Fact Sheets

The Mokelumne, Cosumnes, and American River Watersheds (MCAW) study unit covers approximately 9,000 square kilometers on the western slope of the Sierra Nevada. Groundwater composes about 10 percent of overall water use in the region, but is the sole supply for many individual homes outside the public water-supply infrastructure (Cosumnes, American, Bear, Yuba Integrated Regional Water Management Group, 2014). Many domestic wells in the MCAW study unit were deepened during the recent drought (California Department of Water Resources, 2014), highlighting the vulnerability of groundwater supplies and prompting increased interest in studies of Sierra Nevada groundwater systems. Well water in the MCAW study unit mostly comes from fractured-rock aquifers. The quality of groundwater in these aquifers primarily depends on the type of rock, the age of the groundwater, and the type of human activities at the land surface. Previous groundwater studies in this area found elevated concentrations of nitrate, microbial indicators, and some trace elements in some wells (California State Water Resources Control Board, 2005; Fram and Belitz, 2014).

This study was designed to provide a statistically representative assessment of the quality of groundwater resources used for domestic drinking water in the MCAW study unit. A total of 67 domestic wells and 1 domestic spring were sampled between August 2016 and January 2017 (Shelton and others, 2018). The wells in the study unit typically were 30 to 150 meters deep, and water levels typically were 2 to 42 meters below land surface.