Rock County, Wisconsin: drinking water report. Rock County in southern Wisconsin has about 163,000 residents, with Janesville and Beloit as the primary…
Rock County in southern Wisconsin has about 163,000 residents, with Janesville and Beloit as the primary cities. Most communities draw from groundwater wells tapping the Cambrian-Ordovician aquifer system. The county's agricultural character – dairy, corn, and soybeans dominate the landscape – shapes both the economy and the groundwater quality beneath it.
Nitrate contamination from agricultural activity is the dominant water quality issue in Rock County. According to the Wisconsin DNR's 2024 groundwater monitoring report, 22% of private wells tested in the county exceeded the nitrate MCL of 10 mg/L. The county's thin soils over fractured dolomite bedrock provide minimal filtration between farm fields and the aquifer.
The former Janesville GM Assembly Plant, which closed in 2009 after decades of automotive manufacturing, left groundwater contamination from chlorinated solvents. Wisconsin DNR's 2024 site investigation found TCE at 8 ppb in monitoring wells near the facility – above the MCL of 5 ppb. PFAS from the Southern Wisconsin Regional Airport has been detected at 14 ppt in groundwater downgradient from the facility.
Rock County well owners face a real probability of nitrate contamination. One in five wells tested above the safe limit – those are not good odds for skipping a water test. Infants are most vulnerable to nitrate exposure, but elevated levels indicate broader agricultural contamination that may include pesticides.
Check your water for available data. For nitrate and PFAS together, reverse osmosis handles both effectively. Our water filter guide covers systems certified for agricultural contaminants. Pull your detailed report for local trends, and visit our Wisconsin page for statewide context.