Groundwater in California has been in the spotlight recently, and rightly so: we rely heavily on it. In fact, California’s groundwater usage is the highest in the United States, accounting for about 21% of the nation’s total groundwater extraction.
Groundwater, often referred to as a ‘hidden resource’, plays a crucial role in California’s economy. Its vast supply has significantly contributed to the state’s status as the nation’s largest food and agricultural economy and the world’s fifth-largest overall economy. This economic significance underscores the value of groundwater, a resource that is often overlooked due to its invisible nature.
These economic benefits have not come without many costs: the diminished quantity and degraded quality of groundwater resources, the infrastructure damaged by land subsidence, the decline in ecosystem services such as maintaining wetlands, supporting streamflow, and providing habitat for wildlife, and the increased energy required to bring the groundwater to the surface.
Many of the state’s groundwater basins are being managed sustainably – meaning that withdrawals are, at most, the amount replenished by man or by nature. Still, in some basins, especially those underlying major agricultural regions in the southern Central Valley and Central Coast, groundwater withdrawal far exceeds the amount that is recharged, causing overdraft conditions and threatening this vital resource.
California’s groundwater basins
Groundwater is held in geologic formations known as aquifers. An aquifer is an underground layer of permeable rock, sediment (usually sand or gravel), or soil that stores water in the spaces and voids between the granules. California’s groundwater supplies come from two types of aquifers: fractured rock aquifers and alluvial aquifers.
Fractured-rock aquifers store groundwater in fractures or other void spaces within the rocks; these types of aquifers are typically found in the mountain and foothill areas of the coastal ranges and Sierra Nevada, as well as in the volcanics of the Modoc Plateau. Wells served by these fractured rock formations typically have much smaller yields than an alluvial groundwater basin, with half of all fractured rock wells yielding only enough for individual domestic supplies. In addition, the limited storage capacity of these fractured rock systems can mean that supplies can vary dramatically over the summer and during dry years. (For more on fractured-rock aquifers, click here).
Alluvial aquifers are composed of sand, gravel, and other fine-grained sediments that store the water in the voids and spaces among the sediments. The Department of Water Resources has identified 515 alluvial groundwater basins across the state, which cover 62,000 square miles or about 42% of the state’s area. These basins are highly variable in their water yields, geologic origins, physical and hydrogeological characteristics, production properties, and water quality.
The most significant of the state’s groundwater basins are found in the Central Valley, where a structural trough forms an aquifer system extending from north of Red Bluff to south of Bakersfield, about 400 miles long and from 20 to 70 miles wide. The aquifer system is comprised primarily of sand, gravel, and clay deposits, with most of the freshwater found at depths of less than 2,500 feet. The Sacramento River drains the northern portion of the valley, the San Joaquin River drains the middle portion, while drainage in the Tulare Lake basin is entirely internal. Nearly three-quarters of the state’s groundwater supplies and 83% of the state’s agricultural groundwater use is extracted from the Central Valley aquifer system.
Coastal aquifers, which include several basins in the Bay Area, Central Coast, and Southern California regions, face a significant threat-seawater intrusion. This is a common problem for coastal aquifers, posing a serious challenge to the sustainability of these water sources. Other alluvial aquifers include the Eastern Sierra basins and the Mojave and Colorado deserts.
For more information …
- General facts and concepts about groundwater, by the USGS
- Basic concepts of groundwater hydrology, by Thomas Harter, UC Division of Agriculture and Natural Resources (6 pages)
California’s groundwater use by the numbers
Groundwater is a critically important part of California’s supply, accounting for 41% of total annual agricultural and urban water uses statewide in an average year and up to 58% or more in drought years. About 83 percent of Californians depend on groundwater for some portion of their water supply; for 6 million residents, it is their only supply.
The Department of Water Resources estimates that, on average, 16.5 million acre-feet of groundwater is extracted annually, with 39% going towards agriculture, 41% supporting cities, and 18% being used for managed wetlands. Nearly three-quarters of the state’s groundwater pumping occurs in the Central Valley, with the Tulare Lake region being the largest user of groundwater.
The state’s groundwater storage capacity is more than ten times that of all its surface reservoirs, although not all that water is accessible or of acceptable quality.
However, the state’s groundwater resources are not evenly distributed, creating a stark disparity. Whether or not there is groundwater in a particular area depends on the geology of the underlying soils. Some of the state’s largest cities, such as San Diego and San Francisco, have very little groundwater resources available, while others, such as Bakersfield, rely solely on groundwater to support their population. The Central Coast region is the most dependent on groundwater, with 90% of its supplies coming from groundwater aquifers.
Groundwater use and overuse
Groundwater can be replenished through various methods, such as precipitation, seepage from surface waters into aquifers, intentional recharge efforts, excess irrigation water, and even accidental leaks from pipelines and canals. The balance is maintained when the amount of water pumped out matches the amount recharged. However, if more groundwater is extracted than replenished, the aquifer can become overdrafted.
Overdraft can have many consequences: it can lower groundwater levels, raise energy costs for pumping, cause overlying land to subside, dry up domestic wells, allow saltwater intrusion in coastal areas, and reduce interconnected surface water supplies.
DWR estimates that, on average, 2 MAF is withdrawn from the state’s aquifers per year more than what is being recharged, and much more so during periods of drought. This is nothing new; scientists estimate that since California’s development in the late 1800s, the state’s groundwater reserves have been reduced by 125 MAF or 4.5 times the capacity of Lake Mead. Most of this groundwater depletion has occurred in the San Joaquin Valley.
The San Joaquin Valley is one of the most productive agricultural regions in the nation, producing more than $24 billion in crops and employing around 340,000 people. However, it has a long history of overdraft. Since about 1960, groundwater has been depleted by almost 60 million acre-feet, with the overdraft estimated at 1.5 to 2 MAF per year. During the 2012-2015 drought, the lack of surface water and dwindling groundwater basins caused thousands of domestic wells to go dry, the San Joaquin Valley floor to subside as much as three feet in some areas, and brought California’s agriculture to a halt.
Groundwater use remained unregulated by the state until the passage of the 2014 Sustainable Groundwater Management Act (SGMA).
The Sustainable Groundwater Management Act
In September 2014, Governor Brown signed the Sustainable Groundwater Management Act (SGMA), a three-bill legislative package to sustainably manage California’s groundwater basins. A key aspect of SGMA is the emphasis on local management, mandating the establishment of Groundwater Sustainability Agencies (GSAs) to create and develop groundwater sustainability plans to achieve sustainability within a 20-year timeframe.
Click here to learn more about the Sustainable Groundwater Management Act.
Groundwater quality
A host of natural and human-caused sources can impact groundwater quality. Aquifers can become contaminated by a single point source, such as a decommissioned military base, or by runoff from widespread areas, such as agricultural fields using fertilizers or urban stormwater carrying pollutants from roadways and urban landscapes. Even within a single aquifer, groundwater quality can change with the level of the groundwater table itself.
Groundwater quality generally degrades with increasing depth within an aquifer. Thus, declining groundwater levels can lead to poorer water quality and concentration of contaminants. Groundwater overdraft can create new water quality problems or make existing groundwater pollution worse. As aquifer levels decline from chronic overdraft, natural and man-made pollutants can concentrate in the remaining groundwater, making it unsafe for irrigation or drinking without costly treatment. In some cases, wells must be shut down.
Continued pumping can also cause polluted groundwater or seawater to migrate or be drawn into areas that would otherwise not be impacted.
The Sustainable Groundwater Management Act directs groundwater basins to be managed to avoid ‘significant and unreasonable’ impacts to groundwater quality.
- Community Water Center: Non-profit working to address groundwater issues in the Central Valley and other areas. Website has many resources for residents and others impacted by poor water quality.
- Self-Help Enterprises: Non-profit that works a catalyst in seeking funding and providing technical assistance to communities with contaminated water sources who are working towards community-wide solutions and a permanent source of clean drinking water. Also helps administer replacement drinking programs in applicable areas.
- Protecting Groundwater Quality in California: Management considerations for avoiding naturally occurring and emerging contaminants, from the Environmental Defense Fund
- Water quality degradation, webpage from the USGS
The Safe and Affordable Funding for Equity and Resilience (SAFER) program
The State Water Board’s Safe and Affordable Funding for Equity and Resilience (or SAFER), is a Newsom administration initiative to address the 1 million residents in the state with contaminated drinking water.
SAFER funds will help water systems provide a safe, accessible, and affordable supply of drinking water to communities in both the near and long terms by accelerating implementation of short- and long-term drinking water solutions, moving water systems to more efficient modes of operation, providing short-term operation and maintenance support as a bridge until long-term sustainable solutions are in place, and providing long-term operation and maintenance support when necessary.
Click here to learn more about the SAFER program.
State Water Board programs to protect groundwater quality
The State Water Board and the nine regional water boards protect groundwater through numerous regulatory and planning programs. The key elements of the water board’s approach include identifying and updating beneficial uses and water quality objectives, regulating activities that can impact the beneficial uses of groundwater, and preventing future groundwater impacts through planning, management, education, monitoring, and funding.
The State Water Board and the regional water boards have numerous programs to address this. Some of them are:
- The Irrigated Lands Program regulates discharges from irrigated agricultural lands. These discharges include irrigation runoff, flows from tile drains, and stormwater runoff. These discharges can affect water quality by transporting pollutants, including pesticides, sediment, nutrients, salts, pathogens, and heavy metals, from cultivated fields into surface waters, or by infiltrating down into the underlying groundwater aquifer. Click here to learn more about the Irrigated Lands Program.
- The Drinking Water Source Assessment and Protection (DWSAP) Program addresses both groundwater and surface water sources. The groundwater portion of the Program serves as the state’s wellhead protection program as required by the federal Safe Drinking Water Act. The program has two primary elements: Drinking Water Source Assessment and Source Protection. Since 1997, the Program has with the assistance of others—34 counties, the California Rural Water Association, and more than 500 water systems—completed assessments for nearly all the public drinking water sources in the state. Click here to learn more about the Drinking Water Source Assessment and Protection Program
- The Land Disposal Program regulates the discharge to land of certain solid and liquid wastes. In general, these wastes cannot be discharged directly to the ground surface without impacting groundwater or surface water, and therefore must be contained to isolate them from the environment. Click here to learn more about the Land Disposal Program.
- The Central Valley Salinity Alternatives for Long-Term Sustainability (CV-SALTS) is a collaborative basin planning effort aimed at developing and implementing a comprehensive salinity and nitrate management program. The Salt and Nitrate Control Program provides a new framework for the Regional Water Board to regulate salt and nitrate, while also ensuring a safe drinking water supply. While the CV-SALTS program is not directly related to SGMA, there is potential for significant overlap with GSAs implementing their GSPs. Click here to learn more about CV-SALTS program.
Learn more about how the Water Boards protect groundwater by clicking here.
For more information on California’s groundwater
Page updated August 16, 2024
