Hillsborough County Water Report

Hillsborough County's drinking water received a D grade, scoring 41.5 out of 100. Over the past five years, EPA SDWIS records 89 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 50.0% of assessed waterways are impaired (1 of 2 water bodies) across Hillsborough County's watersheds. The leading impairment causes are mercury - fish consumption advisory and ph. 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 2026-05-14T14:30:00.000-04:00) puts MERRIMACK R at 4.0k cfs — running somewhat below its historical average at 72% of mean. 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. Hillsborough County has extensive coverage with 282 active monitoring sites with 25,750 recent measurements on record. Predominant monitoring categories include physical and microbiological. 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.