Regulatory Guide - Ground Water Issues

Vast stores of underground water played a leading role in transforming California into the nation’s top agricultural producer and most populous state. Today, ground water supplies about 40 percent of the water Californians use or about 16 to 17 million acre-feet per year (acre-foot = the amount of water needed to cover an acre to the depth of one foot or approximately 326,000 gallons). The state’s largest subterranean reserves lie beneath the fertile farmlands in the Central Valley. An estimated 100 million acre-feet of usable water is available. Worldwide, geologists say about 90 percent of the world’s usable water supply occurs as ground water.

It wasn’t until the beginning of this century that ground water came into widespread use for irrigation because advances in drilling and pumping technology made the water easier to access.

Ground water use became so widespread that by the 1930s dropping water tables and escalating pumping costs focused efforts on development of surface waters and construction of extensive storage and conveyance systems such as the State Water Project and the Federal Central Valley Project. Ground water continues to meet many of California’s water needs.

Aquifers

For the most part, ground water is not found in underground lakes or streams but within the pore spaces of sediments such as sand and gravel. Layers of these sediment types and some locally occurring layers of silt and clay, make up an aquifer.

A ground water basin may contain one aquifer or a number of aquifers underlying a depression bounded completely or in part by hills or mountains. Ground water basins may range in size from less than one square mile in mountain ranges to more than a thousand or more square miles in desert or flatlands. Ground water basins underlie nearly half of the state.

The total storage capacity of our ground water basins is estimated at one billion acre-feet. About 250 million acre-feet — representing several times the amount of water stored in all state surface reservoirs combined — is usable (pumpable). Aquifers in basins collect water from rainfall as it infiltrates the soil or it may be placed artificially through man-made percolation ponds or injection wells. Unlike a rushing stream or river current, water in an aquifer moves very slowly. The rate at which water moves through an aquifer depends in part on permeability (water moves more quickly through something coarse like gravel, than through fine-grained sand).

Aquifers act as natural conveyance and water purification systems while the land above a ground water basin is put to other uses. Ground water is not exposed to the elements and is protected from evaporation loss and some forms of pollution. When surface water is depleted during droughts, aquifers serve as a convenient water source although not without a price, for example higher electric bills and lowered water table.

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