Groundwater quality in the North San Francisco Bay area Public-Supply and Shallow Aquifer Systems was investigated by the GAMA-PBP. The North San Francisco Bay Public Supply Aquifer System study unit (NSF-PA) was sampled in 2004 (Kulongoski and others, 2010). The North San Francisco Bay Shallow Aquifer System study unit (NSF-SA) was sampled in 2012 (Bennett, 2018). The NSF-PA and NSF-SA largely coincide areally; however, they represent different parts of the aquifer system vertically. The NSF-PA examined deeper groundwater primarily used for public supply, whereas the NSF-SA examined relatively shallow groundwater primarily used for domestic supply. Both study units were divided into two study areas: (1) alluvium-filled groundwater basins called the Valleys and Plains study area and (2) volcanic, metamorphic, and ultramafic hard-rock highlands surrounding the Valleys
and Plains called the Highlands study area (Bennett, 2018).

Groundwater-quality status and understanding assessments of the study units were chiefly based on data collected by the U.S. Geological Survey. In the NSF-PA, additional inorganic data came from the California State Water Resources Control Board Division of Drinking Water Public Supply Water-Quality Database (Kulongoski and others, 2010).

A grid-based site-selection method was used in both study units, which allowed for an estimation of the proportions of the groundwater resource having constituents at low, moderate, or high concentrations relative to
regulatory and non-regulatory benchmarks for drinking-water quality established by the U.S Environmental Protection Agency (EPA) or California State Water Resources Control Board Division of Drinking Water. The grid-based method provides statistically unbiased results and permits comparison to other GAMA-PBP study areas (Belitz and others, 2010).