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.
Drinking water is one exposure route. These federal datasets also cover Rock County, WI.
The EPA places Rock County, WI in Zone 1 - the EPA's highest-risk category, where the predicted average indoor radon level is above 4 pCi/L. Zone maps predict an area's radon potential from geology and existing indoor readings; they say nothing about a specific house. Testing is the only way to know your own level.
67% of homes in Rock County, WI were built before 1980, and 27% before 1950. Residential lead paint was banned in 1978, so housing age is the strongest available proxy for lead-paint exposure. This describes the county's housing stock, not any one home.
Modelled annual fine-particle pollution (PM2.5) in Rock County, WI averages 8.3 µg/m³, within the EPA's annual standard of 9 µg/m³. These are CDC modelled county estimates rather than readings from a specific monitor.
How radon zones work · Air quality by county · Lead exposure guide
17 public water systems serving Rock County, WI have EPA lead testing on record. The reported lead sample skews toward systems that already flagged a problem, so its exceedance rate is not directly comparable with other areas. 9 water systems serving Rock County, WI carry a health-based EPA Safe Drinking Water Act violation.
| Most-cited violation type | Systems affected |
|---|---|
| Lead and Copper Rule Revisions | 4 |
| Combined Radium (226 + 228) | 2 |
| Ground Water Rule | 2 |
| Gross Alpha (excl. radon & uranium) | 1 |
| Lead and Copper Rule | 1 |
Source: EPA Lead and Copper Rule 90th-percentile tap sampling and EPA SDWIS Safe Drinking Water Act enforcement records, aggregated from the public water systems serving this county. Figures are counts of water systems, not a test of any individual home or tap.