Loudoun County Water Report

Loudoun County earns a B grade for drinking water quality, scoring 68.6 out of 100. Over the past five years, EPA SDWIS records 7 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 54.3% of assessed waterways are impaired (76 of 140 water bodies) across Loudoun County's watersheds. The leading impairment causes are escherichia coli (e. coli) and benthic macroinvertebrates bioassessments. 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 2026-05-14T14:45:00.000-04:00) puts GOOSE CREEK at 64.9 cfs — well below its long-term average at 20% 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. Loudoun County has extensive coverage with 62 active monitoring sites with 15,332 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.