Madison County Water Report

Madison County earns a B grade for drinking water quality, scoring 63.4 out of 100. Over the past five years, EPA SDWIS records 12 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 30.5% of assessed waterways are impaired (131 of 429 water bodies) across Madison County's watersheds. The leading impairment causes are phosphorus, total 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 2026-05-14T12:30:00.000-06:00) puts CAHOKIA CREEK at 75.8 cfs — well below its long-term average at 44% 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. Madison County has moderate coverage with 21 active monitoring sites with 5,926 recent measurements on record. Predominant monitoring categories include inorganics, minor, metals and physical. 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.