Salinity of deep groundwater in California: Water quantity, quality, and protection

National Academy of Sciences (NAS) | July 12th, 2016

Deep groundwater aquifers are poorly characterized but could yield important sources of water in California and elsewhere. Deep aquifers have been developed for oil and g

Deep groundwater aquifers are poorly characterized but could yield important sources of water in California and elsewhere. Deep aquifers have been developed for oil and gas extraction, and this activity has created both valuable data and risks to groundwater quality. Assessing groundwater quantity and quality requires baseline data and a monitoring framework for evaluating impacts. We analyze 938 chemical, geological, and depth data points from 360 oil/gas fields across eight counties in California and depth data from 34,392 oil and gas wells. By expanding previous groundwater volume estimates from depths of 305 m to 3,000 m in California’s Central Valley, an important agricultural region with growing groundwater demands, fresh [<3,000 ppm total dissolved solids (TDS)] groundwater volume is almost tripled to 2,700 km3, most of it found shallower than 1,000 m. The 3,000-m depth zone also provides 3,900 km3 of fresh and saline water, not previously estimated, that can be categorized as underground sources of drinking water (USDWs; <10,000 ppm TDS). Up to 19% and 35% of oil/gas activities have occurred directly in freshwater zones and USDWs, respectively, in the eight counties. Deeper activities, such as wastewater injection, may also pose a potential threat to groundwater, especially USDWs. Our findings indicate that California’s Central Valley alone has close to three times the volume of fresh groundwater and four times the volume of USDWs than previous estimates suggest. Therefore, efforts to monitor and protect deeper, saline groundwater resources are needed in California and beyond.

Saltwater Intrusion in Los Angeles Area Coastal Aquifers— the Marine Connection

U.S. Geological Survey (USGS) | August 14th, 2002

Saltwater from the Pacific Ocean is seeping into some Los Angeles Basin coastal aquifers and replacing freshwater. Without treatment, this ground water does not conform t

Saltwater from the Pacific Ocean is seeping into some Los Angeles Basin coastal aquifers and replacing freshwater. Without treatment, this ground water does not conform to drinking-water or agricultural standards. This problem is significant because much of the water used by the nearly 10 million residents of Los Angeles County comes from ground-water sources. Although not all coastal aquifers in the region are at risk, the existing resources are vital and must be protected to maintain adequate supplies of potable water. In the 1950’s, construction began on the first of three “barriers” in an attempt to halt saltwater intrusion. Each barrier consists of a series of injection wells that essentially form a subsurface wall of freshwater designed to keep saltwater from penetrating further into aquifers. The barriers are only partly effective; saltwater continues to infiltrate in some areas. New studies show that the ground-water geohydrology of the coastal aquifers is more complex than previously imagined. U.S. Geological Survey (USGS) scientists, working in cooperation with the Water Replenishment District of Southern California and the Los Angeles County Department of Public Works, are expanding efforts to understand the geologic processes and fluid pathways that control saltwater intrusion.

San Andreas Fault Stress Change Due To Groundwater Withdrawal in California's Central Valley, 1860-2010

American Geophysical Union (AGU) | December 28th, 2021

We investigate the spatial and temporal effects of groundwater removal in California's Central Valley on ground surface deformation and on stress changes on the nearby ce

We investigate the spatial and temporal effects of groundwater removal in California's Central Valley on ground surface deformation and on stress changes on the nearby central San Andreas fault (CSAF). In particular, we focus on two outstanding questions: (a) What are the spatial distributions of Coulomb failure stress (ΔCFS) on the CSAF? (b) What are the effects of viscoelasticity on uplift and ΔCSF over time? We focus on the well studied 2006–2010 drought, estimating the change in groundwater storage and the resulting uplift and stress change through a numerical model for the Earth. Viscoelastic effects are examined through an estimate of historical groundwater change from 1860-1961 and a hydrological model from 1961-2003 added to the 2006–2010 drought estimates. Here we find that lower crust or upper mantle viscosity variations enhance uplift and stress changes and that CSAF stress changes may accumulate with time, especially with droughts and groundwater consumption.

San Antonio Creek Valley Groundwater Basin Groundwater Sustainability Plan

San Antonio Basin Groundwater Sustainability Agency | December 16th, 2021

The Sustainable Groundwater Management Act (SGMA), effective as of January of 2015, created a new statewide framework for managing California’s groundwater at the loca

The Sustainable Groundwater Management Act (SGMA), effective as of January of 2015, created a new statewide framework for managing California’s groundwater at the local level through the formation of Groundwater Sustainability Agencies (GSAs). SGMA requires the development and implementation of a Groundwater Sustainability Plan (GSP) for each groundwater basin in the state that has been designated as high or medium priority. A GSP presents strategies for maintaining or bringing a groundwater basin into a sustainable condition within the next 20 years. SGMA exempts de minimus pumpers (e.g., individual domestic well owners who extract up to 2 acre-feet per year [AFY]) from most of the SGMA requirements and does not require metering. The San Antonio Basin Groundwater Sustainability Agency (SABGSA) was formed in 2017 for the purpose of sustainably managing groundwater and developing this GSP for the San Antonio Creek Valley Groundwater Basin (Basin). The SABGSA member agencies are the San Antonio Basin Water District and Los Alamos Community Services District. The Basin occupies approximately 123 square miles in western Santa Barbara County (see Figure ES-1). It is bounded on the north by the Casmalia Hills and Solomon Hills, on the east by the San Rafael Mountains and a watershed divide separating the adjoining Santa Ynez River Valley groundwater basin, on the south by the Purisima Hills and Burton Mesa, and the west by the approximate western boundary of Barka Slough. This GSP describes the physical setting of the Basin; quantifies historical, present, and future water budgets; develops quantifiable management objectives that account for the interests of the Basin’s beneficial groundwater uses and users and identifies a group of projects and management actions that will allow the Basin to achieve sustainability within 20 years of plan adoption. This document also includes the list of references and technical studies, documentation of the stakeholder engagement process undertaken in the development of this plan, and several supporting appendices. Due to size, the file has been divided. Part 1, Part 2, Part 3, Part 4, Part 5, Part 6

San Gorgonio Pass Subbasin Groundwater Sustainability Plan

San Gorgonio Pass Groundwater Sustainability Agency | January 28th, 2022

San Joaquin Basin Flood-MAR Watershed Studies Study Design Report

California Department of Water Resources (DWR) | December 3rd, 2025

Click here to be taken to the California Department of Water Resources Watershed Studies home page

Click here to be taken to the California Department of Water Resources Watershed Studies home page

San Joaquin Basin Flood‑MAR Watershed Studies Basin-wide Summary and Next Steps

California Department of Water Resources (DWR) | December 3rd, 2025

The San Joaquin Basin Flood‑MAR Watershed Studies (Watershed Studies) — comprising five watershed reports for Calaveras, Stanislaus, Tuolumne, Merced and Upper San Jo

The San Joaquin Basin Flood‑MAR Watershed Studies (Watershed Studies) — comprising five watershed reports for Calaveras, Stanislaus, Tuolumne, Merced and Upper San Joaquin watersheds and a Basinwide Summary — deliver data-driven assessments of climate change vulnerabilities and adaptation that reveal risks and consequences and call for bold, coordinated solutions. In the San Joaquin Basin, the effects of climate change will exacerbate long-standing water management challenges. As these reports quantify, every water management sector — flood, water supply (surface and groundwater), and ecosystems — is vulnerable. Without bold action, the basin and its watersheds face mounting threats to communities, economies, and ecosystems. Although each watershed has unique characteristics, all confront a future climate that is hotter and characterized by more extreme weather. For flood management, the 2022 Central Valley Flood Protection Plan states clearly: Catastrophic flooding in the San Joaquin Basin is not a question of if, but when. Meanwhile, the reliability of water supplies is expected to worsen, and ecosystems face degradation.  Click here for the California Department of Water Resources Watershed Studies home page

San Joaquin Basin Flood‑MAR Watershed Studies Calaveras Watershed Study Area Report

California Department of Water Resources (DWR) | December 3rd, 2025

The San Joaquin Basin Flood‑MAR Watershed Studies (Watershed Studies) — comprising five watershed reports for Calaveras, Stanislaus, Tuolumne, Merced and Upper San Jo

The San Joaquin Basin Flood‑MAR Watershed Studies (Watershed Studies) — comprising five watershed reports for Calaveras, Stanislaus, Tuolumne, Merced and Upper San Joaquin watersheds and a Basinwide Summary — deliver data‑driven assessments of climate change vulnerabilities and adaptation that reveal risks and consequences and call for bold, coordinated solutions. In the San Joaquin Basin, the effects of climate change will exacerbate long-standing water management challenges. As these reports quantify, every water management sector — flood, water supply (surface and groundwater), and ecosystems — is vulnerable. Without bold action, the basin and its watersheds face mounting threats to communities, economies, and ecosystems. Although each watershed has unique characteristics, all confront a future climate that is hotter and characterized by more extreme weather. For flood management, the 2022 Central Valley Flood Protection Plan states clearly: Catastrophic flooding in the San Joaquin Basin is not a question of if, but when. Meanwhile, the reliability of water supplies is expected to worsen, and ecosystems face degradation. Click here to be taken to the California Department of Water Resources Watershed Studies home page

San Joaquin Basin Flood‑MAR Watershed Studies Merced Watershed Study Area Report

California Department of Water Resources (DWR) | December 3rd, 2025

The San Joaquin Basin Flood‑MAR Watershed Studies (Watershed Studies) — comprising five watershed reports for Calaveras, Stanislaus, Tuolumne, Merced and Upper San Jo

The San Joaquin Basin Flood‑MAR Watershed Studies (Watershed Studies) — comprising five watershed reports for Calaveras, Stanislaus, Tuolumne, Merced and Upper San Joaquin watersheds and a Basin-wide Summary — deliver data‑driven assessments of climate change vulnerabilities and adaptation that reveal risks and consequences and call for bold, coordinated solutions. In the San Joaquin Basin, the effects of climate change will exacerbate long-standing water management challenges. As these reports quantify, every water management sector — flood, water supply (surface and groundwater), and ecosystems — is vulnerable. Without bold action, the basin and its watersheds face mounting threats to communities, economies, and ecosystems. Although each watershed has unique characteristics, all confront a future climate that is hotter and characterized by more extreme weather. For flood management, the 2022 Central Valley Flood Protection Plan states clearly: Catastrophic flooding in the San Joaquin Basin is not a question of if, but when. Meanwhile, the reliability of water supplies is expected to worsen, and ecosystems face degradation. Click here for the California Department of Water Resources Watershed Studies homepage

San Joaquin Basin Flood‑MAR Watershed Studies Stanislaus Watershed Study Area Report

California Department of Water Resources (DWR) | December 3rd, 2025

The San Joaquin Basin Flood‑MAR Watershed Studies (Watershed Studies) — comprising five watershed reports for Calaveras, Stanislaus, Tuolumne, Merced and Upper San Jo

The San Joaquin Basin Flood‑MAR Watershed Studies (Watershed Studies) — comprising five watershed reports for Calaveras, Stanislaus, Tuolumne, Merced and Upper San Joaquin watersheds and a Basin-wide Summary — deliver data‑driven assessments of climate change vulnerabilities and adaptation that reveal risks and consequences and call for bold, coordinated solutions. In the San Joaquin Basin, the effects of climate change will exacerbate long-standing water management challenges. As these reports quantify, every water management sector — flood, water supply (surface and groundwater), and ecosystems — is vulnerable. Without bold action, the basin and its watersheds face mounting threats to communities, economies, and ecosystems. Although each watershed has unique characteristics, all confront a future climate that is hotter and characterized by more extreme weather. For flood management, the 2022 Central Valley Flood Protection Plan states clearly: Catastrophic flooding in the San Joaquin Basin is not a question of if, but when. Meanwhile, the reliability of water supplies is expected to worsen, and ecosystems face degradation. Click here to be taken to the California Department of Water Resources Watershed Studies homepage