The Monterey-Salinas Shallow Aquifer study unit covers approximately 7,820 square kilometers (km²) in Santa Cruz, Monterey, and San Luis Obispo Counties in the Central Coast Hydrologic Region of California. The study unit was divided into four study areas—Santa Cruz, Pajaro Valley, Salinas Valley, and Highlands. More than 75 percent of the water used for drinking-water supply in the Central Coast Hydrologic Region of California is groundwater, and there are more than 8,000 well driller’s logs for domestic wells (California Department of Water Resources, 2013).

The source of groundwater in the shallow aquifer system in Santa Cruz, Pajaro Valley, and Salinas Valley study areas is primarily sediments in the Purisima Formation, the Aromas Sand, the Monterey Formation, and Holocene alluvial deposits; the source of groundwater in the Highlands study area is aquifers in granitic, metamorphic, or lithified sedimentary rocks (California Department of Water Resources, 2003; Hanson, 2003).

This study was designed to provide a statistically representative assessment of the quality of groundwater resources used for domestic drinking water. A total of 100 wells and 70 household tap sites were sampled between October 2012 and May 2013 (Goldrath and others, 2016). Domestic wells in the study unit typically were drilled to depths of 7-275 meters (Goldrath and others, 2016), which is shallower than the depths of public-supply wells in the same area (typically 20-600 meters deep; Kulongoski and Belitz, 2011). Previous studies in the area have indicated water-quality issues, including seawater intrusion and elevated concentrations of nitrate, arsenic, boron, molybdenum, and gross alpha radioactivity (Hanson, 2003; Kulongoski and Belitz, 2011; Harter and others, 2012).