Cerro Gordo County, Iowa: drinking water report. Cerro Gordo County in north-central Iowa has about 44,000 residents, with Mason City as the county seat.
Cerro Gordo County in north-central Iowa has about 44,000 residents, with Mason City as the county seat. The city draws from a mix of shallow and deep wells in the Lime Creek watershed. The county's agricultural economy – corn, soybeans, and hogs – operates on flat, tile-drained terrain that efficiently moves agricultural chemicals from field to waterway to aquifer.
Nitrate is the defining water quality issue in Cerro Gordo County. According to the Iowa DNR's 2024 drinking water data, Mason City's raw water nitrate peaked at 13.5 mg/L during spring 2024 – above the MCL and requiring treatment. The city operates nitrate removal equipment similar to Des Moines, though at a smaller scale.
The Lime Creek aquifer is shallow and unconfined, offering minimal protection from surface contamination. A 2024 Iowa Geological Survey study found that 28% of private wells tested in the county's agricultural areas exceeded the nitrate MCL. The Mason City Municipal Airport has documented AFFF use, with PFOS at 5 ppt in nearby monitoring wells.
Cerro Gordo County's flat terrain, tile drainage, and intensive agriculture create an ideal pathway for nitrate to reach your water supply. If you are on a private well, nitrate testing should be a standing annual commitment – spring is the highest-risk season.
Check your water for data at your address. For nitrate at the concentrations found in this county, reverse osmosis is the most reliable household treatment. Our water filter guide ranks systems by certified nitrate removal. Pull your detailed report for seasonal patterns, and visit our Iowa page for statewide data.