Nitrate contamination is widespread in California
Many wells, especially in the Central Valley, Southern California and Central Coast, have nitrate levels that are contaminated with at least 4 milligrams per liter of nitrate, which comes mostly from farms. The federal standard for drinking water is 10 milligrams per liter.
4-10 MG/L Nitrate as Nitrogen
10+ MG/L Nitrate as Nitrogen
Source: Groundwater Ambient Monitoring and Assessment Database, California Water Boards
Map: John Osborn D'Agostino, CalMatters