Groundwater quality in the Monterey–Salinas Valley Public-Supply and Shallow Aquifer Systems was investigated by the GAMA-PBP. The Monterey–Salinas Valley Public-Supply Aquifer System study unit (MS-PA) initially was assessed in 2005 (Kulongoski and Belitz, 2011). The Mon-terey–Salinas Valley Shallow Aquifer System study unit (MS-SA) was sampled in 2012–13 (Burton and Wright, 2018). The MS-PA and the MS-SA largely coincide areally; however, they represent different parts of the aquifer system vertically. The MS-PA examined deeper groundwater primarily used for public supply, whereas the MS-SA examined relatively shallow groundwater primarily used for domestic supply. The MS-SA was divided into four study areas. Three of these study areas combined (MS-SA3) were equivalent to the MS-PA study unit excluding a small sliver along the Carmel River. The fourth MS-SA study area, Highlands, is composed of the hills and mountains bordering the Salinas Valley.

The MS-PA and the MS-SA study units have warm, dry summers and cool, moist winters. The average annual rainfall is 331 millimeters (mm; 13 inches, in.) with average annual temperatures of14 degrees Celsius (57 degrees Fahrenheit) in Salinas. The study units are drained by three main rivers and their tributaries: the Salinas, Pajaro, and San Lorenzo Rivers.

Groundwater quality status and understanding assessments of the study units were based on data collected by the U.S. Geological Survey (USGS). In the MS-PA, additional inorganic data was obtained from the California State Water Resources Control BoardDivision of Drinking Water Public- Supply Well Water-Quality Database (Kulongoski and Belitz, 2011).

A grid-based method to select wells was applied in both study units, which allows for an estimation of the proportions of the aquifer system with 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 of those results to other GAMA Priority Basin Project study areas (Belitz and others, 2010).