The URL can be used to link to this page

Home My WebLink AboutItem 06 - COUNCIL READING FILE_d_Draft GSP Chapters 3-4Draft Groundwater Sustainability Plan Chapter 3 – Description of Plan Area and Chapter 4 - Basin Setting for the San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies Prepared by 12/2/2019 SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO i TABLE OF CONTENTS Table of Contents ........................................................................................................................................... i List of Figures .............................................................................................................................................. vii Tables ......................................................................................................................................................... viii Appendices ................................................................................................................................................... ix List of Terms Used ......................................................................................................................................... x Executive Summary ....................................................................................................................................... 1 3 Description of Plan Area (§ 354.8) ....................................................................................................... 2 3.1 SLO Basin Introduction .................................................................................................................. 2 3.2 Adjudicated Areas ......................................................................................................................... 5 3.3 Jurisdictional Areas ....................................................................................................................... 5 3.3.1 Federal Jurisdictions .............................................................................................................. 5 3.3.2 Tribal Jurisdiction .................................................................................................................. 5 3.3.3 State Jurisdictions ................................................................................................................. 5 3.3.4 County Jurisdictions .............................................................................................................. 5 3.3.5 City and Local Jurisdictions ................................................................................................... 5 3.3.6 Special Districts ..................................................................................................................... 5 3.4 Land Use ........................................................................................................................................ 5 3.4.1 Water Source Types .............................................................................................................. 9 3.4.2 Water Use Sectors ............................................................................................................... 11 3.5 Density of Wells .......................................................................................................................... 13 3.6 Existing Monitoring and Management Programs ....................................................................... 18 3.6.1 Groundwater Monitoring .................................................................................................... 18 3.6.1.1 Groundwater Level Monitoring ................................................................................. 18 3.6.1.2 Groundwater Quality Monitoring .............................................................................. 18 3.6.1.3 Surface Water Monitoring .......................................................................................... 20 3.6.1.4 Climate Monitoring ..................................................................................................... 20 3.6.2 Existing Management Plans ................................................................................................ 24 3.6.2.1 SLO Basin Characterization and Monitoring Well Installation .................................... 24 3.6.2.2 San Luis Obispo County Master Water Report (2012) ................................................ 24 3.6.2.3 San Luis Obispo County Integrated Regional Water Management Plan (2014) ......... 25 3.6.2.4 City of San Luis Obispo 2015 Urban Water Management Plan (2016) ....................... 25 3.6.3 Existing Groundwater Regulatory Programs ........................................................................ 25 3.6.3.1 Groundwater Export Ordinance (2015) ...................................................................... 25 SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO ii 3.6.3.2 Well Ordinances, County and City .............................................................................. 25 3.6.3.3 Countywide Water Conservation Program Resolution 2015-288 (2015) ................... 26 3.6.3.4 Agricultural Order R3-2017-002 (2017) ...................................................................... 26 3.6.3.5 Water Quality Control Plan for the Central Coast Basins (2017) ................................ 26 3.6.3.6 California DWR Well Standards (1991) ....................................................................... 26 3.6.3.7 Requirements for New Wells (2017) ........................................................................... 27 3.6.3.8 Title 22 Drinking Water Program (2018) ..................................................................... 27 3.6.3.9 Waterway Management Plan – San Luis Obispo Creek Watershed (2003) ................ 27 3.6.3.10 Incorporation Into GSP ................................................................................................ 27 3.6.3.11 Limits to Operational Flexibility .................................................................................. 27 3.7 Conjunctive Use Programs .......................................................................................................... 28 3.8 Land Use Plans ............................................................................................................................ 28 3.8.1 City of San Luis Obispo General Plan .................................................................................. 28 3.8.2 County of San Luis Obispo General Plan ............................................................................. 29 3.8.3 Los Ranchos/Edna Village Plan ............................................................................................ 30 3.8.4 Plan Implementation Effects on Existing Land Use ............................................................. 31 3.8.5 Plan Implementation Effects on Water Supply ................................................................... 32 3.8.6 Well Permitting ................................................................................................................... 32 3.8.7 Land Use Plans Outside of Basin ......................................................................................... 32 3.9 Management Areas ..................................................................................................................... 32 3.9.1 Reason for Creation ............................................................................................................ 32 3.10 Additional GSP Elements, if Applicable ....................................................................................... 32 4 Basin Setting (§ 354.14) ..................................................................................................................... 33 4.1 Basin Topography and Boundaries ............................................................................................. 33 4.2 Primary Users of Groundwater ................................................................................................... 40 4.3 Soils Infiltration Potential............................................................................................................ 40 4.4 Regional Geology ........................................................................................................................ 42 4.4.1 Regional Geologic Structures .............................................................................................. 42 4.4.2 Geologic Formations within the Basin ................................................................................ 42 4.4.2.1 Alluvium ...................................................................................................................... 46 4.4.2.2 Paso Robles Formation ............................................................................................... 46 4.4.2.3 Pismo Formation ......................................................................................................... 46 4.4.3 Geologic Formations Surrounding the Basin ...................................................................... 47 4.4.3.1 Monterey Formation ................................................................................................... 47 4.4.3.2 Obispo Formation ....................................................................................................... 47 SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO iii 4.4.3.3 Franciscan Assemblage ............................................................................................... 47 4.5 Principal Aquifers and Aquitards ................................................................................................ 48 4.5.1 Cross Sections ..................................................................................................................... 48 4.5.2 Aquifer Characteristics ........................................................................................................ 65 4.5.3 Aquitards ............................................................................................................................. 70 4.6 Surface Water Bodies .................................................................................................................. 70 4.7 Subsidence Potential ................................................................................................................... 71 5 Groundwater Conditions (§ 354.16) .................................................................................................. 5.1 Groundwater Elevations and Interpretation .............................................................................. 5.1.1 Groundwater Levels ............................................................................................................ 5.1.2 Groundwater Level Trends .................................................................................................. 5.1.3 Vertical Groundwater Gradients ......................................................................................... 5.1.4 Groundwater Contours ....................................................................................................... 5.2 Change in Storage ....................................................................................................................... 5.3 Seawater Intrusion ...................................................................................................................... 5.4 Groundwater Quality Distribution and Trends ........................................................................... 5.4.1 Anthropogenic Constituents: Diffuse Sources .................................................................... 5.4.2 Anthropogenic Constituents: Point Sources ....................................................................... 5.4.3 Natural Constituents: Diffuse Sources ................................................................................ 5.4.4 Natural Constituents: Point Sources ................................................................................... 5.5 Subsidence .................................................................................................................................. 5.6 Interconnected Surface Water .................................................................................................... 5.6.1 Streams and Lakes .............................................................................................................. 5.6.2 Groundwater-Dependent Ecosystems ................................................................................ 6 Water Budget (§ 354.18) .................................................................................................................... 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 ................................................................................................................. SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO iv 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 .......................................................................................................... 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 .......................................................................................................... SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO v 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 ....................................................................... 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 ........................................................................................................ SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO vi 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 ......................................................................................................................................... SLO Basin Groundwater Sustainability Plan List of Figures County of SLO and City of SLO vii LIST OF FIGURES Figure 1-1: San Luis Obispo Valley Basin and Surround ................................................................................ 1 Figure 3-1: San Luis Obispo Historical Annual Precipitation. ........................................................................ 4 Figure 3-2: San Luis Obispo Valley Basin Existing Land Use Designations. ................................................... 8 Figure 3-3: San Luis Obispo Valley Basin Water Supply Sources. ............................................................... 10 Figure 3-4: San Luis Obispo Valley Basin Water Use Sectors. ..................................................................... 12 Figure 3-5: San Luis Obispo Valley Basin Domestic Well Density. .............................................................. 14 Figure 3-6: San Luis Obispo Valley Basin Production Well Density. ............................................................ 15 Figure 3-7: San Luis Obispo Valley Basin Public Supply Well Density. ........................................................ 16 Figure 3-8: San Luis Obispo Valley Basin Public Supply Well Density. ........................................................ 17 Figure 3-9: Monitored Wells in the San Luis Obispo Valley Basin .............................................................. 19 Figure 3-10: San Luis Obispo Valley Basin Surface Water Features, Weather Stations, and Stream Gauges. .................................................................................................................................................................... 22 Figure 3-11: San Luis Obispo Valley Basin Historical Annual Precipitation and CDFM. .............................. 23 Figure 3-12. City Land Use Map .................................................................................................................. 28 Figure 3-13. County Land Use Map (San Luis Obispo Planning Area) ......................................................... 29 Figure 3-14. County Land Use Map (South County Planning Area) ............................................................ 30 Figure 3-15. Los Ranchos/Edna Land Use Map ........................................................................................... 31 Figure 4-1: Topographic map. ..................................................................................................................... 35 Figure 4-2: Aerial Photograph. .................................................................................................................... 36 Figure 4-3: Annual Precipitation. ................................................................................................................ 37 Figure 4-4: Bottom Elevation of Basin. ....................................................................................................... 38 Figure 4-5: Thickness of Basin Sediments. .................................................................................................. 39 Figure 4-6: Soil Hydrologic Groups.............................................................................................................. 41 Figure 4-7: Stratigraphic Column. ............................................................................................................... 43 Figure 4-8: Geologic Map. ........................................................................................................................... 44 Figure 4-9: Lithologic Data Points and Cross Section Lines......................................................................... 51 Figure 4-10: Cross Section A1-A2. ............................................................................................................... 52 Figure 4-11: Cross Section A2-A3. ............................................................................................................... 53 Figure 4-12: Cross Section A3-A4. ............................................................................................................... 54 Figure 4-13: Cross Section B-B’. .................................................................................................................. 55 Figure 4-14: Cross Section C1-C1’. .............................................................................................................. 56 Figure 4-15: Cross Section C2-C2’. .............................................................................................................. 57 Figure 4-16: Cross Section D-D’. .................................................................................................................. 58 Figure 4-17: Cross Section E-E’. ................................................................................................................... 59 Figure 4-18: Cross Section F-F’. ................................................................................................................... 60 Figure 4-19: Cross Section G-G’. ................................................................................................................. 61 Figure 4-20: Cross Section H-H’................................................................................................................... 62 Figure 4-21: Cross Section I-I’. .................................................................................................................... 63 Figure 4-22: Hydraulic Parameter Data Locations. ..................................................................................... 66 Figure 4-23: Subsidence Potential. ............................................................................................................. 74 SLO Basin Groundwater Sustainability Plan Tables County of SLO and City of SLO viii TABLES Table 3-1: Land use categories defined for the SLO Basin by DWR (2014). ................................................. 7 Table 3-2: Summary of surface water supply sources available to the SLO Basin........................................ 9 Table 3-3: DWR and County Wells .............................................................................................................. 13 Table 3-4: Stream gauges and summary of records available. ................................................................... 20 Table 3-5: Weather station Information and summary of records available. ............................................ 21 Table 3-6. Average Monthly Climate Summary 1987 – 2018 at Cal Poly Weather Station 52. .................. 24 Table 3-7. Los Ranchos/Edna Land Use Acreage ........................................................................................ 31 Table 4-1: SLO Basin Well Aquifer Test Data Summary. ............................................................................. 68 Table 4-2: SLO Basin Well Specific Capacity Data Summary. ...................................................................... 69 SLO Basin Groundwater Sustainability Plan Appendices County of SLO and City of SLO ix APPENDICES SLO Basin Groundwater Sustainability Plan List of Terms Used County of SLO and City of SLO x 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 xi 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. Figure 1-1: San Luis Obispo Valley Basin and Surround SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 2 3 DESCRIPTION OF PLAN AREA (§ 354.8) 3.1 SLO BASIN INTRODUCTION The SLO Basin is oriented in a northwest-southeast direction and is composed of unconsolidated or loosely consolidated sedimentary deposits. It is approximately 14 miles long and 1.5 miles wide. It covers a surface area of about 12,700 acres (19.9 square miles). The SLO Basin is bounded on the northeast by the relatively impermeable bedrock formations of the Santa Lucia Range, and on the southwest by the formations of the San Luis Range and the Edna fault system. The bottom of the SLO Basin is defined by the contact of permeable sediments with the impermeable bedrock Miocene-aged and Franciscan Assemblage rocks. The SLO Basin is commonly referenced as being composed of two distinct valleys, with the San Luis Valley in the northwest and the Edna Valley in the southeast. The San Luis Valley comprises approximately the northwestern half of the SLO Basin. It is the area of the SLO Basin drained by San Luis Obispo Creek and its tributaries (Prefumo Creek and Stenner Creek west of Highway 101, Davenport Creek and smaller tributaries east of Highway 101). Surface drainage in San Luis Valley drains out of the SLO Basin, flowing to the south along the course of San Luis Obispo Creek, toward the coast in the Avila Beach area, approximately along the course of Highway 101. The San Luis Valley includes part of the City and California Polytechnic State University (Cal Poly) jurisdictional boundaries, while the remainder of the San Luis Valley is unincorporated land. Land use in the City is primarily single- and multi-family residential, commercial, industrial, and a small amount of land in agricultural uses. The area in the northwest part of the SLO Basin, along Los Osos Valley Road, has significant areas of irrigated agriculture, primarily row crops. The Edna Valley comprises approximately the southeastern half of the SLO Basin. The primary creeks that drain the SLO Basin are the east and west branches of Corral de Piedras Creek, which join to form Pismo Creek, draining south out of the Edna Valley into Price Canyon. Smaller unnamed tributaries drain south from the SLO Basin in the extreme southeastern part of Edna Valley, ultimately joining Pismo Creek. Some of the unincorporated lands in Edna Valley are served by various private water purveyors. The primary land use in the Edna Valley is agriculture. During the past two decades wine grapes have become the most significant crop type in the Edna Valley. The physical definition of the SLO Basin boundary is the contact between the unconsolidated or loosely consolidated sediments and the basement rock of the Miocene-aged formations and Franciscan Assemblage. There is a topographic high point in the underlying bedrock elevation between the San Luis and Edna sub-basins. The watershed divide and the bedrock divide are not coincident. The sediments of the Edna Valley have significantly greater thickness than those of the San Luis Valley. Precipitation that falls west of that divide ultimately flows to Davenport and San Luis Obispo Creeks, and precipitation that falls east of that divide flows to Corral de Piedras Creek or the other small tributaries, ultimately flowing to Pismo Creek south of the SLO Basin. The primary weather patterns for the SLO Basin derive from seasonal patterns of atmospheric conditions that originate over the Pacific Ocean and move inland. As storm fronts move in from the coast, rainfall in the area falls more heavily in the mountains, and the SLO Basin itself receives less rainfall because of a muted rain shadow effect. Average annual precipitation ranges from approximately 18 inches throughout most of the SLO Basin to about 22 inches in relatively higher elevation areas near the City and Cal Poly. Figure 3-1 presents the time series of annual precipitation for the period of record from 1870 to 2018 at the Cal Poly weather station No. 52. The average historical rainfall at this location to date is 21.69 inches, with a SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 3 standard deviation of 8.75 inches. The historical maximum is 49.99 inches, which occurred in 1884. The historical minimum is 4.56 inches, which occurred in 2013. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 4 Figure 3-1: San Luis Obispo Historical Annual Precipitation. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 5 3.2 ADJUDICATED AREAS The SLO Basin is not an adjudicated basin. 3.3 JURISDICTIONAL AREAS In addition to MOA Parties, there are several entities that have some degree of water management authority in the SLO Basin. Each entity is discussed below. 3.3.1 Federal Jurisdictions There are no federal agencies with land holdings in the SLO Basin. 3.3.2 Tribal Jurisdiction The two prominent Native American tribes in the County are the Obispeño Chumash and Salinan Indian Tribes. The Chumash occupied the coast between San Luis Obispo and northwestern Los Angeles County, inland to the San Joaquin Valley. They were divided into two broad groups, of which the Obispeño were the northern group. The Salinan were northern neighbors of the Chumash, and although the presence of a firm boundary between the Chumash and the Salinan is uncertain, ethnographic accounts have placed Salinan territories in the northern portion of the County. However, these two tribes do not have any recognized tribal land in the SLO Basin. 3.3.3 State Jurisdictions The State of California University system owns and operates land that is associated with California Polytechnic State University, San Luis Obispo (Cal Poly) located in the northern edge of the SLO Basin off Hwy 1. Cal Poly is a significant user of local water sources and manages their water supply in conjunction with the City. The City treats the wastewater generated from Cal Poly. There are no California State Parks or other State-owned lands or entities located within the SLO Basin. 3.3.4 County Jurisdictions The County of San Luis Obispo and the associated San Luis Obispo County Flood Control and Water Conservation District (SLOFCWCD) (see section under Special Districts below) have jurisdiction over the entire County including the SLO Basin. The County owns approximately 300 acres of land in the SLO Basin and is primarily located in the vicinity of the SLO County Airport which makes up the majority of the land owned by the County. 3.3.5 City and Local Jurisdictions The City is centrally located in the SLO Basin and has land and water management authority over its incorporated area. The City has four primary water supply sources including Whale Rock Reservoir, Salinas Reservoir, Nacimiento Reservoir, and recycled water (for irrigation), with groundwater serving as a fifth supplemental source. Three major mutual water companies exist in the SLO Basin: Edna Valley Growers, Varian Ranch, and Edna Ranch Mutual Water Companies. One investor owned utility exists within the SLO Basin: Golden State Water Company (GSWC). GSWC provides groundwater that is pumped from the Edna Valley Basin to residential and agriculture customers. 3.3.6 Special Districts The San Luis Obispo Flood Control and Water Conservation District (SLOFCWCD) is a dependent Special District governed by the County Board of Supervisors. It has jurisdiction over all of the County including the SLO Basin and was established as a resource to help individuals and communities in San Luis Obispo County identify and address flooding problems with the purpose "to provide for control, disposition and distribution of the flood and storm waters of the district and of streams flowing into the district...". SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 6 3.4 LAND USE The County, City and State have land use authority in the SLO basin within their respective jurisdictions. Land use information for the SLO Basin was based on DWR’s land use database (DWR, 2014). The 2014 land use in the SLO Basin is shown on Figure 3-2 and is summarized by group in . All land use categories except native vegetation listed in Table 3-1 are provided by DWR (DWR, 2014). The areas of the basin that did not have a land use designation were assumed to be native vegetation. Table 3-1: Agricultural Land use categories defined for the SLO Basin by DWR (2014). Land Use Category Acres Citrus and subtropical 136 Deciduous fruits and nuts 21 Grain and hay crops 183 Idle 713 Pasture 179 Truck nursery and berry crops 1079 Urban 6,412 Vineyard 1,929 Young perennial 2 Native vegetation <1 Total 10,656 SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 7 Figure 3-2: San Luis Obispo Valley Basin Existing Land Use Designations. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 8 3.4.1 Water Source Types Entities in the SLO Basin utilize three types of water sources to meet the demands: groundwater, surface water, and recycled water. Excluding the City, Cal Poly, and Edna Valley Golf Course, all water demand in the SLO Basin are met with groundwater. Cal Poly has rights to 33.71% of water from Whale Rock Reservoir and the rest of their water supply comes from local groundwater. The City has an entitlement to water from the Nacimiento Water Project, rights to Salinas Reservoir (Santa Margarita Lake), rights to 55.05% of water in Whale Rock Reservoir, SLO Basin groundwater, and recycled water from the City’s Water Resource Recovery Facility (WRRF). The City has imported supplies from Salinas Reservoir, located near the community of Santa Margarita, since 1944, Whale Rock Reservoir, located near the community of Cayucos, since 1961, and Lake Nacimiento since 2011. Table 3-2 summarizes the surface water supply available from each source and Table 3-3 shows the location of water supply source types within the SLO Basin. Table 3-2: Summary of surface water supply sources available to the SLO Basin. Supply Sources Amount Available (AFY) Nacimiento Reservoir- City 5,4821 Salinas Reservoir - City 4,9101 Whale Rock Reservoir - City Recycled Water - City ~1,0001 Total 11,392 1 City of San Luis Obispo, General Plan, Water and Wastewater Management Element, 2018.. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 9 Figure 3-3: San Luis Obispo Valley Basin Water Supply Sources. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 10 3.4.2 Water Use Sectors Water demand in the SLO Basin is organized into the six water use sectors identified in the GSP Emergency Regulations. These include: • Urban- Urban water use is assigned to non-agricultural water uses in the City and census- designated places. Domestic use outside of census-designated places is not considered urban use. • Industrial- There is limited industrial use in the SLO Basin. The DWR land use designations in the SLO Basin does not include industrial uses. • Agricultural- This is the largest groundwater use sector in the SLO Basin by water demand. • Managed wetlands- There are several managed wetlands in the SLO Basin that are managed by both federal, state, and local agencies. In general, wetlands in the area are managed by either of the following agencies: (1) City of San Luis Obispo, (2) California Department of Fish and Wildlife, (3) California State Water Resources Control Board, (4) U.S. Fish and Wildlife Service, and (5) U.S. Army Corps of Engineers. The wetlands and natural vegetation areas that are potentially dependent ecosystems include Laguna Lake and reaches of the San Luis Obispo Creek, Prefumo Creek, Stenner Creek, Davenport Creek, East and West Corral De Piedra Creeks, and Pismo Creek. Water use for these ecologically sensitive areas will be addressed in the water budget and modeling scope of this GSP in order to designate appropriate management actions and proposed projects to provide adequate water supply for natural water use of these areas. • Managed recharge- There is no managed recharge in the SLO Basin. Recycled water discharge to creeks and applied irrigation is included in the urban water use sector. • Native vegetation- This is the largest water use sector in the SLO Basin by land area. This sector includes rural residential areas. Figure 3-4 shows the distribution of the water use sectors and potential groundwater dependent ecosystems in the SLO Basin. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 11 Figure 3-4: San Luis Obispo Valley Basin Water Use Sectors. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 12 3.5 DENSITY OF WELLS Well types, well depth data, and well distribution data were downloaded from DWR’s well completion report map application (DWR, 2018). DWR categorizes wells in this mapping application as either domestic, production, or public supply. These categories are based on the well use information submitted with the well logs to DWR. Well information was also collected from County of San Luis Obispo Environmental Health Services (EHS). The EHS dataset was compiled from information gained from the well construction permit application process. Table 3-3 summarizes the types of wells by use for all well logs submitted to DWR and EHS. Table 3-3: DWR and County Wells Well Data Source Type of Well Total No. of Wells DWR Domestic 75 Production 71 Public Supply 24 Total 170 County EHS Domestic Private 355 Domestic Public 43 Irrigation 231 Total 629 Figure 3-5, Figure 3-6, and Figure 3-7 show the density of wells in the SLO Basin by their types of use. The DWR data used to develop these maps is not necessarily the same set of well data held EHS as shown in Figure 3-8. DWR data was used to develop maps of well densities because they are organized for easy mapping of well density per square mile. These maps should be considered representative of well distributions, but are not definitive. It is also important to note that both the DWR and EHS well databases are not updated with information regarding well status and the well locations are not verified in the field. Therefore, it is uncertain whether the wells in these databases are currently active or have been abandoned or destroyed. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 13 Figure 3-5: San Luis Obispo Valley Basin Domestic Well Density. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 14 Figure 3-6: San Luis Obispo Valley Basin Production Well Density. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 15 Figure 3-7: San Luis Obispo Valley Basin Public Supply Well Density. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 16 Figure 3-8: San Luis Obispo Valley Basin Public Supply Well Density. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 17 3.6 EXISTING MONITORING AND MANAGEMENT PROGRAMS 3.6.1 Groundwater Monitoring Groundwater levels and quality are currently measured in the SLO Basin by the SLOFCWCD and a variety of other agencies as described below. Figure 3-9 shows the locations of monitored wells identified in the Groundwater Ambient Monitoring and Assessment (GAMA) program (i.e. publicly available data) that are monitored by several public agencies, the SLOFCWCD, and the Central Coast Regional Water Quality Control Board (CCRWQCB) Irrigated Lands Program. The monitoring network also includes other wells in the area designated as private that are not shown on this map (Figure 3-8). Additional evaluation of the current monitoring program will be conducted for the GSP to establish a representative monitoring network of public and private wells that will be used during plan implementation to track groundwater elevations and ensure that minimum thresholds have not been exceeded. 3.6.1.1 Groundwater Level Monitoring The SLOFCWCD has been monitoring groundwater levels county-wide on a semi-annual basis for more than 50 years to support general planning and for engineering purposes. Groundwater level measurements are taken once in the spring and once in the fall. The monitoring takes place from a voluntary network of wells. In the SLO Basin, there are 16 active wells in this program (Figure 3-9). The voluntary monitoring network has changed over time as access to wells has been lost or new wells have been added to the network. 3.6.1.2 Groundwater Quality Monitoring Groundwater quality is monitored/reported under several different programs and by different agencies including: • Municipal and community water purveyors must collect water quality samples on a routine basis for compliance monitoring and reporting to the California State Water Resources Control Board (SWRCB) Division of Drinking Water (DDW). • The USGS collects water quality data on a routine basis under the GAMA program. These data are stored in the State’s Geotracker GAMA system. • There are multiple sites that are monitoring groundwater quality as part of investigation or compliance monitoring programs through the CCRWQCB. See Figure 3-9 for CCRWQCB well monitoring locations through the GEotracker GAMA system. • The CCRWQCB under Agricultural Order No. R3-2017-0002, a Conditional Waiver of Waste Discharge Requirements for Discharges from Irrigated Lands, requires all growers to implement groundwater monitoring, either individually or as part of a cooperative regional monitoring program. Growers electing to implement individual monitoring (i.e., not participating in the regional monitoring program implemented by the Central Coast Groundwater Coalition [CCGC] within the SLO Basin) are required to test all on-farm domestic wells and the primary irrigation supply wells for nitrate or nitrate plus nitrite, and general minerals (including, but not limited to, TDS, sodium, chloride, and sulfate). • California Water Data Library contains groundwater level and water quality monitoring stations. The data available from this resource has been used above. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 18 Figure 3-9: Monitored Wells in the San Luis Obispo Valley Basin SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 19 3.6.1.3 Surface Water Monitoring The Water Resources Division of the SLO County Public Works maintains six (6) real-time data monitoring stream gauges within the San Luis Obispo Creek watershed and all except Andrews St. Bridge is located within the SLO Basin. As summarized in Table 3-4, each stream gauge measures stage at 15-minute intervals. Stage-discharge relationships, or rating curves, for each of the five stream gauge stations were generated by Questa Engineering Corps in 2007 as part of the San Luis Obispo Creek Watershed Hydrology and Hydraulic Model Calibration Study. More recently (2018/2019), Central Coast Salmon Enhancement has approximated rating curves for the Andrews St., Elks Lane, and Stenner Creek gauge stations based on recorded stage data and measured flows. The location of the five County gauges are presented in Figure 3-10. In addition to the County gauges, the City of San Luis Obispo routinely estimates flow at four locations (RW- 4, RW-5, RW-7, RW-8) along San Luis Obispo Creek in the vicinity of the City’s WRRF outfall as part of its National Pollutant Discharge Elimination System permitting program. RW-8 at South Higuera Bridge is located outside of the SLO Basin. Flow at the four locations (RW-4, RW-5, RW-7, and RW-8) is calculated weekly from April through the end of October based on the depth measurements recorded along the creek cross-section and are located within the Basin. Table 3-4: Stream gauges and summary of records available. Stream Gage Source Data Recorded Data Interval Year Data Begins Datum1 Andrews St Bridge SLO County Stage 15 Minutes 2006 NAVD 88 Stenner Creek at Nipomo SLO County Stage 15 Minutes 2005 NAVD 88 Elks Ln SLO County Stage 15 Minutes 2005 NAVD 88 Madonna Rd SLO County Stage 15 Minutes 2005 NAVD 88 E. Fork at Jespersen Rd SLO County Stage 15 Minutes 2005 NAVD 88 Marsh Street Bridge SLO County Stage 15 Minutes 2019 NAVD 88 RW-4 City of SLO Depth, Flow Weekly 2005 - RW-5 City of SLO Depth, Flow Weekly 2005 - RW-7 City of SLO Depth, Flow Weekly 2005 - RW-8 City of SLO Depth, Flow Weekly 2005 - 1Prior to 5/23/2017 County data was recorded on NGVD 29 datum. Conversion is 2.86 feet. 3.6.1.4 Climate Monitoring Climate monitoring in the SLO Basin includes stations that collect data related to temperature, evapotranspiration, relative humidity, atmospheric pressure, precipitation, and other climate parameters. Four stations monitored by San Luis Obispo County Public Works collect one or more climate parameters in the SLO Basin. The locations of these stations are shown on Figure 3-10. The National Climatic Data Center has three stations within the County of San Luis Obispo and one station within the SLO Basin that collect climate data. These stations do not have extensive historic data. The station with the most precipitation data not associated with the National Climatic Data Center, Cal Poly Weather Station 52 (CPWS-52), began recording data in 1870. The Cal Poly Weather Station 52 measures daily temperatures and other climate parameters in addition to precipitation. Daily records are available from April 1986 to present. Table 3-5 lists the climate stations and summary of records available. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 20 The long-term precipitation and cumulative departure from the mean (CDFM) measurements at CPWS-52 are shown in Figure 3-11 from 1870 - 2018. Average annual precipitation at this station varies from approximately 7 to 55 inches with a mean annual average precipitation of 21.95 inches. The longest dry period on record occurred from 1943 – 1965 and the longest wet period on record occurred from 1899 – 1916. Table 3-6 provides a summary of average monthly rainfall, temperature, and evapotranspiration (ET0) for the SLO Basin from CPWS-52. Table 3-5: Weather station Information and summary of records available. Station Source Data Recorded Data Interval Year Data Begins Cal Poly Weather Station 52 CIMIS Precipitation, Temperature, Evapotranspiration Daily 1986 SLO Reservoir SLO County Precipitation 12-Hour 2005 The Gas Company SLO County Precipitation 12-Hour 2005 South Portal SLO County Precipitation 12-Hour 2005 SLO County Farm Bureau Weather Element Precipitation, Temperature Daily 2015 SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 21 Figure 3-10: San Luis Obispo Valley Basin Surface Water Features, Weather Stations, and Stream Gauges. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 22 Figure 3-11: San Luis Obispo Valley Basin Historical Annual Precipitation and CDFM. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 23 Table 3-6. Average Monthly Climate Summary 1987 – 2018 at Cal Poly Weather Station 52. Month Average Precipitation (inches)a Average ET0 (inches)a Average Temperature (°F)a January 0.14 0.07 54 February 0.15 0.09 54 March 0.11 0.12 56 April 0.03 0.16 57 May 0.02 0.18 59 June 0.01 0.2 62 July 0 0.2 64 August 0.13 0.19 64 September 0.2 0.16 64 October 0.04 0.13 63 November 0.05 0.09 58 December 0.11 0.07 53 Monthly Average 0.08 0.14 59 Average Calendar Year 21.69b 0.14 59 Notes: a Average of monthly data at Cal Poly SLO Weather Station 52 1987 – 2018. b Average Calendar Year is not the sum of the monthly average, but rather a historical annual average over the period of record from 1871 – 2018. 3.6.2 Existing Management Plans There are numerous groundwater and water management plans and study reports that cover either the whole or portion of the SLO Basin. These plans and reports are described in the following subsections, along with brief descriptions of how they relate to the management of current water supply, projected water supplies, and land use. 3.6.2.1 SLO Basin Characterization and Monitoring Well Installation The SLO Basin Characterization and Monitoring Well Installation documents the available published reports, private well reports, well completion reports, geologic logs, and other data that were reviewed to generate a comprehensive compilation of the current understanding of the hydrogeologic setting of the SLO Basin. This information is intended to provide the basis of knowledge for future planning and management activities performed under the requirements of GMA, including the development of a hydrogeologic conceptual model, construction of a numerical groundwater model, and development of a GSP. 3.6.2.2 San Luis Obispo County Master Water Report (2012) The County’s Master Water Report (MWR) is a compilation of the current and future water resource management activities being undertaken by various entities within the County and is organized by Water Planning Areas (WPA). The MWR explores how these activities interrelate, analyzes current and future supplies and demands, identifies future water management strategies and ways to optimize existing strategies, and documents the role of the MWR in supporting other water resource planning efforts. The MWR evaluates and compares the available water supplies to the water demands for the different water planning areas. This was accomplished by reviewing or developing the following: • Current water supplies and demands based on available information SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 24 • Forecast water demands and water supplies available in the future under current land use policies and designations • Criteria under which there is a shortfall when looking at supplies versus demands • Criteria for analyzing potential water resource management strategies, projects, programs, or policies • Potential water resource management strategies, projects, programs, or policies to resolve potential supply deficiencies 3.6.2.3 San Luis Obispo County Integrated Regional Water Management Plan (2014) The San Luis Obispo County Integrated Regional Water Management Plan (IRWMP) was initially developed and adopted by the SLOFCWCD in 2005 (GEI Consultants, 2005), and has been updated several times. The SLOFCWCD, in cooperation with the SLOFCWCD’s Water Resources Advisory Committee (WRAC), prepared the 2014 IRWMP (San Luis Obispo County, 2014) to align the region’s water resources management planning efforts with the State’s planning efforts. The IRWMP is used to support the region’s water resource management planning and submittal of grant applications to fund these efforts. The IRWMP includes goals and objectives that provide the basis for decision-making and are used to evaluate project benefits. The goals and objectives reflect input from interested stakeholders on the region’s major water resources issues. These goals and objectives help secure and enhance the water supply reliability, water quality, ecosystems, groundwater, flood management and water-related communication efforts across the entire region. In addition, the IRWMP identifies resource management strategies, recognizes other funding opportunities and includes a list of action items (projects, programs, and studies) that agencies around the region are undertaking to achieve and further these goals and objectives. The IRWMP is currently being updated with a DWR submittal target date of December 2019 and adoption by local agencies scheduled for Summer 2020. 3.6.2.4 City of San Luis Obispo 2015 Urban Water Management Plan (2016) The City’s Urban Water Management Plan (UWMP) describes the City’s current and future water demands, identifies current water supply sources, and assesses supply reliability for the City. The UWMP describes the City’s use of groundwater and its support for efforts to avoid overdraft by developing additional sources. The UWMP provides a forecast of future growth, water demand, and water sources for the City through 2035. These sources include water conservation, Nacimiento Water Project, Salinas Reservoir (Santa Margarita Lake), Whale Rock Reservoir, SLO Basin groundwater, and recycled water from the WRRF. The UWMP identifies beneficial impacts to groundwater quality through the use of these sources. 3.6.3 Existing Groundwater Regulatory Programs 3.6.3.1 Groundwater Export Ordinance (2015) In 2015, County of San Luis Obispo adopted an Exportation of Groundwater ordinance (County Code Chapter 8.95) that requires a permit for the export of groundwater out of a groundwater basin or out of the County. An export permit is only approved if the Department of Public Works Director or his/her designee finds that moving the water would not have any adverse impacts to groundwater resources, such as causing aquifer levels to drop, disrupting the flow of neighboring wells, or resulting in seawater intrusion. Export permits are only valid for one year. 3.6.3.2 Countywide Water Conservation Program Resolution 2015-288 (2015) The ordinance also identified areas of severe decline in groundwater elevation and properties overlying these areas would be further restricted from planting new or expanding irrigated agriculture except for those converting irrigated agriculture on the same property into a different crop type. This resolution applies to the Nipomo Mesa Water Conservation Area which is part of the Santa Maria Groundwater Basin, SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 25 the Los Osos Groundwater Basin, and the Paso Robles Groundwater Basin. Therefore, it is not applicable to the SLO Basin. 3.6.3.3 Agricultural Order R3-2017-002 (2017) In 2017 the CCRWQCB issued Agricultural Order No. R3-2017-0002, a Conditional Waiver of Waste Discharge Requirements for Discharges from Irrigated Lands. The permit requires that growers implement practices to reduce nitrate leaching into groundwater and improve surface receiving water quality. Specific requirements for individual growers are structured into three tiers based on the relative risk their operations pose to water quality. Growers must enroll, pay fees, and meet various monitoring and reporting requirements according to the tier to which they are assigned. All growers are required to implement groundwater monitoring, either individually or as part of a cooperative regional monitoring program. Growers electing to implement individual monitoring (i.e., not participating in the regional monitoring program implanted by the Central Coast Groundwater Coalition [CCGC]) are required to test all on-farm domestic wells and the primary irrigation supply wells for nitrate or nitrate plus nitrite, and general minerals (including, but not limited to, TDS, sodium, chloride, and sulfate). 3.6.3.4 Water Quality Control Plan for the Central Coast Basins (2017) The Water Quality Control Plan for the Central Coastal Basin (Basin Plan) was most recently updated in September 2017 by the SWRCB. The objective of the Basin Plan is to outline how the quality of the surface water and groundwater in the Central Coast Region should be managed to provide the highest water quality reasonably possible. The Basin Plan lists beneficial users, describes the water quality that must be maintained to allow those uses, provides an implementation plan, details SWRCB and CCRWQCB plans and policies to protect water quality, and a statewide surveillance and monitoring program as well as regional surveillance and monitoring programs. Present and potential future beneficial uses for inland waters in the SLO Basin are: surface water and groundwater as municipal supply (water for community, military or individual water supplies); agricultural; groundwater recharge; recreational water contact and non-contact; sport fishing; warm fresh water habitat; wildlife habitat; rare threatened or endangered species; and spawning, reproduction, and/or early development of fish. Water Quality Objectives for both groundwater (drinking water and irrigation) and surface water are provided in the Basin Plan. 3.6.3.5 California DWR Well Standards (1991) Under the CWC Sections 13700 to 13806, DWR has the responsibility for developing well standards. DWR maintains these standards to protect groundwater quality. California Well Standards, published as DWR Bulletin 74, represent minimum standards for well construction, alteration, and destruction to protect groundwater. Cities, counties, and water agencies in California have regulatory authority over wells and can adopt local well ordinances that meet or exceed the statewide Well Standards. When a well is constructed, modified or destroyed a well completion report is required to be submitted to DWR. 3.6.3.6 Requirements for New Wells (2017) Senate Bill 252 effective on January 1, 2018. SB 252 requires well permit applicants in critically overdrafted basins to include information about the proposed well, such as location, depth, and pumping capacity. The bill also requires the permitting agency to make the information easily accessible to the public and the GSA. As of 2019, these requirements are under review by DWR. This bill is not applicable because the SLO Basin is not a critically overdrafted basin. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 26 3.6.3.7 Title 22 Drinking Water Program (2018) The 2018 SWRCB DDW regulates public water systems in the State to ensure the delivery of safe drinking water to the public. A public water system is defined as a system for the provision of water for human consumption through pipes or other constructed conveyances that has 15 or more service connections or regularly serves at least 25 individuals daily at least 60 days out of the year. Private domestic wells, wells associated with drinking water systems with less than 15 residential service connections, and industrial and irrigation wells are not regulated by the DDW. Additional information regarding the public water systems can be found using the following link: https://sdwis.waterboards.ca.gov/PDWW/JSP/WaterSystems.jsp?PointOfContactType=none&number=&na me=&county=San%20Luis%20Obispo The SWRCB DDW enforces the monitoring requirements established in Title 22 of CCR for public water system wells, and all the data collected must be reported to the DDW. Title 22 also designates the regulatory limits (e.g., maximum contaminant levels [MCLs]) for various waterborne contaminants, including volatile organic compounds, non-volatile synthetic organic compounds, inorganic chemicals, radionuclides, disinfection byproducts, general physical constituents, and other parameters. 3.6.3.8 Waterway Management Plan – San Luis Obispo Creek Watershed (2003) The San Luis Obispo Creek Watershed Waterway Management Plan was created in response to several damaging floods that occurred in 1969, 1973, and 1995 that caused widespread damage throughout the watershed that includes out-of-bank flooding and extensive bank erosion. This plan identifies management problems and needs of the waterways, detailed hydrologic analyses of the watershed and its main tributaries. The plan also presents a Stream Management and Maintenance Program for the waterways of the watershed that outlines the planning, design, and permitting required to fully implement the program and a Drainage Design Manual that contains revised policies for floodplain and stream corridor management and redesigned flows for stream channels within the City boundary. 3.6.3.9 Incorporation Into GSP Information in these various plans mentioned above has been incorporated into this GSP for consideration in the development of Sustainability Goals, when setting Minimum Thresholds and Measurable Objectives, and was considered during development of Projects and Management Actions to provide consistency among the above listed plans to achieve groundwater sustainability in the SLO Basin. 3.6.3.10 Limits to Operational Flexibility Some of the existing management plans and ordinances will limit operational flexibility. These limits to operational flexibility have already been incorporated into the sustainability projects and programs included in this GSP. Examples of limits on operational flexibility include: • The Groundwater Export Ordinance requires county approval to export of water out of the SLO Basin. This is likely not a significant limitation because exporting water out of the SLO Basin hinders sustainability. • Title 22 Drinking Water Program regulates the quality of water that can be recharged into the SLO Basin. 3.7 CONJUNCTIVE USE PROGRAMS There are no active conjunctive use programs currently operating within SLO Basin. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 27 3.8 LAND USE PLANS The County and City have land use authority in the SLO Basin and the other MOA Parties do not. However, SGMA requires the GSAs to consider land use documents by the overlying governing agencies when making decisions. Government Code Section 65350.5 and 65352 require review and consideration of groundwater requirements before the adoption or any substantial amendment of a city's or county's general plan. The planning agency shall review and consider GSPs and any proposed action should refer to the GSA and GSP. Land use is an important factor in water management as described below. The following sections provide a general description of these land use plans and how implementation may affect groundwater supply. 3.8.1 City of San Luis Obispo General Plan The General Plan is the principal tool the City uses when evaluating municipal service improvements and land use proposals. Every service the City provides to its citizens can trace its roots back to goals and policies found in the General Plan. General Plan goals, policies, and implementation measures are based on an assessment of current and future needs and available resources. The land use element designates the general distribution and intensity of land uses, including the location and type of housing, businesses, industry, open space, and education, public buildings, and parks. Figure 3-12 shows the City’s Land Use Map. Figure 3-12. City Land Use Map The City manages its housing supply growth so that it does not exceed one percent per year on average, excluding dwellings affordable to residents with extremely low, very low or low incomes as defined by the Housing Element. The City decided to adopt a Water and Wastewater Element addressing water resources and wastewater services because of the vital role of these resources and the far-reaching impacts of water policies on community growth and character. This element translates the Land Use Element's capacity for development into potential demand for water supply and wastewater services. This element outlines how the City plans to provide adequate water and wastewater services for its citizens, consistent with the goals SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 28 and policies of other General Plan elements. As stated in the General Plan, the City has an adequate water supply to serve the community’s existing and future water needs. The City envisions groundwater playing an important role in ensuring continued resiliency in its water supply portfolio. 3.8.2 County of San Luis Obispo General Plan The 2014 County General Plan contains three pertinent elements that are related to land use and water supply. Pertinent sections include the Land Use, Agricultural, and Inland Area Plans elements. The County’s General Plan also contains programs that are specific, non-mandatory actions or policies recommended by the Land Use and Circulation Element (LUCE) to achieve community or area wide objectives. Implementing each LUCE program is the responsibility of the County or other public agency that is identified in the program. Programs are recommended actions rather than mandatory requirements. Implementation of any program by the County should be based on consideration of community needs and substantial community support for the program and its related cost. The SLO Basin is within the San Luis Obispo Planning Area and South County Planning Area. The planning areas do not conform to the SLO Basin boundaries but do provide a general representation of the land use in the areas. Figure 3-13 and Figure 3-14 shows the planning areas and land uses. Figure 3-13. County Land Use Map (San Luis Obispo Planning Area) The General Plan Framework for Planning does not provide tabular assessment of land use types and acres, or population projection estimates within the San Luis Obispo Planning Area and South County Planning Area. Therefore, projected demands and supplies based on land use aren’t identified for the SLO Basin in the Land Use element. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 29 Figure 3-14. County Land Use Map (South County Planning Area) 3.8.3 Los Ranchos/Edna Village Plan More specifically, the Los Ranchos/Edna Village Plan establishes a vision for the future that will guide land use and transportation over the next 20 years. This village plan is part of Part III of the LUCE of the County General Plan within the San Luis Obispo Planning Area. The Framework for Planning (LUCE Part I) is the central policy document, while this plan contains programs more specifically applicable to the Los Ranchos/Edna village area. In accordance with the Framework for Planning, allowable densities (intensity of land use) are established (Figure 3-15). The San Luis Obispo Area Plan contains regional land use and circulation goals, policies, and programs that also apply to Los Ranchos/Edna. Table 3-7 and summarize the acreage and distribution of each land use category in Los Ranchos/Edna village. Rural land use acreage is summarized in the Framework for Planning. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 30 Table 3-7. Los Ranchos/Edna Land Use Acreage Figure 3-15. Los Ranchos/Edna Land Use Map 3.8.4 Plan Implementation Effects on Existing Land Use This section to be completed after GSP is complete. SLO Basin Groundwater Sustainability Plan Description of Plan Area (§ 354.8) County of SLO and City of SLO 31 3.8.5 Plan Implementation Effects on Water Supply This section to be completed after GSP is complete. 3.8.6 Well Permitting/Ordinance This section to be completed after GSP is complete. 3.8.7 Land Use Plans Outside of Basin The Parties submitting this GSP have not included information regarding the implementation of land use plans outside of the SLO Basin as adjacent basins are also required to implement SGMA and their GSPs will require them to achieve sustainable groundwater management. 3.9 MANAGEMENT AREAS This section to be completed after GSP is complete. 3.9.1 Reason for Creation 3.10 ADDITIONAL GSP ELEMENTS, IF APPLICABLE SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 32 4 BASIN SETTING (§ 354.14) This section describes the geologic setting of the San Luis Obispo Valley Groundwater Basin (Basin), including the Basin boundaries, geologic formations and structures, principal aquifer units, geologic cross sections, and hydraulic parameter data. The information presented in this chapter, when considered with the information presented in Chapter 5 (Groundwater Conditions) and Chapter 6 (Water Budget), comprises the basis of the Hydrogeologic Conceptual Model (HCM) of the Basin. This section draws upon previously published studies, primarily a hydrogeologic and geologic investigation prepared by GSI for the San Luis Obispo County Flood Control and Water Conservation District (SLOCFCWCD) in 2018, as well as a 1997 draft report, “San Luis-Edna Groundwater Basin Study, Draft Report” (DWR, 1997), which was prepared but never finalized for official publication, and a 1991 report by Boyle Engineering (Ground Water Basin Evaluation) that was prepared for the City. The data and information presented in this section is not intended to be exhaustive but is a summary of the relevant and important aspects of the Basin geology that influence groundwater sustainability. More detailed information can be found in the original reports discussed above. This section presents the framework for subsequent sections on groundwater conditions and water budgets. As part of the GSP process, a numerical groundwater model is being developed for the Basin to use as a tool in the planning process (Appendix ZZ). Much of the information comprising the HCM presented in Chapters 4, 5, and 6 of the GSP is applied directly to the development of the groundwater model. Physical data on the geology and hydrogeologic parameters of the Basin presented in Chapter 4 are used to develop the model structure and parameterization. Data on groundwater conditions and water budget presented in Chapters 5 and 6 are used in model calibration. Multiple sources and types of data are presented in Chapters 4, 5, and 6. Some of this data, such as rainfall amounts, depth to groundwater, and depth to bedrock, is directly measurable and involves a low degree of uncertainty. Other data, such as aquifer transmissivity, is based on calculations and interpretations of observed data, but is not directly measurable, and so involves a greater amount of uncertainty than direct measurements. And finally, values presented in the water budget are primarily derived from analysis of related data; almost none of the water budget components are directly measurable, and so involve more uncertainty than the previously discussed data types. 4.1 BASIN TOPOGRAPHY AND BOUNDARIES The Basin is oriented in a northwest-southeast direction and is composed of unconsolidated or loosely consolidated sedimentary deposits. It is approximately 14 miles long and 1.5 miles wide. It covers a surface area of about 12,700 acres (19.9 square miles). The Basin is bounded on the northeast by the relatively impermeable bedrock formations of the Santa Lucia Range, and on the southwest by the formations of the San Luis Range and the Edna fault system. The bottom of the Basin is defined by the contact of permeable sediments with the impermeable bedrock Miocene-aged and Franciscan Assemblage rocks (DWR, 2003). A topographic map displaying the Basin boundaries is presented in Figure 4-1, which also displays the watershed areas of the San Luis Obispo Creek and Pismo Creek drainages. An aerial photo of the Basin area is presented in Figure 4-2. Elevations within the Basin range from over 500 feet above mean seal level in the southeastern extent of Edna Valley, to under 100 feet above mean sea level where San Luis Obispo Creek flows out of the Basin. The Basin is commonly referenced as being composed of two distinct valleys, with the San Luis Valley in the northwest and the Edna Valley in the southeast. The San Luis Valley comprises approximately the northwestern half of the Basin. It is the area of the Basin drained by San Luis Obispo Creek and its SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 33 tributaries (Prefumo Creek and Stenner Creek west of Highway 101, Davenport Creek and smaller tributaries east of Highway 101). Surface drainage in San Luis Valley drains out of the Basin flowing to the south along the course of San Luis Obispo Creek toward the coast in the Avila Beach area, approximately along the course of Highway 101. The San Luis Valley includes part of the City and California Polytechnic University (Cal Poly) jurisdictional boundaries, while the remainder of the valley is unincorporated land. Land use in the City is primarily municipal, residential, and industrial. The area in the northwest part of the Basin, along Los Osos Valley Road, has significant areas of irrigated agriculture, primarily row crops. The Edna Valley comprises approximately the southeastern half of the Basin. The primary creeks that drain the Basin are the east and west branches of Corral de Piedras Creek; the Corral de Piedras Creek tributaries join to form Pismo Creek, draining south out of the Edna Valley into Price Canyon. Canada de Verde Creek is also a significant tributary that flows south out of the Basin in the extreme southeastern part of Edna Valley, ultimately joining Pismo Creek (Figures 4-1 and 4-2). The Edna Valley includes unincorporated lands, including lands associated with various private water purveyors. The primary land use in the Edna Valley is agriculture. During the past two decades, wine grapes have become the most significant crop type in the Edna Valley. The primary weather patterns for the Basin are derived from seasonal patterns of atmospheric conditions that originate over the Pacific Ocean and move inland. As storm fronts move in from the coast, rainfall in the area falls more heavily in the mountains, and the Basin itself receives less rainfall because of a muted rain shadow effect. Average annual precipitation ranges from approximately 18 inches throughout most of the Basin to about 22 inches in relatively higher elevation areas near the City and Cal Poly (Figure 4-3). The time series of annual precipitation for the period of record from 1871 to 2018 at the Cal Poly weather station is presented in Figure 3-11. The average rainfall at this location is 21.69 inches, with a standard deviation of 8.71 inches. The historical maximum is 49.99 inches, which occurred in 1884. The historical minimum is 4.56 inches, which occurred in 2013. The physical definition of the Basin boundary is the occurrence of unconsolidated or loosely consolidated saturated sediments down to the contact with the basement rock of the Miocene-aged formations and Franciscan Assemblage. (The geologic units will be described in more detail Section 4-4.) Figure 4-4presents a surface defining the bottom boundary of the Basin, based on the elevation of bedrock surface below the Basin sediments. There is a topographic high point in the underlying bedrock elevation between the San Luis Valley and Edna Valley sub-areas. As shown, the watershed divide and the bedrock divide are not coincident. Figure 4-5 presents contours of total thickness of the Basin sediments; the inset figure displays the thickness of sediments in a longitudinal cross section. It is apparent from Figure 4-6 that the sediments of the Edna Valley have significantly greater thickness than those of the San Luis Valley. The longitudinal profile of the Basin from the northwest on the left of the figure to the southeast on the right indicates the watershed divide present in the vicinity of Biddle Ranch Road, indicated on Figure 4-4 and Figure 4-5. Precipitation that falls west of that divide ultimately flows to Davenport and San Luis Obispo Creeks, and precipitation that falls east of that divide flows to Corral de Piedras Creek or the other small tributaries, ultimately flowing to Pismo Creek south of the Basin. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 34 Figure 4-1: Topographic map. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 35 Figure 4-2: Aerial Photograph. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 36 Figure 4-3: Annual Precipitation. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 37 Figure 4-4: Bottom Elevation of Basin. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 38 Figure 4-5: Thickness of Basin Sediments. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 39 4.2 PRIMARY USERS OF GROUNDWATER The primary groundwater users in the Basin include municipal, agricultural, and domestic (i.e., rural residential, small community water systems, and small commercial entities). These entities are discussed in more detail in Chapter 2 of this report. The City currently receives most or all of its supply from surface water sources including Whale Rock Reservoir, Santa Margarita Reservoir, Nacimiento Reservoir, and recycled water (Figure 3-3). However, it maintains its network of production wells in standby mode for emergency supply and intends to utilize groundwater as a resource to meet future water demand. The mutual and private water companies, domestic and agricultural users in the Edna Valley rely almost exclusively on groundwater, although some have water rights along East and West Corral de Piedras Creeks. No surface water points of diversion along San Luis Obispo Creek are present in the Basin. 4.3 SOILS INFILTRATION POTENTIAL Saturated hydraulic conductivity of surficial soils is a good indicator of the soil’s infiltration potential. Soil data from the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) Soil Survey Geographic Database (SSURGO) (USDA NRCS, 2007) is shown by the four hydrologic groups on Figure 4-6. The soil hydrologic group is an assessment of soil infiltration rates that is determined by the water transmitting properties of the soil, which includes hydraulic conductivity and percentage of clays in the soil relative to sands and gravels. The groups are defined as: • Group A – High Infiltration Rate: water is transmitted freely through the soil; soils typically less than 10 percent clay and more than 90 percent sand or gravel. • Group B – Moderate Infiltration Rate: water transmission through the soil is unimpeded; soils typically have between 10 and 20 percent clay and 50 to 90 percent sand • Group C – Slow Infiltration Rate: water transmission through the soil is somewhat restricted; soils typically have between 20 and 40 percent clay and less than 50 percent sand • Group D – Very Slow Infiltration Rate: water movement through the soil is restricted or very restricted; soils typically have greater than 40 percent clay, less than 50 percent sand A higher soil infiltration capacity does not necessarily correlate to higher transmissivity in the underlying aquifer, but it may correlate to greater recharge potential in localized areas. This will be discussed in more detail in Chapter 5. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 40 Figure 4-6: Soil Hydrologic Groups. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 41 4.4 REGIONAL GEOLOGY This section provides a description of the geologic formations and structures in the Basin. These descriptions are summarized from previously published reports. Figure 4-7 displays a stratigraphic column presenting the significant geologic formations within the Basin. Figure 4-8 presents a surficial geologic map of the Basin and surrounding area. Figure 4-9 displays the locations of lithologic data used for this plan, and the section lines corresponding to cross sections in the following figures. Geologic cross sections are presented in Figure 4-10 through 4-22. The selected geologic cross sections illustrate the relationship of the geologic formations that comprise the Basin and the geologic formations that underlie and bound the Basin. The cross sections displayed on Figure 4-10 through Figure 4-21were directly adopted from the SLO Basin Characterization Report (GSI, 2018). 4.4.1 Regional Geologic Structures The primary geologic structures of significance to the hydrogeology of the Basin are the Edna Fault Zone and the adjacent Los Osos Fault Zone, which together form the southwestern boundary of the Basin through the uplift of the Franciscan and Monterey Formation strata in the San Luis Range southwest of the faults. The Edna and Los Osos Faults are normal faults, indicating primary displacement motion is vertical rather than lateral (Figure 4-8). There are some disconnected and unnamed fault splays mapped in the area south of the airport. 4.4.2 Geologic Formations within the Basin For the purpose of this plan, the geologic units in the Basin and vicinity may be considered as two basic groups; the Basin sediments and the consolidated bedrock formations surrounding and underlying the Basin. The consolidated bedrock formations range in age and composition from (1) Jurassic-aged serpentine and marine sediments to (2) Tertiary-aged marine and volcanic depositions. Compared to the saturated sediments that comprise the Basin aquifers, the consolidated bedrock formations are not considered to be significantly water-bearing. Although bedding plane and/or structural fractures in these rocks may yield small amounts of water to wells, they do not represent a significant portion of the pumping in the area. The delineation of the Basin boundaries is defined both laterally and vertically by the contacts of the Basin sedimentary formations with the consolidated bedrock formations. From a hydrogeologic standpoint, the most important strata in the Basin are the sedimentary basin fill deposits that define the vertical and lateral extents of the Basin. These include recent and older deposits of terrestrial sourced sediments, underlain in the Edna Valley by older marine sedimentary units. Figure 4-7 presents a stratigraphic column of the significant local geologic units. Figure 4-8 presents a map of the Basin vicinity (assembled from a mosaic of the Dibblee maps from the San Luis Obispo, Pismo Beach, Lopez Mountain, and Arroyo Grande NE quadrangles) showing where the various formations crop out at the surface. Fault data displayed in Figure 4-8 were acquired via the USGS Earthquake Hazards Program. The Quaternary fault and fold database from which the shapefiles are derived was published in 2006 and cites a wide variety of published sources. Fault traces within the shapefile represent surficial deformation caused by earthquakes during the Quaternary Period (the last 1.6 million years). Figure 4-8 also displays the Basin boundaries defined in DWR Bulletin 118. Inspection of Figure 4-8 indicates that the Bulletin 118 Boundary lines for the Basin boundary do not match up precisely with the most recently mapped extent of the water-bearing formations based on GSI (2018). This is likely an artifact of previous mapping being performed at a larger (statewide) scale. The water-bearing sedimentary formations and the non-water-bearing bedrock formations are briefly described below. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 42 Figure 4-7: Stratigraphic Column. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 43 Figure 4-8: Geologic Map. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 44 4.4.2.1 Alluvium The Recent Alluvium is the mapped geologic unit composed of unconsolidated sediments of gravel, sand, silt, and clay, deposited by fluvial processes along the courses of San Luis Obispo Creek, Davenport Creek, East and West Corral de Piedras Creeks, and their tributaries. Lenses of sand and gravel are the productive strata within the Recent Alluvium. These strata have no significant lateral continuity across large areas of subsurface within the Basin. Thickness of Recent Alluvium may range from just a few feet to more than 50 feet. Well pumping rates may range from less than 10 gallons per minute (gpm) to more than 100 gpm. However, wells screened exclusively in Recent Alluvium are generally less productive than wells that screen significant thicknesses of the Paso Robles and/or Pismo Formations. 4.4.2.2 Paso Robles Formation The Paso Robles Formation underlies the Recent Alluvium throughout most of the Basin, and overlies the Pismo Formation where present. It is composed of poorly sorted, unconsolidated to mildly consolidated sandstone, siltstone, and claystone, with thin beds of volcanic tuff in some areas. The Paso Robles Formation was deposited in a terrestrial setting on a mildly sloping floodplain that has been faulted, uplifted, and eroded since deposition. The Paso Robles Formation is exposed at the surface throughout much of the Edna Valley, except in areas where existing streams have deposited Recent Alluvium on top of it. It is not readily distinguishable from alluvium in geophysical well logs. Locally, the Paso Robles Formation is sometimes distinguished as being yellow in color, with sticky clay. DWR Well Completion Reports with these types of descriptions generally were identified as Paso Robles Formation for the purpose of interpreting the geology in the cross sections. However, it was sometimes difficult to distinguish between Recent Alluvium and Paso Robles Formation in driller’s descriptions, and professional judgment and broader context within the Basin were often used when defining the contact between these two units. Wells that screen both the Recent Alluvium and Paso Robles Formation have reported yields from less than 100 to over 500 gpm. 4.4.2.3 Pismo Formation The oldest geologic water-bearing unit with significance to the hydrogeology of the Basin is the Pismo Formation. The Pismo Formation is a Pliocene-aged sequence of marine deposited sedimentary units composed of claystone, siltstone, sandstone, and conglomerate. There are five recognized members of the Pismo Formation (Figure 4-7 ). While all members are part of the Pismo Formation, each member reflects different depositional environments, and the variations in geology may affect the hydrogeologic characteristics of the strata. From the oldest to youngest, the members are: • The Edna Member, which lies unconformably atop the Monterey Formation, and is locally bituminous (hydrocarbon-bearing) • The Miguelito Member, primarily composed of thinly bedded grey or brown siltstones and claystones • The Gragg Member, usually described as a medium-grained sandstone • The Bellview Member, composed of interbedded fine-grained sandstones and claystones • The Squire Member, generally described as a medium- to coarse-grained fossiliferous sandstone of white to grey sands Previous reports have identified the significant thicknesses of sand at depth beneath the Paso Robles Formation in the Edna Valley as the Squire Member of the Pismo Formation. However, it is not clear SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 45 whether these are accurately assigned as Squire. Other members of the Pismo Formation may be part of the sequence, and there is some ambiguity as to the actual member assignment. Even in the adjacent Pismo Beach and Arroyo Grande NE quadrangle geologic (Dibblee 2006a, 2006b), there is ambiguity in the geologic nomenclature. In the adjacent geologic maps these quadrangles, a continuous exposure of this unit across the boundary between the two maps is referred to as Pismo Formation in one map (Dibblee 2006b), and Squire Sandstone in the other (Dibblee 2006a). Therefore, it is probably more accurate to generally refer to these units as the Pismo Formation, and not to specifically identify the member designations. This convention will be followed for the remainder of this report. The Pismo Formation is extensive below the Paso Robles Formation in the Edna Valley. Thicknesses of Pismo Formation up to 400 feet are reported or observed in well completion reports and in the cross sections (Figure 4-5). The presence of sea shells in the lithologic descriptions of well completion reports is clearly diagnostic of the Pismo Formation because of its marine origin. Many of the well completion reports in the Edna Valley document the presence of water-bearing blue and green sands beneath the Paso Robles Formation, and these are considered to be largely diagnostic of the Pismo Formation as well. Wells that are completed in both the Paso Robles and Pismo Formations are reported to yield from less than 100 gpm to approximately 700 gpm. 4.4.3 Geologic Formations Surrounding the Basin Older geologic formations that underlie the Basin sediments typically have lower permeability and/or porosity and are generally considered non-water-bearing. In some cases, these older beds may occasionally yield flow adequate for local or domestic needs, but wells drilled into these units are also often dry or produce groundwater less than 10 gpm. Generally, the water quality from the bedrock units is poor in comparison to the Basin sediments. In general, the geologic units underlying the basin include Tertiary-age consolidated sedimentary and volcanic beds (Monterey and Obispo Formations), and Cretaceous-age sedimentary and metamorphic rocks (Franciscan Assemblage). 4.4.3.1 Monterey Formation The Monterey Formation is a thinly bedded siliceous shale, with layers of chert in some locations. In other areas of the County outside of the Basin, the Monterey Formation is the source of significant oil production. While fractures in consolidated rock may yield small quantities of water to wells, the Monterey Formation is not considered to be an aquifer for the purposes of this GSP. Regionally, the unit thickness is as great as 2,000 feet, and the unit is often highly deformed. Water wells completed in the Monterey Formation are occasionally productive if a sufficient thickness of highly deformed and fractured shale is encountered. More often, however, the Monterey shale produces groundwater to wells in very low quantities. Groundwater produced from the Monterey Formation often has high concentrations of Total Dissolved Solids (TDS), hydrogen sulfide, total organic carbon, and manganese. 4.4.3.2 Obispo Formation The Obispo Formation and associated Tertiary volcanics are composed of materials associated with volcanic activity along tectonic plate margins approximately 20 to 25 million years ago. The Obispo Formation is composed of ash and other material expelled during volcanic eruptions. Although fractures in consolidated volcanic rock may yield small quantities of water to wells, the Obispo Formation is not considered to be an aquifer for the purposes of this GSP. 4.4.3.3 Franciscan Assemblage The Franciscan Assemblage contains the oldest rocks in the Basin area, ranging in age from late Jurassic through Cretaceous (150 to 66 million years ago). The rocks include a heterogeneous collection of basalts, which have been altered through high-pressure metamorphosis associated with subduction of the oceanic SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 46 crust beneath the North American Plate before the creation of the San Andreas Fault. The current assemblage includes ophiolites, which weather to serpentinites and are common in the San Luis and Santa Lucia Ranges. Although fractures may yield small quantities of water to wells, the Franciscan Assemblage is not considered to be an aquifer for the purposes of this GSP. 4.5 PRINCIPAL AQUIFERS AND AQUITARDS Water-bearing sand and gravel beds that may be laterally and vertically discontinuous are generally grouped together into zones that are referred to as aquifers. The aquifers can be vertically separated by fine-grained zones that can impede movement of groundwater between aquifers, referred to as aquitards. Three aquifers exist in the Basin: • Alluvial Aquifer – A relatively continuous aquifer comprising alluvial sediments that underlie the San Luis Obispo Creek and tributary streams, as well as East and West Corral de Piedras Creeks and tributary streams; • Paso Robles Formation Aquifer – An interbedded aquifer comprised of terrestrially-derived sand and gravel lenses in the Paso Robles Formation. • Pismo Formation Aquifer - An interbedded aquifer comprised of marine sand and gravel lenses in the Pismo Formation. There are no significant aquitards that vertically separate the three aquifers in the Basin over large areas. There may be deposits of clay and silt that are not laterally extensive that locally separate two aquifers, but there is no recognized aquitard in the Basin that separates the aquifers over significant areas. 4.5.1 Cross Sections Eleven cross sections were prepared for this report; three (A1-A2, A2-A3, A3-A4) are oriented along the longitudinal axis of the Basin and eight (B-B’ through I-I’) are oriented across the Basin, perpendicular to the longitudinal axis (Figure 4-9). All lithologic data was reviewed during the selection of the section line locations. The cross sections display lithology, interpretations of geologic contacts based on available data, well screen intervals, and interpreted and mapped faults. If the geologic interpretation was not clear from the points on the cross section lines, nearby data from other locations was reviewed to provide broader geologic context. Each geologic cross section is discussed in the following paragraphs. The longitudinal axis of the Basin is much longer than the cross basin section lines, the longitudinal axis was divided into three separate cross sections for the sake of clarity and presentation of detail. As part of the work performed for the GSP, CHG performed a passive seismic geophysical plan in the area along Buckley Road south of the airport (Appendix ZZ). Data from this plan resulted in slight adjustments in three of the previously developed cross sections. These data have been incorporated into the cross sections. • Cross Section A1-A2 (Figure 4-10) extends approximately 6.5 miles from the northwest extent of the Basin at its boundary with the Los Osos Basin to about 1 mile east of Highway 101. Land surface elevation is about 200 feet AMSL at the northwest extent, and slopes gently downward to about 120 feet AMSL at the southeast extent. Recent Alluvium is exposed at the surface for the entire length of this cross section, ranging in thickness from less than 50 feet near the Los Osos Valley Basin boundary to about 80 feet near the center of the section. The Paso Robles Formation is relatively thin in the northeast where it has been significantly eroded by the alluvium, but thickens to approximately 70 feet in the southeastern part of the section. Marine sands of the Pismo SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 47 Formation occur below the Paso Robles Formation in the southeastern part of the section, with a maximum thickness of about 50 feet. • Cross Section A2-A3 (Figure 4-11) extends approximately 4 miles along the longitudinal Basin axis, starting near Tank Farm Road and cutting obliquely across Buckley Road to just past Edna Road in the southeast. Land surface elevation ranges from approximately 120 feet AMSL in the northwest to more than 270 feet AMSL in the southwest. Along the northwest half of the section line, alluvium is exposed at the surface, with an approximate thickness of 40 to 50 feet. The alluvium is primarily underlain by the Paso Robles Formation with thicknesses ranging from approximately 40 to 80 feet. Just southeast of the airport, the Paso Robles Formation is exposed at the surface, beginning at the point where there is a noticeable rise in land surface elevation. This is approximately coincident with the maximum elevation of the underlying bedrock formations (the bedrock divide that approximates the dividing line between the Edna Valley and the San Luis Valley). A recent geophysical investigation by Cleath-Harris Geologists in the area of the high bedrock elevation has provided greater detail on the Basin geometry in this area. The thickness of the Paso Robles Formation in this area is up to 120 feet. Pismo Formation sediments underlie the Paso Robles Formation in this area, with thickness of about 50 feet in the area of Davenport Creek. The Pismo Formation thickness starts to increase significantly along this section line to the southeast, with about 250 feet of Pismo sediments evident at the southeastern extent of the section line. Several of the borings in this section indicate wells are partially or completely screened in bedrock formations, indicating that the relatively thin saturated portions of the water-bearing sediments did not yield enough water for the purposes of the wells. • Cross section A3-A4 (Figure 4-12) extends about 6.5 miles along the Basin axis from approximately Biddle Ranch Road to the southeast extent of the Basin. Land surface elevation rises from about 250 feet AMSL on the northwest end of the section to over 500 feet AMSL in the southeast. Relatively thin occurrences (40 feet or less) of Recent Alluvium associated with Corral de Piedras Creek and its tributaries are evident in some areas on the western half of this section. In the southeastern extent of the section, the Paso Robles Formation crops out at the surface where the land is beginning to rise to the northern mountains, and is dissected by small streams and valleys in this area. The Pismo Formation sediments reach their maximum thickness of more than 400 feet along the northwestern extent of this section; the thickness of the Pismo gradually thins to about 90 feet at the southwestern extent of the section. • Cross section B-B’ (Figure 4-13) extends about 1.5 miles across the Basin perpendicular to the Basin axis in the vicinity of Foothill Boulevard and Los Osos Valley Road. The section line has a land surface elevation of about 180 feet AMSL on the northern end, sloping downward to about 130 feet AMSL along the Basin’s long axis, and rising again to about 230 feet AMSL on the southern end. Recent Alluvium is exposed at the surface along this entire section, with thicknesses of about 20 to 30 feet. In the northern half of the section, alluvium is deposited directly on underlying basement rock. In the southern half of the section, the Paso Robles Formation underlies the alluvium with a maximum thickness of about 45 feet. The southern extent of the section crosses the Los Osos Fault Zone. • Cross Section C1-C1’ (Figure 4-14) extends from the northern lobes of the Basin boundary, which are formed from alluvium from Stenner and San Luis Obispo Creeks, and trends southward approximately 5.5 miles across the Basin from Cal Poly through the City, approximately along the path of Highway 101. Land surface elevation is about 350 feet at the northern end of the section line on some noticeable hilltops along the line, and slopes downward to an approximate altitude of 80 feet on the southern end. Most of the northern extent of this section has alluvium of about 20 to 40 feet of thickness deposited directly on underlying bedrock. Only in the southernmost 1½ miles of the section line, where it crosses the main body of the Basin, do Paso Robles Formation sediments underlie the alluvium. The Paso Robles Formation is about 90 feet thick here, and it is in turn underlain by about 60 feet of Pismo Formation sediments. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 48 • Cross Section C2-C2’ (Figure 4-15) extends about 1½ miles southward through the eastern lobe of the northern part of San Luis Valley. Alluvium is deposited directly on top of basement rock along this section. Alluvium is thin here, ranging from less than 10 feet to about 40 feet. • Cross Section D-D’ (Figure 4-16) extends about 2.5 miles southward from a prominent serpentine ridge in the north to the southern Basin boundary. Land surface elevation is about 160 feet on the northern end of the section, sloping down to about 110 feet in the Basin center, and rising to about 180 feet on the southern end. Recent Alluvium is exposed at the surface along most of this section, reaching a maximum thickness of about 80 feet. The alluvium is deposited directly on basement rock through the northern half of the section. In the southern half of the section, approximately 20 to 30 feet of Paso Robles Formation underlies the alluvium. Near the southern extent of the Basin, the section line crosses into the combined Edna-Los Osos Fault Zone, at which point the land surface elevation rises steeply and the Paso Robles Formation crops out at the surface due to the upthrown formations south of the faults. • Cross Section E-E’ (Figure 4-17) extends about 2½ miles across the Basin in the vicinity of the airport and the area south of Buckley Road. Land surface elevation ranges from about 170 feet on the northern end to 230 feet in the southern end. In the northern half of this section, Recent Alluvium are exposed at the surface. In the southern half, the Paso Robles Formation is exposed. Alluvial thickness in the northern half of the section ranges from about 20 to 70 feet, and is underlain by about 30 to 35 feet of Paso Robles Formation. In the southern half of the section, it crosses into the Edna-Los Osos Fault Zone, and the Paso Robles Formation is upthrown to the point that it is exposed at the surface. Paso Robles Formation thickness ranges from 50 feet to about 100 feet. Sediments of the Pismo Formation underlie the Paso Robles Formation in this area, and are about 25 to 70 feet thick. • Cross Section F-F’ (Figure 4-18) extends about 2 miles north to south in the western extent of the Edna Valley area. The Paso Robles Formation is exposed at the surface along most of this section. One small pod of alluvium associated with Davenport Creek is evident in the center of the section. The Paso Robles Formation has a maximum thickness of about 175 feet in this section. It is underlain by about 50 to 60 feet of Pismo Formation sediments in the area north of the Edna Fault Zone. To the south, the section line extends into the Edna Fault Zone. South of the fault, the formations are upthrown, resulting in a small area of Pismo Formation sediments exposed at the surface. • Cross Section G-G’ (Figure 4-19) extends about 2 miles through the heart of the Edna Valley area. Land surface elevation ranges from about 300 feet on the north end to more than 350 feet on the south end. A thin veneer of alluvium, about 20 feet thick, that is associated with Corral de Piedras Creek and tributaries is exposed at the surface along much of this section. The Paso Robles Formation crops out in the north of the section, and underlies the alluvium with an average thickness of about 50 to 60 feet. The Pismo Formation displays its largest thickness along this section, with a maximum thickness of about 450 feet near where this section intersects with cross section A3-A4. The southern end of the section line crosses into the Edna Fault zone, and sediments are displaced such that the Pismo Formation sediments are exposed at the surface on the southern slopes of the Basin in this area. • Cross Section H-H’ (Figure 4-20) extends approximately 2½ miles through the Edna Valley. Land surface is approximately 350 feet on the northern end, sloping downward to about 230 feet near Corbett Canyon Road, then quickly rising to nearly 400 feet on the south end of the section on the upthrown side of the Edna Fault. The Paso Robles Formation is exposed at the surface for nearly the entire section. The section line crosses a small exposure of Recent Alluvium associated with Corral de Piedras Creek. In the northern half of the section, the Paso Robles Formation sediments are deposited directly on the basement rock formations, with a maximum thickness of about 80 feet. In the southern half of the section, the basement rock elevation plunges and the thickness of the Paso Robles Formation is about 150 to 230 feet. The Pismo Formation underlies the Paso SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 49 Robles Formation sediments in the southern half of the section, with a maximum thickness of about 200 feet. In the Corbett Canyon area, the section crosses the Edna Fault; south of the fault the basement rock formations are thrust up to the surface, and represent the boundary of the Basin. • Cross Section I-I’ (Figure 4-21) crosses the southern extent of the Edna Valley. The northern part of the section lies along the lower slopes of the Santa Lucia Range, and displays Paso Robles Formation sediments deposited on top of bedrock formations. A small pod of Recent Alluvium associated with Corral de Piedras Creek is displayed. Along the center of the Edna Valley, the Paso Robles Formation thickness is about 200 feet, and is underlain by about 100 feet of Pismo Formation sediments. The section crosses the Edna Fault Zone, which shows Pismo Formation sediments upthrown to land surface on the south side of one fault splay, and bedrock of the Monterey Formation upthrown to land surface elevation south of a second fault splay. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 50 Figure 4-9: Lithologic Data Points and Cross Section Lines. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 51 Figure 4-10: Cross Section A1-A2. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 52 Figure 4-11: Cross Section A2-A3. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 53 Figure 4-12: Cross Section A3-A4. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 54 Figure 4-13: Cross Section B-B’. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 55 Figure 4-14: Cross Section C1-C1’. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 56 Figure 4-15: Cross Section C2-C2’. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 57 Figure 4-16: Cross Section D-D’. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 58 Figure 4-17: Cross Section E-E’. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 59 Figure 4-18: Cross Section F-F’. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 60 Figure 4-19: Cross Section G-G’. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 61 Figure 4-20: Cross Section H-H’. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 62 Figure 4-21: Cross Section I-I’. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 63 4.5.2 Aquifer Characteristics The relative productivity of an aquifer can be expressed in terms of transmissivity, hydraulic conductivity, or specific capacity. The most robust method is measuring transmissivity using a long-term (frequently 24 hours or more) constant-rate pumping test. Water level drawdown data collected during this test can be analyzed and used to calculate transmissivity. Specific capacity is a simple measure of flow rate (gpm) divided by drawdown (feet), routinely measured by well service contractors during well maintenance and reported in units of gpm per foot of drawdown (gpm/ft). Specific capacity measurements may be affected by well construction details, and, therefore, are not only related to aquifer characteristics. Nevertheless, the following commonly accepted empirical relationships allows transmissivity to be estimated from specific capacity measurements. T (gpd/ft) = SC (gpm/ft) * (1,500 – 2,000), where T = Transmissivity (gpd/ft) SC = Specific Capacity (gpm/ft) 1500 – 2000 = Empirical factor, (1,500 used for unconfined, 2,000 for confined aquifer) Data describing these data from water wells throughout the Basin were compiled. The data was obtained from Previous regional studies or reports, previous pumping tests and well service information provided by local stakeholders. All available reports and documents that were made available through data requests, report reviews, etc., were reviewed for technical information, and included in this summary if the data were judged to be sufficient. DWR (1958) reports a range of irrigation well pumping rates from 300 to 600 gpm, and a range of specific capacity values of 15 to 20 gpm/ft for the Basin, corresponding to transmissivity estimates from 22,500 to 40,000 gallons per day per foot (gpd/ft). Boyle (1991) evaluated five constant-rate aquifer tests for City wells, all in the San Luis Valley, and reported transmissivity values ranging from 11,200 to 71,000 gpd/ft, with an average of 41,240 gpd/ft. DWR (1997) discussed the range of hydraulic conductivity values used in the preparation of its groundwater model, which averaged about 15 ft/day in the San Luis Obispo Creek Valley, and about 6 ft/day in the Edna area. Figure 4-22 displays the spatial distribution of the available data locations for well tests in the Basin. Inspection of Figure 4-22 indicates a good spatial coverage of locations, with reasonable data density throughout the Basin. Table 4-1 presents a compilation of all constant rate aquifer test data compiled during the preparation of this GSP. Table 4-2 presents a compilation of the specific capacity data. This information is used in the groundwater model development, and in the technical work supporting preparation of the GSP for the Basin. Table 4-1 presents a data summary for the constant rate aquifer test that was available, including information on pumping rate, static and pumping water levels, screened intervals, total depth, and formations screened. It was not always readily apparent which formations are screened from the available data, and sometimes well screens may span more than one formation. If there is uncertainty regarding this designation, it is indicated with a question mark in Table 4-1. Calculated transmissivity values range from less than 1,000 gpd/ft to a maximum of 158,400 gpd/ft. (The highest reported transmissivity value of 158,400 gpd/ft is an outlier, and was likely influenced by recharge from a nearby stream. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 64 Table 4-2 presents all available information for the specific capacity well tests identified. Table 4-2 includes a transmissivity estimate based on the empirical relationship discussed previously. Data presented tables 4-1 and 4-2 indicate that wells screened in the Alluvium and Paso Robles Formation have transmissivities ranging from about 5,000 to 158,000 gallons per day per foot (gpd/ft), and averaging over 42,000 gpd/ft. Wells screened in Paso Robles and Pismo Formations have transmissivities ranging from less than 1,000 to about 40,000 gpd/ft, and average about 10,000 gpd/ft. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 65 Figure 4-22: Hydraulic Parameter Data Locations. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 66 SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 67 Table 4-1: SLO Basin Well Aquifer SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 68 4.5.3 Aquitards An aquitard is a layer of low permeability, usually comprised of fine-grained materials such as clay or silt, which vertically separates adjacent layers of higher permeability formations that may serve as aquifers. Although there is some amount of clay present in nearly all of the boring logs reviewed for this plan, there are no formally defined or laterally continuous clay layers that function as aquitards within the Basin. In the San Luis Valley, wells are commonly screened across both the Recent Alluvium and the underlying Paso Robles Formation, and these two formations essentially function as a single hydrogeologic unit is this area. Similarly, in the Edna Valley, wells are commonly screened across both the Paso Robles Formation and the underlying Pismo Formation, and these two formations essentially function as a single hydrogeologic unit is this area. 4.6 SURFACE WATER BODIES Surface water/groundwater interactions represent a small, but significant, portion of the water budget of an aquifer system. In the Basin, these interactions occur primarily at streams and lakes. As previously discussed, there are several named creeks that flow across the Basin. In the San Luis Valley area of the Basin, these include San Luis Obispo Creek, Stenner Creek, Prefumo Creek, Froom Creek, and Davenport Creek, in addition to smaller unnamed tributaries. In the Edna Valley these include East and West Corral de Piedras Creeks (which join to form Pismo Creek just south of the Basin Boundary), and Canada de Verde Creek in southeastern Edna Valley. The watersheds support important habitat for native fish and wildlife, including the federally threatened South-Central California Coast steelhead (Oncorhynchus mykiss) (Stillwater Sciences et al. 2012, Stillwater Sciences 2014). Laguna Lake is the only lake in the Basin. It is a naturally occurring lake just north of Los Osos Valley Road and west of Highway 101. The downstream outlet of the lake flows into the Prefumo Creek culvert under Madonna Road. In the past, flashboards were used to maintain water elevation in the lake to support recreation and maintain wildlife habitat. However theseare no longer used.. The water in the lake is partially supplied by seasonal flow in Prefumo Creek, which flows into Laguna Lake. and at least partially supplied by subsurface groundwater inflow. Groundwater interaction with streams in the Basin is not well quantified, but it is recognized as an important component of recharge in the water budget. Where the water table is above the streambed and slopes toward the stream, the stream receives groundwater flow from the aquifer; this is known as a gaining reach (i.e., the stream 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; this is known as a losing reach. During seasonal dry flow conditions, it is clear that groundwater elevation is deeper than the streambed. Therefore, it is generally understood that the streams in the Basin discharge to the underlying aquifer, at least in the first part of the wet-weather flow season. If there is constant seasonal surface water flow, it is possible that groundwater elevations may rise to the point that they are higher than the stream elevation, and the creek may become a seasonally gaining stream in some reaches. Groundwater modeling can help evaluate surface water groundwater interaction.. The amount of flow in surface water/groundwater interaction is difficult to quantify. Boyle (1991) assumed that 10 percent of the measured surface water flow coming into the Basin in San Luis Obispo Creek and Stenner Creek was recharged to the aquifer, and used an average rate of 430 acre-feet/yr (AFY). In its draft SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 69 report, DWR (1997) reports model-generated estimates ranging from streams gaining 2,700 AFY from the aquifer, to streams losing 680 AFY to the aquifer. The County, through its coordination with Zone 9 and the City, maintains a network of five stream gauges in the San Luis Valley Basin to record heights of flow throughout the year for flood warning purposes (Figure 3-10). The gauges were constructed in November 2001 and have periods of record from that year to the present. Continuous data monitoring of height of flow at the gages is recorded, but equivalent discharge (cubic feet per second) is not recorded. 4.7 SUBSIDENCE POTENTIAL Subsidence is the gradual settling or sinking of the earth’s surface due to material movement at depth in a location, and is frequently associated with groundwater pumpage, and is one of the undesired results identified in SGMA. Subsidence has been documented in parts of the San Luis Valley. The most severe subsidence that has occurred in the Basin was in the 1990s along the Los Osos Valley Road corridor. Subsidence occurred within young organic soil (i.e., peat) in response to extraction of groundwater within a relatively shallow aquifer and resulted in significant settlement of the ground surface. The settlement caused local damage to businesses and homes in that area as local groundwater pumping dewatered the soft soil units beneath buildings and the surrounding area. Subsidence of more than 1 foot of settlement of the ground surface in some locations damaged buildings and resulted in reconstruction or retrofitting buildings. Another area of known subsidence is along the shores of Laguna Lake. Homes located along the shoreline have experienced settlement that has cracked foundations, patios, and window and door openings. Many homes in that area have been retrofitted to address the settlement. While the subsidence near Laguna Lake is not specifically related to extraction of groundwater, lowering of the groundwater table in that area could result in further settlement and subsidence. The historical manifestation of subsidence generally has been limited to a the area along Los Osos Valley Road and downstream, where compressible soil types that were particularly vulnerable to large settlements in response to lowering of the local groundwater table. This history emphasizes the importance of considering subsurface conditions that may be associated with subsidence. Not all soil and rocks are vulnerable to the type of subsidence that occurred along Los Osos Valley Road. The potential for subsidence to occur, and the severity of the subsidence, is dependent on the geology, groundwater levels, and the properties of the soil and rock that may be dewatered in association with groundwater pumping. The subsidence evaluation consisted of a review of published data and studies performed by local, state, and federal agencies, as well as a familiarity of local geology and soil. The following is a summary of the key findings. DWR identifies the Basin as having a low subsidence potential. However, historical subsidence is known to have occurred in specific geographic areas of the Basin because of groundwater pumping. The Basin was evaluated on the basis of the extent of known and mapped geologic units within the Basin (Yeh, 2018). The relative potential for subsidence was divided into three categories and delineated as shown in Figure 4-23. • Category 1. Category 1 has the highest likelihood of future subsidence if subject to lowered groundwater levels in the future. Based on a review of public data and consultant reports, alluvium mapped in these areas contains young organic soil known in areas around Los Osos Valley Road, Laguna Lake, and low-lying wetland areas near Tank Farm Road. These areas are known to have experienced historical subsidence or to contain soft or organic soil and were identified as having a potential for subsidence in relation to geology and groundwater pumping. These areas are SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 70 identified as Category 1 in in Figure 4-23, with star symbols marking approximate areas of known historical subsidence. Extraction of groundwater resources in these areas could cause further subsidence. • Category 2. Low-lying topographic areas in the Basin that are mapped as young alluvial soil were identified as potentially containing soft or organic soil layers that may have a potential for subsidence in relation to groundwater pumping, but currently there is no historical or subsurface information to further evaluate those areas. Those areas are mostly located along Prefumo Creek and San Luis Obispo Creek and the main drainages through the west end of the Edna Valley near Price Canyon. These areas are identified as Category 2 in in Figure 4-23. This screening criteria recognizes the unconsolidated nature typical of young alluvium that has been mapped in these areas potentially could subside because of compaction of the aquifer if groundwater levels were lowered. • Category 3. Geographic areas in the Basin that were mapped as bedrock or older surficial sediments, and are not known to be underlain by young organic soil or young alluvium, were identified as Category 3 in in Figure 4-23. These areas were evaluated and characterized as not having factors known to be susceptible to subsidence in relation to groundwater pumping. Generally, these are upland areas where bedrock is shallow or where bedrock is mapped at the ground surface, such as in the areas around the airport and Orcutt Road (in Figure 4-23). SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 71 Figure 4-23: Subsidence Potential. SLO Basin Groundwater Sustainability Plan Basin Setting (§ 354.14) County of SLO and City of SLO 72 SLO Basin Groundwater Sustainability Plan References County of SLO and City of SLO 73 5 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. —. 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. SLO Basin Groundwater Sustainability Plan References County of SLO and City of SLO 74 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.