Is Ennis Lake Polluted?
Yes — Ennis Lake in Madison County, Montana is on the EPA's Clean Water Act 303(d) impaired-waters list (2021 assessment cycle). It is cited for Habitat alterations, Hydrologic alteration, Metals (other than mercury). 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 | 2021 |
| EPA IR category | 5 |
| Uses not supported | Drinking water, Aquatic life |
| EPA names this lake in the listing | Yes |
| Location | Madison County, Montana |
Pollutants Cited
- Habitat alterations — listed by EPA as exceeding water-quality standards for at least one designated use.
- Hydrologic alteration — listed by EPA as exceeding water-quality standards for at least one designated use.
- Metals (other than mercury) — listed by EPA as exceeding water-quality standards for at least one designated use.
Specific parameters in the EPA record: Arsenic, Flow Regime Modification, Other Anthropogenic Substrate Alterations, Physical Substrate Habitat Alterations.
What this means for using Ennis Lake
Ennis Lake's listing is driven by contaminants that build up in fish (Metals (other than mercury)), so the practical impact is on eating the fish, not on swimming. Follow the state fish-consumption advisory for Montana. 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. See the full breakdown on the lake report. The official EPA assessment is available in the ATTAINS waterbody report.
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