Santa Clara County Water Report

Santa Clara County earns a B grade for drinking water quality, scoring 67.5 out of 100. Over the past five years, EPA SDWIS records 45 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 46.0% of assessed waterways are impaired (40 of 87 water bodies) across Santa Clara County's watersheds. The leading impairment causes are mercury and diazinon. 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 2026-05-14T12:00:00.000-07:00) puts COYOTE C AB HWY 237 A MILPITAS CA at 13.8 cfs — well below its long-term average at 24% 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. Santa Clara County has moderate coverage with 45 active monitoring sites with 11,048 recent measurements on record. Predominant monitoring categories include physical and organics, pesticide. 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.