The Madera–Chowchilla and Kings shallow aquifer study unit consists of groundwater resources in the Madera, Chowchilla, and Kings groundwater subbasins of the San Joaquin Valley that are used for private domestic drinking water supply. Over 150,000 people in the study unit rely on private domestic wells for their water supply (Johnson and Belitz, 2017). These domestic wells generally are shallower than public drinking-water supply wells in the same areas (Shelton and Fram, 2017; Shelton and others, 2013; Burton and others, 2012). Groundwater quality can vary with depth in aquifer systems; in particular, shallower groundwater can be more susceptible to contamination from human activities at the land surface.

Land use in the study unit is approximately 72 percent agricultural, 14 percent urban, and 14 percent natural, and the largest urban area is the city of Fresno. The dominant crop types are orchards and vineyards. Irrigation return flows are the major source of groundwater recharge, and pumping for irrigation and municipal supplies is the major source of discharge.

Previous groundwater studies in this region have identified several water-quality constituents of concern in the shallow groundwater resources used by domestic wells, including nitrate, uranium, and fumigants (for example, Burow and others, 1999; Jurgens and others, 2010; Harter and Lund, 2012).

This study was designed to provide a statistically representative assessment of the quality of groundwater resources used for domestic drinking water in the Madera–Chowchilla and Kings subbasins. A total of 77 wells were sampled by the GAMA Priority Basin Project between August 2013 and April 2014 (Shelton and Fram, 2017). The wells in Madera-Chowchilla typically were 52 to 122 meters deep, and wells in Kings typically were 35 to 87 meters deep. Data from 27 domestic wells sampled by the USGS National Water Quality Assessment Project between April 2009 and April 2015 also were included in this study.