San Mateo County Water Report

San Mateo County earns a B grade for drinking water quality, scoring 67.1 out of 100. Over the past five years, EPA SDWIS records 19 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 notable 23.8% of assessed waterways carry an impairment designation (25 of 105 water bodies) across San Mateo County's watersheds. The leading impairment causes are pathogens and mercury. 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 SAN GREGORIO C A SAN GREGORIO CA at 13.1 cfs — well below its long-term average at 38% 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 Mateo County has extensive coverage with 71 active monitoring sites with 32,510 recent measurements on record. Predominant monitoring categories include microbiological and nutrient. 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.