Approximately 2 million California residents rely on privately owned domestic wells for drinking water. During the California drought of 2012?16 groundwater levels declined in many parts of the state and wells were deepened in response. Most of the wells deepened during this time were domestic wells that were drilled into fractured bedrock throughout the Sierra Nevada foothills region of northern California. To understand the impacts of extreme drought on groundwater supply availability and quality in this setting, the United States Geological Survey completed a geochemical survey of domestic wells throughout the Yuba and Bear River watersheds during 201516 as part of the State Water Boards Groundwater Ambient Monitoring and Assessment Program Priority Basin Project (GAMA-PBP). This fact sheet highlights key findings from the GAMA-PBP assessment.