The Yuba River and Bear River Watersheds study unit (Yuba-Bear Watersheds) covers approximately 4,400 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 beyond the limits of public water supply infrastructure (Cosumnes, American, Bear, Yuba Integrated Regional Water Management Group, 2014). Recent drought conditions highlighted the vulnerability of private wells to diminished groundwater supplies in the study area and many wells required deepening (California Department of Water Resources, 2014).

Well water in the Yuba-Bear Watersheds 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, 2010; 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 Yuba-Bear Watersheds. A total of 71 wells and 4 springs were sampled between October 2015 and May 2016 (Jasper and others, 2017). The wells in the study unit typically were 30–150 meters deep, and water levels typically were 7–25 meters below land surface.