The North San Francisco Bay Shallow Aquifer study unit covers over 1,800 square miles near the coast in Central California. The study unit consists of 6 groundwater basins, some of which have multiple sub-basins (California Department of Water Resources, 2003), as well as the surrounding highland terranes and is split into two study areas (Bennett and Fram, 2014). Alluvial-filled valleys in the study unit were combined to form the Valleys and Plains study area. Volcanic, metamorphic, and ultramafic hard-rock highlands surrounding the Valleys and Plains form the Highlands study area.

The shallow aquifer system in the study unit is defined as those parts of the aquifer system shallower than the perforation intervals for public-supply wells used in the deeper public-supply aquifer system (Bennett, 2018). An assessment of the deeper public-supply aquifer system was done by Kulongoski and others (2010). Sampled wells in the shallow aquifer system ranged from 22 to 755 feet deep and were typically constructed using solid casings that extended below land surface to open or perforated intervals that ranged between 8 and 550 feet deep. Water quality in the shallow aquifer system can differ from that in the deeper public-supply aquifer system.

The study unit has warm, dry summers and cool, moist winters. Average annual rainfall is about 30 inches. The study unit is drained by several rivers and streams along with their principal tributaries. Land use in the study unit is approximately 73 percent natural (forest, grasslands, and bare rock), 16 percent urban, and 11 percent agricultural. Primary agricultural uses include pasture, vineyards, flower nurseries, and orchards (Bennett, 2018). The largest urban areas are the cities of Santa Rosa, Petaluma, Rohnert Park, and Napa. Recharge to the groundwater system is primarily by stream-channel infiltration from the major rivers and their tributaries and by infiltration of precipitation. The primary sources of discharge are water pumped for municipal supply, evaporation, and discharge to streams.