The U.S. Geological Survey collected groundwater samples from 38 wells used for domestic and small system drinking water supplies in Butte, Sutter, and Yuba counties of California during 2021. The wells were sampled for the Butte-Sutter-Yuba Domestic-Supply Aquifer Study Unit of the State Water Resources Control Board Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project’s assessment of the quality of groundwater resources used for domestic and small system drinking water supplies. The study unit was defined by the extent of the Butte, Los Molinos, North and South Yuba, Sutter, Vina, and Wyandotte Creek groundwater subbasins of the Sacramento Valley and an uplands area adjacent and to east of these subbasins within Butte County. The study unit was divided into a total of 40 grid cells across two study areas with 25 grid cells in the Sacramento Valley study area and 15 in the Upland study area. One or more domestic or small system wells were sampled in 38 of the 40 of the grid cells. Groundwater samples were analyzed for field water-quality parameters, volatile organic compounds, pesticides and pesticide degradants, stable isotopes of water, nutrients, major ions and trace elements, hexavalent chromium, perchlorate, per- and polyfluoroalkyl substances (PFAS), and microbial indicators. Groundwater levels were measured in 25 of the 38 wells sampled.