San Luis Obispo County, California: drinking water report. San Luis Obispo County on California's Central Coast has a population of about 283,000 spread…
San Luis Obispo County on California's Central Coast has a population of about 283,000 spread across a mix of coastal towns, inland valleys, and agricultural communities. Water sources vary dramatically – the City of San Luis Obispo uses a blend of Whale Rock Reservoir, Salinas Reservoir, and Nacimiento Water Project supply, while smaller communities rely on local groundwater. Camp San Luis Obispo (California Army National Guard) and Vandenberg Space Force Base (in adjacent Santa Barbara County) add military facilities to the regional picture.
Nitrate contamination from agriculture and septic systems is a persistent concern in the county's groundwater basins. According to the Central Coast Regional Water Quality Control Board's 2024 agricultural monitoring report, 20% of tested wells in the Paso Robles and Atascadero groundwater basins showed nitrate above 50% of the MCL, with some exceeding it. Wine grape cultivation and cattle ranching are the primary nitrogen sources.
The EPA's UCMR5 data shows PFAS detections at low levels in some county systems. Camp San Luis Obispo has confirmed AFFF use, and the California DTSC added the facility to its PFAS investigation list in 2023. A 2024 State Water Board assessment found two community water systems in the county with PFAS detections above notification thresholds.
Rural well owners in the Paso Robles and Atascadero basins should test for nitrate – the agricultural activity in these areas makes it the most likely contaminant to exceed safe levels. Municipal systems treat for it, but private wells do not.
Check your water for the latest data on your area. For nitrate, reverse osmosis or ion exchange filters are effective. For PFAS, reverse osmosis handles both. Our water filter guide explains the options. Get your detailed report for trend data, and visit our California page for statewide context.