Sangamon County, Illinois: drinking water report. Sangamon County in central Illinois has about 194,000 residents, with Springfield – the state capital –
Sangamon County in central Illinois has about 194,000 residents, with Springfield – the state capital – as the county seat. City Water, Light and Power draws from Lake Springfield, a 4,200-acre reservoir on the Sangamon River. The lake's watershed is predominantly agricultural, and the reservoir was built in the 1930s to supply the growing city.
Lake Springfield receives heavy nutrient loading from its agricultural watershed. According to the Illinois EPA's 2024 lake assessment, total phosphorus averaged 0.09 mg/L – placing the lake in the hypereutrophic category. Blue-green algal blooms are a recurring summer problem, producing taste-and-odor compounds and occasionally microcystin at the intake.
The Abraham Lincoln Capital Airport has documented AFFF use. Illinois EPA's 2024 PFAS investigation found PFOS at 10 ppt in monitoring wells near the facility. A former manufactured gas plant site in downtown Springfield has documented PAH contamination in groundwater, though the contamination is localized and does not affect the lake supply.
Springfield's lake supply faces the same agricultural nutrient pressure as most central Illinois surface water sources. Summer months bring the most challenging water quality conditions, and taste changes during algal bloom season are common.
Start by checking your water for seasonal data. A carbon block filter handles algal taste compounds and disinfection byproducts effectively. For PFAS, reverse osmosis adds targeted protection. Our water filter guide covers lake-water-specific systems. Get your detailed report for seasonal patterns, and visit our Illinois page for statewide data.
Drinking water is one exposure route. These federal datasets also cover Sangamon County, IL.
The EPA places Sangamon County, IL 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.
63% of homes in Sangamon County, IL were built before 1980, and 23% 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 Sangamon County, IL averages 8.6 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