Ammonia spikes persist as summer heat amplifies toxicity

Water temperatures have climbed to 26.6°C, entering the seasonal window when ammonia becomes acutely toxic to fish — and two upstream sanitary districts have been discharging far above their permitted levels. St. Joseph Sanitary District ran more than four times its ammonia limit in the fourth quarter of 2025, with exceedances continuing through spring at 58 percent over permit. Milwaukee Metro Sewer is also in significant noncompliance. Neither facility has faced penalties.

The two rivers draining this landscape are already federally listed as impaired — 18 of 40 assessed stream segments in the area carry that designation. Summer is when stressors compound: heat accelerates ammonia's toxicity, while high discharge volumes stretch treatment capacity. The margin between technical noncompliance and ecological failure narrows week by week.