Marathon County, Wisconsin: drinking water report. Marathon County in central Wisconsin has a population of about 138,000, centered on Wausau.
Marathon County in central Wisconsin has a population of about 138,000, centered on Wausau. The county draws water from deep and shallow wells across multiple municipal systems. The region's dairy farming industry – one of the most intensive in the state – shapes the groundwater quality picture. Marathon County has more dairy farms per square mile than most counties in Wisconsin, and that density translates to nitrate loading in shallow aquifer zones.
Nitrate contamination from dairy manure application and fertilizer use is the primary water quality concern. According to the Wisconsin DNR's 2024 groundwater assessment, 25% of private wells tested in Marathon County during the 2022-2024 monitoring cycle exceeded 10 mg/L for nitrate – the EPA's MCL. The percentage has increased from 18% a decade earlier, tracking the intensification of dairy operations in the county.
The municipal systems in Wausau and surrounding communities draw from deeper wells that are generally better protected from surface contamination. The EPA's UCMR5 data shows PFAS at low levels in the Wausau system. The Central Wisconsin Airport has used AFFF firefighting foam, and PFAS has been documented in shallow monitoring wells near the facility.
Private well owners in Marathon County face genuine nitrate risk. Unlike municipal systems with treatment, private wells deliver whatever the aquifer contains. Annual testing is the minimum recommendation, and the county health department may offer free or subsidized testing.
Check your water for public system data in your area. For nitrate, reverse osmosis at the kitchen tap removes it effectively – boiling concentrates nitrate rather than removing it. Our water filter guide covers nitrate-specific options. Pull your detailed report for trends, and visit our Wisconsin page for statewide context.