The Tulare Shallow Aquifer study unit covers approximately 2,320 square miles and consists of the Tulare, Kaweah, and Tule groundwater basins of the southern San Joaquin Valley and adjacent highlands of the Sierra Nevada (California Department of Water Resources, 2003; Bennett and others, 2017). The freshwater aquifer system in the groundwater basins is composed of Quaternary-age alluvial and fluvial sediments primarily derived from the Sierra Nevada to the east and is divided into upper and lower zones by a clay-rich lacustrine unit, the Corcoran Clay (Faunt, 2009). The highlands have fractured rock aquifers composed primarily of Mesozoic-age granitic rocks. The hydrology of the southern San Joaquin Valley has been extensively altered by human activity, especially by agricultural development (Faunt, 2009). This study examined the quality of groundwater resources used for domestic drinking-water supply. Domestic wells in the study unit typically are drilled to depths of 150 to 500 feet (Bennett and others, 2017), which is shallower than the depths of public-supply wells in the same area (typically 300 to 1,400 feet deep; Burton and others, 2012). Water quality varies with depth in the groundwater system.

This study was designed to provide a statistically representative assessment of groundwater resources used for domestic drinking water (Bennett and others, 2017). A total of 96 wells, distributed across the study unit, were sampled between November 2014 and April 2015. Previous groundwater studies in this area have found that nitrate, fumigants, microbial indicators, and several trace elements are present at elevated concentrations in some domestic wells (Burow and others, 1998; Jurgens and others, 2010; Harter and Lund, 2012; California State Water Resources Control Board, 2016).