The Klamath Mountains study unit covers more than 8,800 square miles and consists primarily of hard rock terranes characterized by high peaks and deep river gorges with relatively few groundwater basins (Bennett and others, 2014). Most of the study unit consists of metamorphic, ultramafic, and granitic rocks. The primary aquifer system sampled in the study unit, in contrast to most other study units evaluated by the GAMA Priority Basin Project, consisted of localized areas of fractured bedrock. 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). The primary aquifer system in the study unit is defined as those parts of the aquifer corresponding to the open or perforated intervals of wells listed in the California Department of Public Health (CDPH) database.

Drinking water in the Klamath Mountains comes mainly from springs (approximately 18 percent [%] of sampled sites) and wells (approximately 82% of sites) screened in sediment from glacial, alluvial, or fluvial sources. Most wells that were sampled are drilled to depths of 25 to 400 feet, consist of solid casing or seal from the land surface to a depth of about 24 to 300 feet, and are open or perforated below the solid casing or seal. Water quality in the primary aquifer system may differ from that in the shallow and deep water-bearing zones.

Land use in the study unit is approximately 97% natural (forest, grasslands, and bare rock), 2% urban, and 1% agricultural.

The study unit has warm, dry summers and cool, moist winters. Average annual rainfall ranges from 69 inches on the western edge of the study unit to 40 inches in the central and eastern parts of the study unit. The study unit is primarily drained by the Klamath-Trinity, Sacramento, and Smith River drainage systems.

Groundwater recharge in the study unit occurs from the ambient recharge of runoff and snowmelt that penetrates the bedrock through faults and fractures at higher elevations. The primary sources of groundwater discharge are water pumped for municipal supply, evaporation, and discharge to streams.