Water Education Foundation

Groundwater

Posted by: Aquafornia on August 19, 2008 at 4:18 pm

Groundwater typically accounts for about 30% of statewide water use in a year; it can be up to 40% in a drought year. Groundwater is not distributed evenly across the state; some cities, such as San Francisco and San Diego, have very little groundwater resources. Other cities, such as Bakersfield, have abundant groundwater resources. Along the central coast, 90% of the drinking water is from groundwater.

California has about 450 groundwater basins, which have been estimated to store about 850 million acre-feet of water; however, only about half of that is close enough to the surface to be pumped economically.

Even in average rainfall years, more groundwater is used than is replaced by precipitation, stream seepage, or artificial recharge systems. Annual statewide overdraft is estimated by the DWR to be approximately 1.4 million acre-feet in a normal year. Most of the overdraft occurs in the San Joaquin Valley.

Groundwater is recharged by precipitation, surface runoff, and irrigation. It can also be recharged by using imported water injected back into the aquifer, or by using imported water instead of pumping groundwater. When there is no rainfall, snowfall, or other source available for recharge, the aquifer can become overdrafted. DWR defines overdraft as “a condition in which ground levels decline over a period of years, and never fully recover, even in wet years.

Overdrafting can result in lowered water tables, poor water quality, and increased energy costs for pumping. Overdraft can also lead to land subsidence, as well as cause sea water and other contaminants to invade the aquifer. Subsidence occurs when overpumping leads to the collapse of the earth's surface overlying the aquifer; it is often irreversible. Subsidence can also cause salt water intrusion into coastal aquifers. Continued overdraft will eventually lead to exhaustion of the supply.

California is only one of two states left that has yet to enact a comprehensive statewide groundwater management system. Currently there are no controls over the amount drawn from underground aquifers. Only 19 groundwater basins have been adjudicated, most of those in Southern California.

To some, such a program equals a state-dictated system for a resource that historically has been considered a right of the overlying land owners. Agricultural interests oppose any regulation as they fear it would constrain pumping in drought years.

The quality of groundwater is of concern as well. Many of the state's groundwater basins are contaminated to some degree. Up until as recently as the 1970s, it was felt that groundwater basins did not need to be protected because of the widespread belief at the time that all contaminants, including toxic chemicals, were removed by percolation through soil and sediment. However, it has since been learned that a large variety of contaminants can migrate into groundwater supplies, and groundwater supplies must be carefully protected.

Groundwater contamination can come from landfills, leaked toxins, solvents, microbial agents, acid mine drainage and agricultural chemicals. The contaminants are usually concentrated in small sections of the basin, and can force some communities to abandon their wells and rely on imported surface water supplies.

Unfortunately, once contamination has been detected, it is costly and difficult to remove. Some aquifers may remain contaminated for hundreds of years, or quite possibly forever. It is therefore much cheaper to prevent contamination of groundwater basins then to remediate them afterwards.

Perchlorate is used in rocket propellants, road flares, fireworks, and air bag inflation systems. It is a chemical contaminant which is very mobile once it gets into water. Perchlorate has been showing up in an increasing number of Southern California’s groundwater sources, including Santa Clarita, Rialto, and Simi Valley. Experts have been unable to agree on a drinking water standard for perchlorate.   Some state agencies have set different minimums, and the EPA has declined to regulate it.   Environmentalists have been pushing for more stringent standards.

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