Is Delavan Lake Polluted?
Yes — Delavan Lake in Walworth County, Wisconsin is on the EPA's Clean Water Act 303(d) impaired-waters list (2024 assessment cycle). It is cited for Nutrients (phosphorus, nitrogen). 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 | 2024 |
| EPA IR category | 5 |
| Uses not supported | Recreation |
| EPA names this lake in the listing | Yes |
| Location | Walworth County, Wisconsin |
Pollutants Cited
- 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.
Specific parameters in the EPA record: Phosphorus, Total.
What this means for using Delavan Lake
Because Delavan 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. Delavan Lake measures algae (chlorophyll-a) at 9.1 µg/L and phosphorus at 52.5 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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