Is Green Lake Polluted?
Yes — Green Lake in Isanti County, Minnesota is on the EPA's Clean Water Act 303(d) impaired-waters list (2024 assessment cycle). It is cited for Impaired biota (cause unknown), Mercury, Nutrients (phosphorus, nitrogen), PCBs. 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 | Isanti County, Minnesota |
Pollutants Cited
- Impaired biota (cause unknown) — listed by EPA as exceeding water-quality standards for at least one designated use.
- 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.
- 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.
- PCBs — Polychlorinated biphenyls are legacy industrial pollutants that persist in sediment and bioaccumulate in fish. Like mercury, PCB listings drive fish-consumption advisories.
Specific parameters in the EPA record: FISH BIOASSESSMENTS, MERCURY IN FISH TISSUE, NUTRIENTS, PCBS IN FISH TISSUE.
What this means for using Green Lake
Because Green 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. Green Lake carries an overall water-quality grade of C, with algae (chlorophyll-a) at 11.7 µg/L and phosphorus at 44 µg/L — see the full breakdown on the lake report. The official EPA assessment is available in the ATTAINS waterbody report.