Is Horsetooth Reservoir Polluted?
Yes — Horsetooth Reservoir in Larimer County, Colorado 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). A formal cleanup plan — a Total Maximum Daily Load (TMDL) — has been written. "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) | Completed |
| Assessment cycle | 2024 |
| EPA IR category | 5 |
| Uses not supported | Drinking water, Aquatic life |
| EPA names this lake in the listing | Yes |
| Location | Larimer County, Colorado |
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.
Specific parameters in the EPA record: Arsenic, Mercury in Fish Tissue.
What this means for using Horsetooth Reservoir
Horsetooth Reservoir's listing is driven by contaminants that build up in fish (Mercury, Metals (other than mercury)), so the practical impact is on eating the fish, not on swimming. Follow the state fish-consumption advisory for Colorado. 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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