Is Mauvaise Terre Lake Polluted?
Yes — Mauvaise Terre Lake in Morgan County, Illinois is on the EPA's Clean Water Act 303(d) impaired-waters list (2024 assessment cycle). It is cited for Mercury, Metals (other than mercury), Nutrients (phosphorus, nitrogen), Turbidity. A formal cleanup plan — a Total Maximum Daily Load (TMDL) — has been written. "Impaired" is a legal designation, separate from the lake's A–F water-quality grade: it means at least one designated use (such as swimming, aquatic life, or fish consumption) does not meet state standards for the listed pollutant.
EPA 303(d) Listing
| On 303(d) impaired list | Yes |
| Cleanup plan (TMDL) | Completed |
| Assessment cycle | 2024 |
| EPA IR category | 5 |
| Location | Morgan County, Illinois |
Pollutants Cited
- Mercury — Mercury accumulates in fish tissue and is the single most common impairment in northern lakes. It usually arrives via atmospheric deposition from coal combustion rather than local discharge — which is why even remote, clear lakes can be mercury-impaired. It triggers fish-consumption advisories, not swimming closures.
- Metals (other than mercury) — listed by EPA as exceeding water-quality standards for at least one designated use.
- Nutrients (phosphorus, nitrogen) — Excess phosphorus and nitrogen fuel algae growth and summer blooms. This is the classic agricultural-runoff and shoreline-development impairment, and the one most likely to affect swimming via blue-green algae.
- Turbidity — listed by EPA as exceeding water-quality standards for at least one designated use.
Specific parameters in the EPA record: IRON, MERCURY, PHOSPHORUS, TOTAL, TOTAL SUSPENDED SOLIDS (TSS), TURBIDITY.
What this means for using Mauvaise Terre Lake
Because Mauvaise Terre Lake is listed for Nutrients (phosphorus, nitrogen), swimming can be affected — check for posted beach advisories, especially after heavy rain or during visible algae. An impairment listing does not mean the lake is closed — most impaired lakes remain open for boating and swimming. It means a specific pollutant exceeds a standard for a specific use. Mauvaise Terre Lake carries an overall water-quality grade of F and phosphorus at 162 µg/L — see the full breakdown on the lake report. The official EPA assessment is available in the ATTAINS waterbody report.