Addressing Nitrate in California’s Drinking Water Technical Report 4: Groundwater Nitrate Occurrence

This report reviews the hydrogeology and groundwater quality in the Tulare Lake Basin (TLB) and Salinas Valley (SV). We also assembled groundwater quality data from nearly two dozen local, state, and federal agencies and other sources into a dataset, here referred to as the (Central) California Ambient Spatio-Temporal Information on Nitrate in Groundwater (CASTING) dataset. The dataset combines nitrate concentrations from 16,709 individual samples taken at 1,890 wells in the Salinas Valley (SV) and from 83,375 individual samples taken at 17,205 wells in the Tulare Lake Basin (TLB) collected from the 1940s to 2011, accounting for a total of 100,084 samples from 19,095 wells. Almost 70% of these samples were collected from 2000 to 2010; only 15% of the samples were collected prior to 1990. Half of all wells sampled had no recorded samples prior to 2000.