Napa County Water Report

Napa County's water systems carry a failing grade, scoring 33.4 out of 100. Over the past five years, EPA SDWIS records 67 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 19.5% of assessed waterways carry an impairment designation (8 of 41 water bodies) across Napa 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 2026-05-14T12:00:00.000-07:00) puts NAPA R at 37.6 cfs — well below its long-term average at 19% 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. Napa County has moderate coverage with 14 active monitoring sites with 5,284 recent measurements on record. Predominant monitoring categories include physical 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.