Ground water in Segment 1 is contained in five major aquifers (fig. 11), four of which consist primarily of basin-fill deposits that occupy structural depressions caused by deformation of the Earth's crust. The four basin-fill aquifers are the Basin and Range aquifers, the Central Valley aquifer system, the Coastal Basins aquifers, and the northern California basin-fill aquifers. The fifth major aquifer is the northern California volcanic-rock aquifers. Few of these aquifers extend over an area large enough to be termed "regional." An exception is the Central Valley of California, which is a very large basin-fill aquifer best described as a "regional" aquifer. Some water in Segment 1 is obtained from areally-extensive volcanic and carbonate rocks, but water within these rocks is mostly in fractures or solution openings and, consequently, the rocks generally yield little water. One notable exception is in eastern Nevada, where some alluvial basins are hydraulically connected by widespread deposits of permeable carbonate rocks that underlie the alluvium. In some places, consolidated rocks are hydraulically connected to overlying unconsolidated deposits and, thus, are part of the same aquifer or aquifer system.
Because of the scattered, local nature of most of the aquifers, it is difficult to classify them. The grouping used herein is based on geology, physiography, and climate but is not the only one possible. The areas designated "not a principal aquifer" lack sufficient basin-fill sediments or permeable consolidated rock to yield significant amounts of water to wells.
The Basin and Range aquifers are located in an area that comprises most of Nevada and the southern California desert (fig. 11). The water-yielding materials in this area are in valleys and basins, and consist primarily of unconsolidated alluvial-fan deposits, although locally flood plain and lacustrine (lake) beach deposits may yield water to wells. Also, the consolidated volcanic and carbonate rocks that underlie the unconsolidated alluvium are a source of water if the consolidated rocks are sufficiently fractured or have solution openings. Many of these valleys and basins are internally drained; that is, water from precipitation that falls within the basin recharges the aquifer and ultimately discharges to the land surface and evaporates within the basin. Ground water is generally under unconfined, or water-table, conditions at the margins of the basins, but as the unconsolidated deposits become finer grained toward the centers of the basins, the water becomes confined. Rarely, basins might be hydraulically connected in the subsurface by fractures or solution openings in the underlying bedrock. These multiple-basin systems end in a terminal discharge area, or sink, from which water leaves the flow system by evaporation. Also, several basins or valleys may develop surface-water drainage that hydraulically connects the basins, and ground water flows between the basins, mostly through the unconsolidated alluvial stream/flood plain sediments.
The Central Valley aquifer system (fig. 11)occupies most of a large basin in central California between the Sierra Nevada and the Coast Range Mountains. The Central Valley is the single most important source of agricultural products in the United States, and ground water for irrigation has been essential in the development of that industry. The basin contains a single, large, basin-fill aquifer system, the largest such system in the Nation. Although the valley is filled with tens of thousands of feet of unconsolidated sediments, most of the fresh ground water is at depths of less than 2,500 feet. Ground water in the valley is under unconfined to confined (artesian) conditions, primarily depending on depth; most of the shallow ground water is unconfined.
The Coastal Basins aquifers occupy a number of basins in coastal areas from northern to southern California (fig. 11). These basins have similar morphology and a Mediterranean climate. All are in structural depressions formed by folding and faulting, all are filled with marine and alluvial sediments, and all are drained by streams that contain water at least part of the year. Nearly all the large population centers in Segment 1 are located in these basins, and the available ground water is used primarily for municipal supplies. In most of the basins, however, population has grown to such an extent that local ground-water supplies are no longer adequate, and surface water must be transported from distant sources to meet demand. Ground water in the basins is under unconfined to confined conditions, and two or more vertically sequential aquifers can be present in a basin, separated by confining units that consist of fine-grained sediments. In nearly all basins that contain more than one aquifer, however, the aquifers are hydraulically connected to some degree. Seawater intrusion is a common problem in nearly all the Coastal Basins aquifers.
Interior northern California is sparsely populated, and most ground-water demand is for agricultural irrigation. The most productive and highly-utilized aquifers in the area are the northern California basin-fill aquifers (fig. 11). These aquifers are in unconsolidated alluvial sediments. However, in some basins, wells drilled into underlying volcanic rocks might produce large quantities of water, often more than wells completed in the unconsolidated sediments.
The northern California volcanic-rock aquifers consist of volcanic rocks that yield water primarily from fractures and locally from intergranular spaces in porous tuffs. Because water-yielding zones in these rocks are unevenly distributed, wells that yield water are outnumbered by dry holes; however, in some areas, wells completed in the volcanic-rock aquifers yield large volumes of water. The northern California volcanic-rock aquifers are relatively unexplored and undeveloped.
