The Sierra Nevada Regional (SNR) study unit covers approximately 25,000 square miles and includes the Sierra Nevada mountain and foothill regions. The study unit was divided into four study areas corresponding to the four major rock types present in the region: granitic rocks, metamorphic rocks, sedimentary deposits, and volcanic rocks (Shelton and others, 2010). In contrast to most other study units evaluated by the GAMA PBP, the SNR study unit consists primarily of fractured-bedrock aquifers.

Most of the Sierra Nevada consists of granitic and metamorphic rocks. Groundwater is in fractures in the rocks. The fracture systems may be interconnected or isolated, resulting in variability in water levels, well yields, and water quality on local and regional scales (California Department of Water Resources, 2003). Sedimentary basins contain glacial, alluvial fan, fluvial, and lake sediments. Aquifers composed of different materials commonly contain groundwater with different chemical compositions.

The primary aquifer is defined as those parts of the aquifer system tapped by wells and springs listed in the State of California database of public drinking-water supply sources. In the SNR study unit about 25 percent (%) of these sources are springs. Most wells are drilled to depths of 150 to 600 feet, consist of solid casing or a seal from the land surface to a depth of about 50 to 200 feet, and are open or have perforated casing below that depth. Water quality in the primary aquifer system may differ from that in the shallower and deeper parts of the aquifer system.

Average annual precipitation ranges from 11 inches at low elevations to more than 80 inches at high elevations, and the majority of precipitation falls as snow. Rivers flowing from the Sierra Nevada provide a large part of the surface water used elsewhere in the State. About 90% of the land in the study unit is natural, and about 60% consists of national forests, national parks, and other federally managed lands. Urban land use is about 5% and is rapidly growing, placing increasing demands on groundwater supplies. The remaining 5% is in agricultural use.

Recharge to fractured-bedrock aquifers is mainly from stream-channel infiltration and direct infiltration of precipitation and snow melt. Sedimentary basin aquifers also are recharged by mountain-front recharge at the margins of the basins. Groundwater exits the aquifer system when it is pumped for water supply, flows into streams and lakes, discharges from springs, or leaves areas with a shallow depth to groundwater by evapotranspiration.