The GAMA-PBP is a comprehensive assessment of statewide groundwater quality in California. The first phase of the GAMA-PBP in 2004–15 assessed groundwater resources used for public drinking-water supplies. The second phase is assessing groundwater resources used for domestic drinking-water supplies. An estimated 2 million Californians rely on individual domestic wells or State small-system wells for drinking water, and far less information is available about these resources than about public-supply resources. The U.S. Geological Survey (USGS) began sampling wells for this second phase in 2012. Domestic wells typically are drilled to shallower depths in the groundwater system than public-supply wells. Shallow groundwater may respond more quickly and be more susceptible to contamination from human activities at the land surface than groundwater in deeper aquifers.