Winnebago County, Wisconsin: drinking water report. Winnebago County in east-central Wisconsin has about 171,000 residents, with Oshkosh and Neenah as the…
Winnebago County in east-central Wisconsin has about 171,000 residents, with Oshkosh and Neenah as the largest cities. Lake Winnebago – the largest inland lake in Wisconsin – provides the primary water source for the region. The Fox River system feeds the lake and carries agricultural and industrial inputs from across the upper Fox River valley.
Lake Winnebago has experienced increasingly severe blue-green algae blooms driven by phosphorus loading from agricultural runoff. According to the Wisconsin DNR's 2024 lake monitoring report, total phosphorus concentrations in Lake Winnebago averaged 0.064 mg/L – above the 0.04 mg/L threshold for eutrophic conditions. Cyanotoxin sampling during summer 2024 found microcystin at 3.1 ug/L at the Oshkosh intake during the peak bloom period.
The Oshkosh area also has a PFAS concern. The former Wittman Regional Airport and industrial sites in the Fox Valley used AFFF. Wisconsin DNR's 2024 PFAS action plan documented detections in Winnebago County groundwater at 18 ppt combined PFAS. Wisconsin has proposed PFAS standards of 20 ppt for PFOA and PFOS individually.
Lake Winnebago's algal bloom season – typically July through September – is when source water quality faces the most pressure. If your water develops an earthy taste or smell during summer months, that is likely algal-derived and a signal to pay attention to your filtration.
Check your water for current and seasonal data. A quality carbon filter handles algal taste compounds, while reverse osmosis adds PFAS and broader contaminant protection. Our water filter guide covers systems suited to lake-sourced water. Get your detailed report for seasonal trends, and visit our Wisconsin page for statewide context.