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impermeable and contain highly saline water. On top of the manne deposits lie sand, gravel, clay and silt that washed oul of the surrounding mountains. These layers of alluvium now form vast fresh water under- ground aquifers capped with fertile topsoil. California's coastal region and interior deserts also have aquifers that developed in the earthen debris that eroded from adjacent mountains. 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 all have intensely utilized alluvial aquifers, In many places in California, groundwater moves through sand and gravel that are not overlain by impervious materials. These aquifers are unconfined. In some places, the water is bound by impervious layers of earth and is said to be in confined aquifers. Increasingly, geologists are recognizing that confine- ment is a matter of degree where in some places, water travels freely within and among aquifers, and elsewhere the groundwater moves slowly between aquifers, if at all. Water makes its way into underground basins - or is said to "recharge" - whenever precipitation and irrigation exceed the amount of water that evapo- rates from the sailor is consumed by plants. Precipitation and irrigation water that exceeds the percolation rate of the ground runs off into streams and rivers. Some of the water that recharges valley groundwater basins is precipitation that falls in the valley and soaks directly into the soil. But much of the water that fills the valley aquifers comes from the same place as the sand and gravel alluvium - the surrounding mountains. California's coastal and interior mountains capture rain and snow from Pacific-borne storms sweeping eastward onto the continent. The mountain soil can hold moisture like a sponge, allowing water to slowly seep into the earth. Since much of winter's precipi- tation falls as snow, the runoff can extend into the early summer - allowing snow melt to percolate into Ihe mountains, seep into streambeds and eventually make its way into valley aquifers. Recharge generally occurs from the infiltration of water from the rivers and small streams that flow out of the surrounding mountains and from direct seepage of water from the soils and rocks that make up the mountains to the alluvium. Once this recharge water enters the alluvium, it slowly migrates toward the lowest points in the water table. Prior to modern development, this recharge water generally moved through the aquifer toward the lowest parts of the valley floOL In these areas the land surtace was often below the top of the water table and water would seep upward to Ihe land surface forming marshes, contributing to the flow of the streams in these low-lying areas, Human development altered this regime. Ground- water pumping has lowered the water table in many areas - drawing groundwater toward the areas of intense pumping rather than to marshes and streams. Agricultural and landscape irrigation, in turn, have increased the amount and distribution of water that percolates through the valley soils and into the aquifers, While percolating irrigation water helps to restore aquifers, it also can carry fertilizers, pesticides and minerals into the groundwater. Following a single drop of snowmelt reveals the dynamic connections between surface water and groundwater. Freed from the snow pack by a warm ray of sun, the drop seeps into a crack of Sierra granite, bubbles out of the fracture into a spring into a cascading stream, seeps back into the cobbles, reemerges into the sunlight as the river spills into the valley, soaks into the sands underlying a slow- moving pool. and flows through buried gravels into a low-lying marsh. H ISTORV OF USE California's first European settlers relied on water from streams and springs to meet their needs, including irrigation. The drought of 1880, however, prompted farms and communities to tap the ground- water for the first time in a significant way. Resorting to technologies that had existed for thousands of years, settlers dug shallow wells to expose shallow water tables. At first. the pressure in these lull aquifers - the "head" that moved groundwater into low-lying marshes and streams - was enough to push water up to the surface, creating what are known as flowing artesian wells. As more groundwater was used and water tables fell, windmills and piston pumps were employed to lift the water to the surtace. In the 1920s, the inven- tion of the deep-well turbine pump and the electrifi- cation of rural California put water hundreds of feel below the surface within reach for the first time. This allowed people to pump larger volumes of water. In the 1940s and 1950s, pumping increased sharply as agricultural operations expanded, particularly in the Central Valley. Groundwater Well .~ , ~ . :~ , I :~ I Casing ,: I Filterpa(k~ ... . ;JI;; I I .. ~ .. ) ~ Water Table ,I q :;..; d ~I Pump Column ; ~ " """ ~\". , . " ... : i <j' ~ ~41 ~ , . . , , Turbine Pump t .. ',' ~ - r!'- !: t .: , " l ..... .. ~~... . - ........ s