Cook County Water Report

Cook County earns an A grade for drinking water quality, scoring 70.8 out of 100. Over the past five years, EPA SDWIS records 30 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 53.7% of assessed waterways are impaired (523 of 974 water bodies) across Cook 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 2026-05-14T12:45:00.000-06:00) puts DES PLAINES RIVER at 320.0 cfs — well below its long-term average at 50% 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. Cook County has extensive coverage with 258 active monitoring sites with 43,464 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.