Decatur Water Hardness & Quality Report (2026)

The City of Decatur Water Division supplies drinking water to approximately 75,000 residents in Macon County, Illinois, primarily from Lake Decatur, a 24-square-mile reservoir impounded by the Decatur Dam on the Sangamon River. Emergency backup sources include the DeWitt County well field and a former sand/gravel pit near the South Water Treatment Plant. The utility operates the South Water Treatment Plant, Nitrate Removal Facility, Raw Water Pump Station, South Booster Pump Station, William Street Pump Station, three elevated storage tanks, and lime residuals lagoons. Water is treated via coagulation, sedimentation, filtration, and disinfection with additional nitrate removal; annual Consumer Confidence Reports are published at decaturil.gov.

The Lake Decatur Watershed spans over 1,000 square miles in the Sangamon River basin, draining agricultural lands across Macon, DeWitt, and Piatt counties. Surficial geology is dominated by Pleistocene-age glacial till, outwash sands, and gravels overlying Pennsylvanian shale, sandstone, and coal seams, with deeper Mississippian limestone beneath. Runoff dissolves carbonates, dolomite, and gypsum from these bedrock formations into the reservoir, yielding a hard supply. Karst-influenced hydrology accelerates mineral leaching through bedrock fractures, while agricultural activity contributes nitrates that require specialized removal treatment.

Hard water in Decatur accelerates scale buildup in water heaters, dishwashers, boilers, and coffee makers, reducing efficiency by up to 20–30% and shortening appliance lifespan. Faucet aerators, showerheads, and pipes accumulate chalky deposits restricting flow and raising energy costs; laundry detergents underperform, leaving stiff towels and dingy fabrics. Monthly vinegar descaling of fixtures, annual heater flushing, and scale-inhibiting shower filters are recommended; a water softener is advised especially for households with front-loading washers or low-flow appliances. Treated water maintains pH around 7.5–8.5; no PFAS detected above limits; arsenic has been flagged above health guidelines from natural soil and bedrock sources; nitrate-nitrogen is managed by the Nitrate Removal Facility via ion exchange.