Is Lake St Louis Polluted?
Yes — Lake St Louis in St. Charles County, Missouri is on the EPA's Clean Water Act 303(d) impaired-waters list (2022 assessment cycle). It is cited for Algal growth. No cleanup plan (TMDL) has been completed yet. "Impaired" is a legal designation, separate from the lake's EPA condition class water-quality readings: 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) | Not yet written |
| Assessment cycle | 2022 |
| EPA IR category | 5 |
| Uses not supported | Aquatic life |
| EPA names this lake in the listing | Yes |
| Location | St. Charles County, Missouri |
Pollutants Cited
- Algal growth — listed by EPA as exceeding water-quality standards for at least one designated use.
Specific parameters in the EPA record: Chlorophyll-a.
What this means for using Lake St Louis
Review the EPA assessment and any state advisories before relying on Lake St Louis for swimming, fishing, or other uses. 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. Lake St Louis measures algae (chlorophyll-a) at 74.3 µg/L and phosphorus at 50 µg/L. See the full breakdown on the lake report. The official EPA assessment is available in the ATTAINS waterbody report.
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