Groundwater
If California were flat, the volume of its groundwater would be enough to flood the entire state 8 feet deep. The enormous cache of underground water helped the state become the nation’s top agricultural producer. Groundwater also provides a critical hedge against drought to sustain California’s overall water supply.
In years of average precipitation, about 40 percent of the state’s water supply comes from underground. During a drought, the amount can approach 60 percent.
Sources
Groundwater comes mostly from snowmelt and rain percolating through soil and rock and from overland rivers, streams and other waterways. The infiltration is gradual, driven by gravity. Groundwater will flow toward rivers, lakes or the ocean to begin the cycle anew. Groundwater can be pumped or flow naturally to the surface through seepage or springs.
Groundwater can be thousands of years old, although typically it is pumped within years or decades after it originally moved underground through porous material called aquifers.
Aquifers can be several feet, or several thousand feet thick. California’s alluvial aquifers are composed of gravel, sand, silt and clay eroded from surrounding rocks and deposited by running water and wind.
California’s Underground Supply
It’s unclear exactly how much water can be stored within California’s 515 alluvial groundwater basins and subbasins. The state Department of Water Resources (DWR) estimates their total storage capacity at more than 850 million acre-feet, more than 10 times that of all the state’s surface reservoirs. One acre-foot is enough to supply two to three households for a year in California.
About 83 percent of Californians rely on groundwater for some portion of their water supply. The amount of groundwater used in a community’s water supply varies considerably, depending on the availability of surface water supplies and the cost to pump, treat and distribute groundwater.
California’s largest and most heavily used groundwater basins are in the Central Valley, where aquifers are generally very permeable. DWR estimates as many as 2 million wells tap the underground supply.
Groundwater is also heavily used in the Salinas and Santa Clara valleys, the coastal plains of Los Angeles and Orange counties, and the interior basins of Riverside, San Bernardino and Inyo counties.
Interconnection with Surface Water
The state’s surface water and groundwater are intimately connected. Rivers and creeks are fed from groundwater, which, in turn, is recharged by surface water. Surface water diversions can deplete groundwater just as groundwater pumping can reduce the flows of nearby waterways.
Polluted surface water degrades groundwater and groundwater contaminants affect surface water quality. The use, transfer, depletion or contamination of one can directly affect the other.
Flood and furrow irrigation and unlined irrigation canals are important sources of groundwater recharge. The rate of absorption of rainwater into the ground is influenced by many factors. The amount of water that reaches the water table is called natural groundwater recharge.
How much recharge occurs depends upon river conditions, the rate and duration of rainfall and irrigation, soil moisture conditions, the water table depth and the soil type. It generally takes several years of average precipitation to recharge aquifers in California to pre-drought levels.
The time it takes for surface infiltration to reach an aquifer as deep as 400 feet may take hours, days, or even years. In some flood-irrigated areas, groundwater levels in nearby domestic wells rise within a few hours to days of flood-up.
Challenges
Some communities including Orange County and Sonoma County have figured out ways to better manage their groundwater. Other communities have wrestled with the consequences of pumping too much groundwater, including seawater intrusion into coastal aquifers, reduced stream flows, land subsidence, drying of wells and loss of wetlands, riparian habitat and associated aquatic species.
Some groundwater is polluted with natural elements leached from the earth, including chromium, radon, boron and arsenic. Other contamination comes from industrial and agricultural wastes in the form of synthetic pesticides, herbicides, fertilizers, cleaning solvents and fuel ingredients. [See also Groundwater Treatment.]
Land subsidence from groundwater pumping continues in many areas of California, some places at a rate of more than 1 foot a year. In the San Joaquin Valley, subsidence has damaged and slowed the conveyance of surface water through the California Aqueduct, Delta Mendota Canal, Friant-Kern Canal and San Luis Canal, as well as private canals, bridges, pipelines and storm sewers.
Management
Until recently, California did not regulate groundwater use beyond the general dictum of putting it to a “beneficial” use. [See Groundwater Law.] A new era of groundwater management began after Sept. 17, 2014, when Gov. Jerry Brown signed historic legislation called the Sustainable Groundwater Management Act (SGMA).
The law empowers local officials to halt the trend of “critically overdrafted” or depleted basins. Under SGMA, high- and medium-priority groundwater basins were required to establish local Groundwater Sustainability Agencies (GSA) to manage their basins or submit an alternative plan that meets the goals of the law.
Those agencies must create and adopt state-approved plans that halt overdraft and bring groundwater basins into balanced levels of pumping and recharge. For critically overdrafted basins, balance must be achieved by 2040. For high- and medium-priority basins, 2042 is the deadline. The first of the local sustainability plans began to be implemented in 2020.
Improving recharge capabilities is a central focus of most plans submitted to the state, but this includes several moving parts – finding the ideal geologic sites, erecting conveyance and having the upstream storage to hold the peak flows that are to be sent to the recharge basin. The promise is that recharge facilities can be built much faster than new surface storage, which takes as long as 20 years.