Areas of Elevated Contaminants in Groundwater Determined From Midwest Regional Assessment
Many of the public and private wells sampled contained natural or manmade contaminants
At least one contaminant was found at levels of human health concern in about one-third of untreated groundwater samples collected from wells in the Cambrian-Ordovician aquifer system, according to a recent report by the U.S. Geological Survey (USGS). When radon concentrations greater than 300 picocuries per liter are included, 64% of wells sampled contain a contaminant concentration above a human health benchmark.
The Cambrian-Ordovician aquifer system is ranked ninth in the nation for public supply water withdrawals from principal aquifers. The aquifer supplies water to many parts of the northern Midwest, including areas of Minnesota, Wisconsin, Illinois, Iowa, Missouri and the Upper Peninsula of Michigan, as well as the major cities of Minneapolis, Rockford,Ill., and Chicago.
Many of the public and private wells sampled contain natural or manmade contaminants, including radium, radon, boron, strontium, manganese, barium, nitrate, pesticides and volatile organic compounds. Radon and radium are naturally occurring radioactive elements and known carcinogens. The deeper parts of the aquifer system in Illinois, Iowa and eastern Wisconsin are vulnerable to high concentrations of radium, boron and strontium. The shallow areas of the aquifer system in Minnesota, Wisconsin and the Upper Peninsula of Michigan are vulnerable to radon and manganese.
The study was conducted as part of an ongoing systematic assessment of some of the nation’s most important aquifer systems by the USGS National Water-Quality Assessment (NAWQA) Program. Results are available online.
Human health benchmarks used to evaluate the significance of contaminant concentrations in raw water samples included U.S. Environmental Protection Agency (EPA) Maximum Contaminant Levels and USGS Health-Based Screening Levels for unregulated contaminants, developed by USGS in collaboration with EPA. Concentrations also were compared to EPA Secondary Drinking Water Regulations established for aesthetic quality or other non-health reasons. In relating measured concentrations to health benchmarks, this study offers a preliminary assessment of potential health concerns and identifies conditions that may warrant further investigation. The research is not a substitute for comprehensive risk and toxicity assessments.
Radium and strontium levels in domestic and public supply wells from the Cambrian-Ordovician aquifer system are higher and more frequently exceed the human health benchmark than in any of the other 30 principal aquifers studied by NAWQA. Arsenic levels frequently exceed the human-health benchmark in domestic and public-supply wells of many of the other principal aquifers studied by NAWQA, but arsenic did not exceed the benchmark in any samples from the Cambrian-Ordovician aquifer system.