San Diego County Water Report

San Diego County's drinking water earned a C grade, scoring 54.6 out of 100. Over the past five years, EPA SDWIS records 378 health-based violations — a pattern that public water utilities are required to disclose and correct.

Under the Clean Water Act §303(d), EPA ATTAINS tracks whether waterways meet quality standards for drinking, recreation, and aquatic life (reporting cycle: 2022). A substantial 38.8% of assessed waterways are impaired (193 of 497 water bodies) across San Diego County's watersheds. The leading impairment causes are pathogens and nitrogen. Impairment does not mean tap water is unsafe — it measures ambient waterway conditions upstream of treatment, not finished drinking water.

USGS NWIS gauge data (as of 2026-05-14T11:15:00.000-07:00) puts SANTA MARGARITA R A YSIDORA CA at 0.0 cfs — well below its long-term average at 0% of mean — low-flow conditions worth noting for water-dependent ecosystems. Streamflow is a leading indicator of drought stress, sediment load, and dilution capacity: low flows concentrate pollutants and warm water temperatures, stressing aquatic life and, in surface-water-dependent systems, the source water quality for treatment plants.

EPA's Water Quality Portal (WQP) aggregates monitoring data from federal, state, and tribal agencies. San Diego County has extensive coverage with 284 active monitoring sites with 78,731 recent measurements on record. Predominant monitoring categories include microbiological and physical. More monitoring sites generally indicate greater scientific attention to local water conditions — and provide the baseline data that regulators use to set future impairment listings.