concentrations in wells that are currently below the SMCL. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 42 Figure 5-17: Distribution of TDS in Basin. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 43 5.9.3.2 Nitrate Nitrate (as Nitrogen) is a widespread contaminant in California groundwater. Although it does occur naturally at low concentrations, high levels of nitrate in groundwater are associated with agricultural activities, septic systems, confined animal facilities, landscape fertilizers and wastewater treatment facilities. Nitrate is the primary form of nitrogen detected in groundwater. It is soluble in water and can easily pass through soil to the groundwater table. Nitrate can persist in groundwater for decades and accumulate to high levels as more nitrogen is applied to the land surface each year. It is a Primary Drinking Water Standard constituent with an MCL of 10 mg/l. Nitrate is a constituent of concern in groundwater because it has been detected at concentrations greater than its RWQCB Basin Objectives of 5 mg/l (as N) in the Basin. The Nitrate MCL has been established at 10 mg/l (as N). Overall, nitrate water quality results ranged from below the detection limit to 80 mg/l (as N) with an average of 3.9 mg/l (as N) and a median value of 2.0 mg/l (as N). Figure 5-18 presents occurrences and trends for nitrate in the Basin groundwater. Wells with the most sampling data over time were selected for presentation. The color-coded symbols represent the average result if multiple samples are documented. Most of the chemographs displayed on Figure 5-18 indicate concentrations of nitrate well below the MCL, and do not indicate trends of increasing concentrations with time. Chemographs labelled number 4 and 5 on Figure 5-18 do appear to indicate a slight upward trend in nitrate (as nitrogen) concentrations over the data period of record. Sustainability projects and management actions implemented as part of this GSP are not anticipated to increase nitrate concentrations in groundwater in a well that would otherwise remain below the MCL to increase above the MCL. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 44 Figure 5-18: Distribution of Nitrate in Basin. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 45 5.9.3.3 Arsenic Arsenic is also a common contaminant in California groundwater. Although it does occur naturally at low concentrations, elevated levels of arsenic in groundwater may be associated with pesticide use, mining activities, and release of industrial effluent. Arsenic has a Primary Drinking Water Standard with an MCL of 10 ug/l. Overall, arsenic concentrations ranged from below the detection limit to 28 ug/l, with an average value of 2.5 ug/l and a median value of 2 ug/l. Figure 5-19 presents occurrences and trends for arsenic in the Basin groundwater from wells with the most arsenic analytical data over time. The color-coded symbols represent the average result if multiple samples are documented. Wells screened in the bedrock aquifers may be expected to have higher natural arsenic concentrations than wells screened in Basin sediments due to increased degrees of mineralization in these waters. Most of the chemographs displayed show stable or decreasing concentrations of arsenic over the data period of record. (Graph number 1 shows a slight increase over time, but is still below the MCL). Sustainability projects and management actions implemented as part of this GSP are not anticipated to directly cause arsenic concentrations in groundwater in a well that would otherwise remain below the MCL to increase above the MCL. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 46 Figure 5-19: Distribution of Arsenic in Basin. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 47 5.9.3.4 Boron Boron is an unregulated constituent and therefore does not have a regulatory standard. However, boron is a constituent of concern because elevated boron concentrations in water can damage crops and affect plant growth. Boron has been detected at concentrations greater than its RWQCB Basin Objective of 200 micrograms per liter (ug/l). Boron water quality results ranged from non-detect to 2,500 ug/l with an average of 0.16 ug/l and a median value of 0.12. Boron concentrations in the Alluvial Aquifer have been relatively consistent throughout the period of record. Boron concentrations in the Paso Robles Formation Aquifer have generally remained steady or declined slightly over the period of record. Sustainability projects and management actions implemented as part of this GSP are not anticipated to directly cause boron concentrations in groundwater in a well to increase. 5.9.3.5 Other Constituents Other constituents found in exceedance of their respective regulatory standard include arsenic, iron, gross alpha, manganese, selenium, and sulfate. Each of these exceedances occurred in samples from a small number of wells, indicating isolated occurrences of these elevated constituent concentrations rather than widespread occurrences, affecting the entire Basin. Isolated concentrations of arsenic, iron, gross alpha, and sulfate in the Basin have been relatively consistent throughout the period of record. Selenium concentrations have generally declined since 2007. There are not enough data to determine the trend of the elevated manganese concentrations in the Basin. Sustainability projects and management actions implemented as part of this GSP are not anticipated to directly cause concentrations of any of these constituents in groundwater to increase. REFERENCES Boyle Engineering. 1991. City of San Luis Obispo Groundwater Basin Evaluation. January. 1991. Carollo. 2012. San Luis Obispo County Master Water Report. 2012. City of San Luis Obispo. 2016. 2015 Urban Water Management Plan. 2016. —. 2018. General Plan. 2018. —. 2015. Water Resources Status Report. 2015. Cleath & Associates, Inc. 2001. Well Construction and Testing Report for Lewis Lane #4, Edna Valley, San Luis Obispo County. Prepared for Southern California Water Company. July. 2001. —. 2003. Well Construction and Testing Report for Water Supply and Irrigation Wells, City of San Luis Obispo, Hayashi Irrigation Wells and Highway 101 Water Supply Well. March. 2003. Cleath-Harris Geologists, Inc. 2010. Edna Valley Water System Groundwater Study. Prepared for Golden State Water Company. May. 2010. —. 2013. Summary of Drilling,. Testing, and Destruction of the Golden State Water Company Country Club Test Well, Edna Road System, 6110 Lewis Lane, San Luis Obispo, California. Prepared for Golden State Water Company. June. 2013. —. 2013. Summary of Exploration and Testing, 5061 Hacienda Avenue, San Luis Obispo, California. Prepared for Golden State Water Company. February. 2013. SLO Basin Groundwater Sustainability Plan Groundwater Conditions (§ 354.16) County of SLO and City of SLO 48 —. 2014. Summary of Exploration and Testing, Blodgett parcel, Whiskey Run Lane, Country Club Area, San Luis Obispo, California. Prepared for Golden State Water Company. July. 2014. County of San Luis Obispo. 2014. San Luis Obispo County Integrated Regional Water Management Plan (IRWMP). 2014. Cuesta Engineering Corporation. 2007. San Luis Obispo Creek Watershed Calibration Study. 2007. Dibble, T.W. 2004. Geologic Map of the Lopez Mountain Quadrangle, San Luis Obispo County, California. s.l. : Dibble Geology Center Map, 2004. #DF-130. Dibblee, T.W. 2006. Geologic Map of the Arroyo Grande NE Quadrangle, San Luis Obispo County, California. s.l. : Dibble Geology Center Map, 2006. #DF-211. —. 2006. Geologic Map of the Pismo Beach Quadrangle, San Luis Obispo County, California. s.l. : Dibblee Geology Center Map, 2006. #DF-212. —. 2004. Geologic Map of the San Luis Obispo Quadrangle, San Luis Obispo County, California. . s.l. : Dibble Geology Center Map, 2004. #DF-129. DWR. 2003. California’s Groundwater: Bulletin 118 – Update 2003, Groundwater Basin Descriptions. 2003. —. 2016. California's Groundwater: Bulletin 118 - Interim Update 2016, Working Towards Sustainability. 2016. —. 2003. California's Groundwater: Bulletin 118 - Update 2003, Groundwater Basin Descriptions. 2003. —. 2014. DWR Atlas - Aglricultural Lang Use and Irrigated Areas. [Online] 2014. gis.water.ca.gov. —. 1964. San Luis Obispo and Santa Barbara Counties Land and Water Use Survey, 1959. . s.l. : California Department of Water Resources (DWR), 1964. —. 1958. San Luis Obispo County Investigation. State Water Resources Board Bulletin No. 18. . s.l. : California Department of Water Resources (DWR). May., 1958. —. 1997. San Luis-Edna Valley Groundwater Basin Study, Draft Report. . s.l. : California Department of Water Resources (DWR)., 1997. —. 2019. Sustainable Groundwater Management Act 2019 Basin Prioritization - Process and Results Document. 2019. ESA Consultants, Inc. 1994. Hydrologic Investigation, Edna Valley Well Location Study. September. 1994. GSI Water Solutions. 2018. San Luis Obispo Valley Basin Characterization and Monitoring Well Installation. 2018. Hall, C.A. 1979. Geologic map of the San Luis Obispo – San Simeon Region, California. s.l. : U.S. Geological Survey, 1979. Map I-1097. —. 1973. Geology of the Arroyo Grande Quadrangle, California. . s.l. : California Division of Mines and Geology, 1973. Map Sheet 24. SLO-FCWCD. 2014. CASGEM Monitoring Plan for High and Medium Priority Groundwater Basins in the San Luis Obispo County Flood Control & Water Conservation District. September. s.l. : San Luis Obispo Flood Control & Water Conservation District, 2014. Stillwater Sciences. 2015. Percolation Zone Study of Pilot-Study Groundwater Basins in San Luis Obispo County, California. September. 2015. TEAM Engineering & Management. 2000. Groundwater Yield Analysis. July. 2000. USBR. 1955. Reconnaissance Report San Luis Obispo County Basin, California. . s.l. : U.S. Bureau of Reclamation, Region 2, Sacramento., 1955. USDA-NRCS. 2007. Soil Survey Geographic Database (SSURGO). s.l. : U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS), 2007. WSC. 2018. Salinas and Whale Rock Reserviors Safe Annual Yield TM. 2018.