The Central Sierra Nevada study unit is approximately 370 square miles and includes no California Department of Water Resources defined groundwater basins. The study unit was divided into two study areas: the Coarse Gold study area corresponding to the watershed of the Fresno River above Hensley Lake, and the Wishon study area corresponding to the watershed of Willow Creek, a tributary of the San Joaquin River (Ferrari and others, 2008).

The primary aquifers in the study unit are fractured bedrock. Most of the bedrock is granitic rock, which is the most abundant rock type in the Sierra Nevada, with the remainder being metamorphic rock. Groundwater occurs in open joints, fractures, and exfoliation planes in the bedrock. These 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).

The primary aquifers in the study unit are defined as those parts of the aquifers corresponding to the open intervals of wells listed in the California Department of Public Health database. In the Central Sierra Nevada study unit, these wells typically are drilled to depths between 300 and 750 feet, consist of solid casing from land surface to a depth of about 50 to 75 feet, and are open below the solid casing.

In the Sierra Nevada, ore deposits commonly are associated with the contacts between granitic and metamorphic rocks. The Central Sierra Nevada study unit contains many small mining prospect sites (nearly all inactive), mainly for gold or tungsten (U.S. Geological Survey, 2005). Minerals associated with naturally-occurring ore deposits commonly contain high concentrations of constituents that may adversely affect groundwater quality.

The Central Sierra Nevada study unit has warm, dry summers and cold, wet winters. Average annual precipitation ranges from 20 inches at Hensley Lake to 50 inches in the mountains in the northern part of the study unit. Land use in the Central Sierra Nevada study unit is approximately 95 percent (%) undeveloped (forests and grasslands), and 5% urban. The undeveloped lands are used mostly for recreation and open-range grazing. The largest urban area is the city of Oakhurst.

Municipal and community water supply accounts for nearly all of the total water use in the Central Sierra Nevada study unit. Groundwater provides nearly all of the water supply, with limited use of surface water in some areas. Recharge to the groundwater flow system is mainly from stream-channel infiltration and direct infiltration of precipitation. Groundwater leaves the aquifer system mainly when it is pumped for water supply or flows into streams or lakes.