Groundwater Quality in the Northern Coast Ranges Groundwater Basins, California

U.S. Geological Survey (USGS) | February 1st, 2015

The Northern Coast Ranges (NOCO) study unit is 633 square miles and consists of 35 groundwater basins and subbasins (California Department of Water Resources, 2003; Matha

The Northern Coast Ranges (NOCO) study unit is 633 square miles and consists of 35 groundwater basins and subbasins (California Department of Water Resources, 2003; Mathany and Belitz, 2015). These basins and subbasins were grouped into two study areas based primarily on locality. The groundwater basins and subbasins located inland, not adjacent to the Pacific Ocean, were aggregated into the Interior Basins (NOCO-IN) study area. The groundwater basins and subbasins adjacent to the Pacific Ocean were aggregated into the Coastal Basins (NOCO-CO) study area (Mathany and others, 2011). The primary aquifer system in the NOCO-IN study area occurs in alluvial basins made up of sand, silt, gravel, clay, and thin volcanic ash layers or lenses. Groundwater movement in the NOCO-IN study area follows the topography and direction of surface-water features. In the NOCO-CO study area, groundwater is present in alluvial fan, floodplain, and terrace deposits. Groundwater movement in the NOCO-CO study area is from east to west towards the Pacific Ocean. The primary aquifer system in the study unit generally is defined as those parts of the aquifer system corresponding to the perforated intervals of sites listed in the California Department of Public Health (CDPH) database of public drinking-water supply sources. Well depths in the NOCO-IN study area ranged from 36 to 400 feet below land surface (ft bls), and depths to top-of-perforations ranged from 15 to 148 ft bls. In the NOCO-CO study area, well depths ranged from 15 to 400 ft bls, and depths to top-of-perforations ranged from 10 to 356 ft bls. Water quality in the primary aquifer system may differ from that in the shallower and deeper parts of the aquifer system. Average annual rainfall in the NOCO study unit ranges from 22 to 79 inches. In the NOCO-IN study area, the climate is classified as Mediterranean, with warm to hot, dry summers and cold, wet winters. In the NOCO-CO study area, the climate is influenced by the Pacific Ocean and is characterized by cool to mild summers and cold, wet winters. The study unit is drained by several rivers and their principal tributaries: the Eel, Russian, Mad, Navarro, Smith, Klamath, Noyo, and Big Rivers. Land use in the study unit is about 60 percent (%) natural (mostly grassland and forest), 29% agricultural, and 11% urban. The primary uses of agricultural lands are pasture, row crops, hay, vineyards, and timberlands. The largest urban areas are the cities of Crescent City, Arcata, Eureka, Fort Bragg, Willits, Ukiah, and Lakeport. Recharge to the groundwater system is primarily from mixture of ambient sources, including direct percolation of precipitation and irrigation waters, infiltration of runoff from surrounding hills/areas, seepage from rivers and creeks, and subsurface inflow (from non-alluvial geologic units that bound the alluvial basins). The primary sources of discharge are evaporation, discharge to streams, and water pumped for municipal supply and irrigation.

Groundwater Resource Management Fundamentals: Groundwater Basin Connectivity

United Water Conservation District (UWCD) | May 14th, 2014

As Ventura County and the rest of the State of California continue to deal with drought conditions there is an increased interest and reliance on groundwater reso

As Ventura County and the rest of the State of California continue to deal with drought conditions there is an increased interest and reliance on groundwater resources. Over the past decade or so the CA Department of Water Resources and others have promoted water supply planning by watershed groups or other local entities, and this approach has generally been accepted by planners and stakeholders. In Ventura County, there is significant groundwater flow between defined groundwater basins both within and between watersheds, and stream flow in the dry seasons is commonly dependent on groundwater discharges to stream channels. Groundwater flow between connected subbasins has long been recognized in southern Ventura County, most notably in the groundwater basins that underlie the watersheds of the Santa Clara River and Calleguas Creek. The intent of this Open-File Report is to document the historical evolution of the concept of the hydrologic connectivity of groundwater basins in the Santa Clara River valley and the Oxnard Plain coastal area, and to summarize the underlying scientific data supporting the hydrologic connectivity principle. The idea of groundwater basin connectivity is not new, and in fact, predates the formation of United Water Conservation District (United), but is a concept that is embraced by United as a key tenet of their groundwater management activities. As a primary groundwater management entity for the Santa Clara River valley and Oxnard Plain area, United has extensive datasets supporting the concept of groundwater flow or movement (i.e., connectivity) between the subbasins in its jurisdiction. The current drought conditions being experienced in Ventura County have reinvigorated a desire by several entities to develop basin-wide or regional water supply management plans. Clearly, the hydrologic relationships between the groundwater basins are important factors for recognition in water supply management plans. The effects of groundwater extraction or wastewater discharge, for example, impact downgradient subbasins and respective stakeholders. Recognition of this important hydrologic relationship is fundamental to creating long-term sustainable groundwater resource management strategies. The current regulatory setting in California includes several attempts to “manage” groundwater resources or water resource, in general, but those attempts are often focused at a local level (e.g., Urban Water Management Plans, Agricultural Water Management Plans) that do not account for the regional nature of this resource. These “entity-specific” plans are not, by themselves, sufficient for water resource sustainability planning purposes as they frequently ignore or minimize the importance of the regional nature of the hydrologic system. This Open-File Report has been prepared to provide Ventura County water resource management agencies (e.g., Fox Canyon Groundwater Management Agency, Ventura County Watershed Protection District, United Water Conservation District) and local municipalities and businesses that may be stakeholders in the development of water resource management and sustainability plans a guiding framework for thinking about how the hydrologic systems in the Santa Clara River valley and the greater Oxnard Plain coastal area function.

Groundwater Sustainability Plan for the Las Posas Valley Basin

Fox Canyon Groundwater Management Agency (FCGMA) | December 13th, 2019

The Fox Canyon Groundwater Management Agency (FCGMA, or the Agency), has developed this Groundwater Sustainability Plan (GSP) for the Las Posas Valley Basin (LPVB; DWR B

The Fox Canyon Groundwater Management Agency (FCGMA, or the Agency), has developed this Groundwater Sustainability Plan (GSP) for the Las Posas Valley Basin (LPVB; DWR Basin 4-008) in compliance with the 2014 Sustainable Groundwater Management Act (SGMA) (California Water Code, Section 10720 et seq.). FCGMA is one of three Groundwater Sustainability Agencies (GSAs) in the LPVB. The other two GSAs are the Camrosa Water District GSA–Las Posas Valley and the Las Posas Valley Outlying Areas GSA. This GSP is the sole GSP prepared for the LPVB, and covers the entire LPVB, including all areas of the LPVB outside of FCGMA’s jurisdiction. The purpose of this GSP is to define the conditions under which the groundwater resources of the entire LPVB, which support agricultural, municipal and industrial, and environmental uses, will be managed sustainably in the future. Although the California Department of Water Resources (DWR) has defined the LPVB as a single groundwater basin, the western and eastern parts of the basin are hydraulically separated from each other by the Somis Fault, a geologic feature that inhibits groundwater flow across it. As a result, groundwater conditions on the west side of the fault in the Fox Canyon Aquifer and Grimes Canyon Aquifer, two primary aquifers in the LPVB, differ from conditions on the east side of the fault. Furthermore, the Epworth Gravels Aquifer, located on the east side of the fault is hydrologically separated from the Fox Canyon Aquifer and Grimes Canyon Aquifer. Hydrologic differences in the controls on, and responses to, both recharge and groundwater production necessitate the definition of three separate management areas in the LPVB. These three management areas are the West Las Posas Management Area (WLPMA), the East Las Posas Management Area (ELPMA), and the Epworth Gravels Management Area. The hydrologic conditions, sustainable yield, and sustainability criteria are discussed and defined by management area throughout this GSP. Historical groundwater production in the LPVB has resulted in chronic declines in groundwater levels and loss of groundwater in storage in parts of each of the three management areas. In the WLPMA, the average rate of groundwater production between 2015 and 2017 was approximately 14,000 acre-feet per year (AFY). In the ELPMA and the Epworth Gravels Management Area, the average rate of groundwater production between 2015 and 2017 was approximately 20,500 AFY and 1,500 AFY, respectively. Numerical groundwater simulations indicate that if these production rates were carried into the future, groundwater elevations in each of the management areas of the LPVB would not recover during multi-year cycles of drought and recovery. In order to determine the sustainable yield of each management area, combinations of projects and management actions were explored to estimate the rate of groundwater production that would prevent chronic declines in groundwater elevation and ongoing loss of groundwater storage in the future. Additionally, in the WLPMA, the numerical groundwater model simulations were used to assess the influence of groundwater conditions on the adjacent Oxnard Subbasin. In the ELPMA, numerical groundwater simulations were also used to assess zones of the Fox Canyon Aquifer that are most prone to conversion from confined to unconfined conditions. The rate of groundwater production that avoids chronic water level declines, loss of storage, potential land subsidence, and impacts to adjacent basins is referred to as the sustainable yield for each management area. With the currently available projects and management actions, the sustainable yield of the WLPMA is approximately 12,500 AFY, with an uncertainty estimate of ±1,200 AFY (Table ES-1). In the ELPMA, the total sustainable yield (including the Epworth Gravels Management Area) is estimated to be 17,800 AFY ±2,300 AFY. For the Epworth Gravels Management Area only, the sustainable yield is estimated to be approximately 1,300 AFY. Except for the Epworth Gravels Management Area, both the historical (1985–2015) and recent (2015–2017) groundwater production rates exceeded the upper end of the future sustainable yield estimates (Table ES-1). Adoption of this GSP represents the first step in achieving groundwater sustainability within the. LPVB, as required by SGMA. SGMA requires that groundwater in each of the management areas of the LPVB be managed sustainably within 20 years of adoption of the GSP. SGMA also requires that this GSP be evaluated at a minimum of every 5 years after adoption. As part of the 5-year evaluation process, the sustainable yield will be refined and adjusted. These refinements will be based on new data, additional studies undertaken to fill data gaps, and groundwater modeling. Refinements and adjustments will also be made to the minimum threshold groundwater levels developed to avoid undesirable results, the measurable objective groundwater levels that account for the need to continue groundwater production during drought cycles and the associated interim milestones to help gauge progress toward sustainability over the next 20 years. The required 5-year evaluations will also examine both new water supply projects, and the potential impacts of extractions rates on groundwater elevations and sustainability in the LPVB. Additional modeling is recommended during the 5-year update process to understand how changes in pumping and additional new water supply projects can increase the overall sustainable yield of the different management areas of the LPVB. As this understanding improves, projects to support increases in the overall sustainable yield can be developed.

Groundwater Sustainability Plan for the Oxnard Subbasin

Fox Canyon Groundwater Management Agency (FCGMA) | December 13th, 2019

The Fox Canyon Groundwater Management Agency (FCGMA, or the Agency) has developed this Groundwater Sustainability Plan (GSP) for the Oxnard Subbasin (Subbasin; DWR Basin

The Fox Canyon Groundwater Management Agency (FCGMA, or the Agency) has developed this Groundwater Sustainability Plan (GSP) for the Oxnard Subbasin (Subbasin; DWR Basin 4-004.02) of the Santa Clara River Valley Groundwater Basin (DWR Basin 4-004), in compliance with the 2014 Sustainable Groundwater Management Act (SGMA; California Water Code, Section 10720 et seq.). The purpose of this GSP is to define the conditions under which the groundwater resources of the entire Oxnard Subbasin, which support agricultural, municipal and industrial, and environmental uses, will be managed sustainably in the future. Historical groundwater production has resulted in seawater intrusion in the five primary aquifers of the Subbasin. These aquifers have been divided into an Upper Aquifer System, which comprises the Oxnard and Mugu Aquifers, and a Lower Aquifer System, which comprises the Hueneme, Fox Canyon, and Grimes Canyon Aquifers. The average rate of groundwater production from the Upper Aquifer System between 2015 and 2017 was approximately 40,000 acre-feet per year (AFY). The average production from the Lower Aquifer System between 2015 and 2017 was approximately 29,000 AFY. Numerical groundwater simulations indicate that if these production rates were carried into the future, seawater intrusion would continue in the Subbasin and the area currently impacted by concentrations of chloride greater than 500 milligrams per liter would grow. The landward extent of this area is referred to as the saline water impact front. Combinations of projects and management actions were explored to estimate the rate of groundwater production that would prevent future expansion of the area of the Subbasin currently impacted by concentrations of chloride greater than 500 milligrams per liter. This rate of groundwater production is referred to as the sustainable yield. With the currently available projects and management actions, the sustainable yield of the Upper Aquifer System, was calculated to be approximately 32,000 AFY, with an uncertainty of ± 4,100 to 6,000 AFY. The sustainable yield of the Lower Aquifer System was calculated to be approximately 7,000 AFY, with an uncertainty of ± 2,300 to 3,600 AFY. Adoption of this GSP represents the first step in achieving groundwater sustainability within the Oxnard Subbasin by 2040, as required by SGMA. Evaluation of this GSP is required at a minimum of every 5 years following submittal to the California Department of Water Resources (DWR). As part of the 5-year evaluation process, the sustainable yield for each aquifer system will be refined and adjusted. These refinements will be based on new data, additional studies undertaken to fill data gaps, and groundwater modeling. Refinements and adjustments will also be made to the minimum threshold water levels developed to avoid undesirable results, the measurable objective water levels that account for the need to continue groundwater production during drought cycles and the associated interim milestones to help gauge progress toward sustainability over the next 20 years. In order to minimize the pumping reductions required to achieve sustainable management of the Subbasin, investment in large-scale projects to increase water supply, provide the infrastructure to redistribute pumping, and/or directly control seawater intrusion should be investigated. Basin optimization studies, groundwater modeling, and project feasibility studies will be conducted over the next 5 years to explore practicable processes and approaches to increasing the sustainable yield of the Oxnard Subbasin.

Groundwater Sustainability Plan for the Pleasant Valley Basin

Fox Canyon Groundwater Management Agency (FCGMA) | December 13th, 2019

The Fox Canyon Groundwater Management Agency (FCGMA, or the Agency) has developed this Groundwater Sustainability Plan (GSP) for the Pleasant Valley Basin (PVB; DWR Basi

The Fox Canyon Groundwater Management Agency (FCGMA, or the Agency) has developed this Groundwater Sustainability Plan (GSP) for the Pleasant Valley Basin (PVB; DWR Basin 4-006), in compliance with the 2014 Sustainable Groundwater Management Act (SGMA) (California Water Code, Section 10720 et seq.). FCGMA is one of three Groundwater Sustainability Agencies (GSAs) in the PVB. The other two GSAs are the Camrosa Water District–Pleasant Valley GSA and the Pleasant Valley Outlying Areas GSA. This GSP is the sole GSP prepared for the PVB, and covers the entire PVB, including all areas of the PVB outside of FCGMA’s jurisdiction. The purpose of this GSP is to define the conditions under which the groundwater resources of the entire PVB, which support agricultural, municipal and industrial, and environmental uses, will be managed sustainably in the future. The PVB shares a boundary and is in hydraulic communication with Oxnard Subbasin (Subbasin; DWR Basin 4-004.02) to the west. The boundary between the PVB and the Oxnard Subbasin is associated with a change in character of recent and older alluvial deposits. In the PVB, these deposits are finer grained and are, in general, less suitable for groundwater production than the coarser-grained sediments of the same age in the Oxnard Subbasin. There is no corresponding change in character in the deeper aquifers, including the Fox Canyon Aquifer, which are continuous across the boundary between the PVB and the Oxnard Subbasin. Groundwater production from wells on either side of the boundary between the PVB and the Oxnard Subbasin influences groundwater elevations and the direction of groundwater flow across this boundary. Historical groundwater production from the PVB and Oxnard Subbasin combined has resulted in seawater intrusion in the aquifers of the Subbasin. In the PVB, the average rate of groundwater production between 2015 and 2017 was approximately 13,200 acre-feet per year (AFY). In 2015, approximately 53% of the production from the Lower Aquifer System, which comprises the Hueneme, Fox Canyon, and Grimes Canyon Aquifers, and 47% of the production from the Upper Aquifer System, which comprises the older alluvium in the PVB, and the Oxnard and Mugu Aquifers in the Oxnard Subbasin. Numerical groundwater simulations indicate that if these production rates were carried into the future, groundwater elevations in the PVB would not recover during multi-year cycles of drought and recovery, and seawater intrusion would continue in the Oxnard Subbasin. The landward extent of the area in the Subbasin currently impacted by concentrations of chloride greater than 500 milligrams per liter is referred to as the “saline water impact front.” Combinations of projects and management actions were explored to estimate the rate of groundwater production that would allow groundwater elevations in the PVB to recover during multi-year cycles of drought and recovery, and prevent future landward migration of the saline water impact front. This rate of groundwater production is referred to as the sustainable yield. With the currently available projects and management actions, the sustainable yield of the PVB was estimated to be approximately 11,600 AFY, with an uncertainty of ± 1,200 AFY. At the upper bound of the uncertainty estimate (12,600 AFY), the estimated sustainable yield of the PVB is 600 AFY lower than the 2015–2017 average production rate. Adoption of this GSP represents the first step in achieving groundwater sustainability within the PVB by 2040, as required by SGMA. Evaluation of this GSP is required at a minimum of every 5 years following submittal to the California Department of Water Resources (DWR). As part of the 5-year evaluation process, the sustainable yield will be refined and adjusted. These refinements will be based on new data, additional studies undertaken to fill data gaps, and groundwater modeling. Refinements and adjustments will also be made to the minimum threshold groundwater levels developed to avoid undesirable results, the measurable objective groundwater levels that account for the need to continue groundwater production during drought cycles and the associated interim milestones to help gauge progress toward sustainability over the next 20 years. In order to minimize the pumping reductions required to achieve sustainable management of the PVB, investment in projects to increase water supply, provide the infrastructure to redistribute pumping, and/or directly control seawater intrusion in the Oxnard Subbasin should be investigated. Inter-basin optimization studies, groundwater modeling, and project feasibility studies are recommended over the next 5 years to explore practicable processes and approaches to increasing the sustainable yield of the PVB. Due to size, this file has been divided. Part 1, Part 2, Part 3, Part 4, Part 5, Part 6

Groundwater Vulnerability Study Santa Clara County, California

Santa Clara Valley Water District | October 18th, 2010

The Santa Clara Valley Water District (District) is the primary water resources agency in Santa Clara County. This Groundwater Vulnerability Study was conducted to predic

The Santa Clara Valley Water District (District) is the primary water resources agency in Santa Clara County. This Groundwater Vulnerability Study was conducted to predict the vulnerability of groundwater to potentially contaminating land use activities to aid the District in its management and protection activities. Groundwater vulnerability is comprised of two key components: 1) groundwater sensitivity and 2) potentially contaminating activities. Groundwater sensitivity is generally defined as the relative ease with which a contaminant on or near the land surface can migrate to the aquifer of interest based on the intrinsic characteristics of the aquifer and the overlying unsaturated materials. Groundwater sensitivity is combined with the potentially contaminating activities risk to characterize overall groundwater vulnerability. The Study Area is comprised of three groundwater subbasins: the Santa Clara, the Coyote, and the Llagas. Each has unique hydrogeologic characteristics as well as unique current and historic land uses. Water-bearing units in the Study Area have been grouped into two major aquifer systems, the Shallow Aquifer and Principal Aquifer. Groundwater vulnerability was assessed separately for each aquifer. Generally, the Shallow Aquifer occurs above regional confining layers. The Principal Aquifer lies beneath the Shallow Aquifer and supplies most of the groundwater produced for beneficial uses in the Study Area. The Principal Aquifer occurs under semi-confined to confined conditions. In areas where confining layers do not exist or are not laterally and vertically extensive, only the Principal Aquifer occurs. Accordingly, in the recharge zones, depth to water was characterized as the first encountered groundwater for both the Shallow and Principal aquifers. In contrast, the Shallow and Principal aquifers were uniquely defined in the confined areas with the Shallow Aquifer characterized by the first encountered groundwater and the Principal Aquifer defined by the top screened interval of wells tapping the primary groundwater production zone. The southern area and lateral margins of the Santa Clara Subbasin are unconfined recharge areas. An extensive regional aquitard occurs in the northern interior portion of the subbasin. The Santa Clara Subbasin is currently highly developed with residential, commercial, and industrial areas. Due to the high density of urban land uses including major industrial manufacturing and processing facilities, point-source contamination is prevalent but generally contained in the Shallow Aquifer. This is due, in part, to the protection offered by the significant confining layers found in the northern portion of the subbasin. The Santa Clara Subbasin is currently undergoing continued urban expansion and redevelopment of formerly industrial areas to residential use. No significant laterally extensive confining layers exist in the Coyote Subbasin, and groundwater generally occurs under unconfined conditions. Compared with the Santa Clara Subbasin, the Coyote Subbasin is relatively rural, undeveloped, and mostly unincorporated with far fewer industrial/commercial contaminant release sites. Due to generally unconfined conditions, the Coyote Subbasin is hydrogeologically sensitive to groundwater contamination. Existing water quality impacts are related to agricultural practices and rural (e.g., septic) land use. Coyote Valley has a high potential for future residential and commercial development. Groundwater in the Llagas Subbasin occurs under unconfined to confined conditions. Generally unconfined recharge areas are found in the northern portion of the subbasin, while confining layers become more frequent and laterally and vertically extensive in the southern areas. Accordingly, the northern portion of the subbasin is relatively more hydrogeologically sensitive to groundwater contamination compared with the southern subbasin. The Llagas Subbasin has urban development focused in the north and south with the central portion of the subbasin comprised predominantly of agricultural development and large residential parcels. The Llagas Subbasin is currently impacted by high levels of nitrate associated with rural land use and agriculture and perchlorate from historic releases from a flare manufacturer. A comprehensive literature review was conducted to identify established methods to conduct vulnerability assessments. Based on the evaluation of available groundwater sensitivity assessment methods and an understanding of available hydrogeologic and water quality information for the Study Area, a statistical method was selected to quantify the sensitivity to contamination of the Shallow Aquifer and Principal Aquifer. Water quality data (i.e., nitrate concentrations and distribution) were used for calibration and verification purposes to identify and rank groundwater sensitivity factors. Based on the logistic regression statistical analysis, four factors were found to be the most important in characterizing groundwater sensitivity. These include 1) soil media characteristics in the vadose zone, 2) groundwater recharge, 3) depth to top of screen, and 4) annual groundwater production. Groundwater recharge in the Study Area includes a significant amount of artificial recharge conducted by the District. Traditionally, artificial recharge and groundwater production would not be considered intrinsic aquifer characteristics. However, for this Study, groundwater sensitivity did include consideration of quasiintrinsic characteristics such as artificial recharge and production. In addition to the objective statistical analysis, subjective refinements relative to artificial recharge areas and characterization of depth to water/aquifer were applied based on observation, technical judgment, and limitations in the nitrate data set. These refinements included subjectively ranking areas near artificial recharge areas as highly sensitive and using depth to first encountered groundwater to represent the depth to water factor for the Principal Aquifer in the recharge zones. This results in higher sensitivity in the recharge areas for the Principal Aquifer. Figures ES-1 and ES-2 show the sensitivity of the Shallow and Principal aquifers, respectively. Not surprisingly, the sensitivity assessment found that the Shallow Aquifer is more sensitive than the Principal Aquifer. In addition, the analysis indicates that the sensitivity of the Shallow and Principal aquifers is generally highest in the Llagas Subbasin, followed by the Coyote Subbasin, and Santa Clara Subbasin. Despite the protection afforded by the regional confining layer in the southern portion of the Llagas Subbasin, both the Shallow and Principal aquifers are highly sensitive to contamination due to high recharge rates and permeable soils. The sensitivity of the Shallow and Principal aquifers in the Coyote Subbasin are also relatively high due primarily to shallow aquifer conditions, high recharge rates, and large amounts of groundwater production. Although the confined zone in the Santa Clara Subbasin affords relatively good protection from surface contamination, the outer western confined zone appears to be highly sensitive to contamination due to the significant groundwater production in this area. Unlike the sensitivity assessment, it is not possible to statistically calibrate potentially contaminating activities risk. Accordingly, potentially contaminating activities are commonly identified and ranked based on subjective observation and experience (e.g., California Drinking Water Source Assessment and Protection Program). For the potentially contaminating activities risk ranking, emphasis was placed on accurately characterizing the maximum risks so as not to underestimate potential risks. The potentially contaminating activities risk analysis included four main risk factors or categories—general plan land use, potentially contaminating business activities, known contaminated sites, and supplemental data. Twenty-nine land use categories were selected and ranked with respect to groundwater impacts. Relative risk ranks were developed for 48 categories of business activities. Known and open contamination sites were given the highest ranking with a lower ranking for closed sites. Additional supplemental data for irrigated agriculture, septic system density, mines, landfills, and petroleum pipelines were also ranked and included in the analysis. These various risk factors were combined to generate the overall potentially contaminating activities risk distribution map. The potentially contaminating activities risk analysis found large portions of the Santa Clara Subbasin are at high risk due to its high level of development with commercial and industrial areas and many associated industrial and commercial contaminant release sites along with the lingering impacts of agricultural releases. Relatively lower overall risks are associated with the Coyote Subbasin, which is relatively rural, undeveloped, and mostly unincorporated with far fewer industrial/commercial contaminant release sites. Nonetheless, most of the subbasin shows a moderate level of risk associated with irrigated agriculture. It is important to note that the Coyote Valley has the most potential for future development and thus the most potential for an increase in risk in the future. Relatively lower overall potentially contaminating activities risk (compared with the Santa Clara Subbasin) is found in the Llagas Subbasin due to its more rural nature. Areas of relatively higher risk are associated with commercial and industrial development in the vicinity of the cities of Morgan Hill and Gilroy. Moderate risk is found in the central portion of the subbasin associated with irrigated agriculture. While continued conversion of rural to urban land use in the vicinity of the cities of Morgan Hill and Gilroy in the future will likely increase risk in these areas, the central portion of the subbasin is expected to remain relatively unchanged with respect to risk, given the established zoning. The sensitivity assessment and potentially contaminating activities risk were combined to create the overall vulnerability maps for the Shallow and Principal aquifers. Figures ES-3 and ES-4 show the vulnerability of the Shallow and Principal aquifers, respectively. As might be expected, the vulnerability of the Shallow Aquifer is greater than the Principal Aquifer in areas of confinement in the Santa Clara Subbasin. The density of commercial/industrial sites and known contamination release sites in the northern Santa Clara Subbasin make the Shallow Aquifer highly vulnerable to contamination. The Principal Aquifer has fewer areas of very high vulnerability compared with the Shallow Aquifer due to the relative lower sensitivity of the Principal Aquifer. The Llagas and Coyote subbasins exhibit high to very high groundwater vulnerability in both the Shallow and Principal aquifers. The high vulnerability is driven primarily by the high sensitivity in these two subbasins. Given the potential for future development in the Coyote Subbasin, the high degree of vulnerability of the subbasin requires the highest level of effort directed toward protection. Following completion of the vulnerability assessment, a web-based geographical information system tool was developed, which incorporated the sensitivity, potentially contaminating activities risk, and vulnerability maps. Additional maps are also provided to enhance the usefulness of the tool. The tool enables District staff to work interactively with the vulnerability study analysis. The tool enables District staff to evaluate potential impacts of land use changes, prioritize basin management activities, prioritize review of known contamination sites, update existing data layers, update or modify risk factors, and add supplemental data layers.

Historical Seawater Intrusion Map Pressure 180-Foot Aquifer - 500 mg/L or Greater Chloride Areas

Monterey County Water Resources Agency (MCWRA) | April 11th, 2018

Hydrogeologic Controls and Geochemical Indicators of Groundwater Movement in the Niles Cone and Southern East Bay Plain Groundwater Subbasins, Alameda County, California

U.S. Geological Survey (USGS) | January 1st, 2018

Beginning in the 1970s, Alameda County Water District began infiltrating imported water through ponds in repurposed gravel quarries at the Quarry Lakes Regional Park, in

Beginning in the 1970s, Alameda County Water District began infiltrating imported water through ponds in repurposed gravel quarries at the Quarry Lakes Regional Park, in the Niles Cone groundwater subbasin, to recharge groundwater and to minimize intrusion of saline, San Francisco Bay water into freshwater aquifers. Hydraulic connection between distinct aquifers underlying Quarry Lakes allows water to recharge the upper aquifer system to depths of 400 feet below land surface, and the Deep aquifer to depths of more than 650 feet. Previous studies of the Niles Cone and southern East Bay Plain groundwater subbasins suggested that these two subbasins may be hydraulically connected. Characterization of storage capacities and hydraulic properties of the complex aquifers and the structural and stratigraphic controls on groundwater movement aids in optimal storage and recovery of recharged water and provides information on the ability of aquifers shared by different water management agencies to fulfill competing storage and extraction demands. The movement of recharge water through the Niles Cone groundwater subbasin from Quarry Lakes and the possible hydraulic connection between the Niles Cone and the southern East Bay Plain groundwater subbasins were investigated using interferometric synthetic aperture radar (InSAR), water-chemistry, and isotopic data, including tritium/helium-3, helium-4, and carbon-14 age-dating techniques.

Hydrogeologic Investigation Arroyo Seco Cone

Monterey County Water Resources Agency (MCWRA) | April 15th, 1994

This report presents the findings, analysis, conclusions, and recommendations developed as part of an investigation to characterize the hydrogeology of the Arroyo Seco Co

This report presents the findings, analysis, conclusions, and recommendations developed as part of an investigation to characterize the hydrogeology of the Arroyo Seco Cone area of the Salinas Valley. The purpose of this investigation is to assess the feasibility of developing a seasonal surface water spreading operation utilizing water diverted from the Arroyo Seco in order to enhance ground water recharge in the area. The goal of a project to store water in the ground for subsequent pumping and use is to increase the yield of the Salinas Basin. This investigation defines the hydrogeologic characteristics of the Arroyo Seco Cone area, characterizes the hydrology of the Arroyo Seco and the availability of divertable flows, and estimates the annual recharge that could be achieved through the development of spreading basins.

Hydrogeology and Geochemistry of Aquifers Underlying the San Lorenzo and San Leandro Areas of the East Bay Plain, Alameda County, California

U.S. Geological Survey (USGS) | June 7th, 2003