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The Groundwater Resource As Ca/~f()rni(l scarches (or wars to halallce ~mfer Sllf)jJly and demand, there is more interest in gmwu/- It'afer hanking - storillg water IfllderKI"OIl1/(! in wet Yl'ursIor wirhdrmral in dry ycars. Abol'l', the Kern WaleI' Hank receil'cd local jloot/flows ill 1995. 4 The state's enormous groundwater aquifers are estimated to hold nearly 20 times the amount of water that can be stored behind all of California's dams - in total, some 850 million acre.feet of water. (An acre-toot at water is 325,900 gallons, enough to meet the indoor and outdoor needs of one to two average households for a year.) Put another way, an acre-foot of water is enough to flood a football field - which is roughly an acre in size - 1 foot deep. If California were flat, the volume at its groundwater would be enough to flood the entire state 8 feet deep. WHAT IS GROUNDWATER? Groundwater resides in the pore spaces of soils and openings in geologic formations. Some of the water is contained within unsaturated lones, soils that contain air and gas in addition to some water. As a result, the water cannot be drawn into wells and pumped to the surface. Where impervious layers of material within the earth prevent the water from percolating farther down, water accumu- lates, creating a saturated zone that can be tapped by wells. Saturated zones - or aquifers - can be several feet thick or several thousand feet thick. The top of the saturated zone is known as the water table. Groundwater can be pumped from two general types of geologic formations: sediments and tractured hard rock. Sediments primarily are made up of sand, gravel, silt and clay known as alluvium that were deposited by rivers eroding the surrounding area. It might seem improbable then to say that California needs to conserve water. The problem, according to the state Department of Water Resources (DWR), is that only between 250 million acre-feet and 450 million acre-feet of the state's groundwater can be economically reached or is of a high enough quality. In addition. local aquifers are not always large enough to meet local demands. On average, about 15 million acre-feet of ground- water are pumped statewide each year. The supply has proven to be a reliable and resilient resource. The aqUifers are particularly valuable because unlike the state's surface water, which occurs predominantly in the northern and eastern mountains, groundwater is widely distributed throughout the state, underlying the land where it is needed. Groundwater also is available year-round unlike surface water runoff that flows heaviest each spring. About half of the state is underlain by a groundwater basin - 450 of them in all. Many are within 100 teet of the surface while others lie thousands of teet below ground. Some regions are overwhelmingly dependent on natural underground reservoirs Ihal the public does not see. More than 9 million Californians - nearly one in three - rely solely on groundwater to meet their needs, including the cities at Woodland, Fresno and Bakersfield. In rural areas and along California's central coast 90 percent of the drinking water is supplied by groundwater. Valleys and coastal plains are typically comprised of alluvium, and can hold large quantities of water. More limited sources of groundwater can be found in small fractures within the hard rock at the Sierra Nevada, coastal and other mountain ranges that were fractured as they were pushed upward by volcanic forces or by the collision of shifting plates of the earth's crust Though generally smaller in volume, many small wells tap these hard-rock sources. California's largest and most heavily used ground- water basins are in the Central Valley. where a huge volume at sediments - as much as 50.000 feet thick in the Sacramento Valley and 32,000 feet thick in the Tulare Basin - accumulated over millions of years. Much of the sediment was laid down during a time when the Pacific Ocean inundated the valley. The marine sediments are largely consolidated.