The Bear Valley and Lake Arrowhead Watershed study unit covers about 112 square miles (290 square kilometers) in the San Bernardino Mountains of southern California. The Bear Valley study area is an alluvium-filled valley surrounding Big Bear and Baldwin Lakes and corresponds to the Bear Valley groundwater basin (California Department of Water Resources, 2003). The Lake Arrowhead Watershed study area consists primarily of granitic bedrock and includes parts of six watersheds around Lake Arrowhead (Mathany and Burton, 2017).

This study examined the quality of groundwater resources used for public drinking water supply in the two study areas. In 2010, the U.S. Geological Survey (USGS) collected samples from 38 wells and springs distributed across the two study areas. Data from 2007 to 2010 were compiled from the California State Water Resources Control Board (SWRCB) Division of Drinking Water database for 78 other public-supply sources in the study unit.

In the Bear Valley study area, public-supply wells are typically drilled to depths between 250 and 550 feet and screened in sedimentary deposits, but sometimes extend into the underlying metamorphic and granitic basement rocks. Groundwater is primarily recharged by winter precipitation in the mountains surrounding the valley, and groundwater pumping for municipal use has resulted in decreased groundwater levels in the Bear Valley study area (Flint and Martin, 2012). In the Lake Arrowhead Watershed study area, more than half of the public-supply sources are springs or horizontal wells, and vertical wells typically are drilled to depths of 230 to 500 feet. Wells and springs tap groundwater in the fracture systems of granitic bedrock.

Land use in both study areas is a mixture of natural and urban. Natural land cover is predominately forests. The Bear Valley study area has much more urban land use than the Lake Arrowhead Watershed study area, at 58 percent and 21 percent, respectively.