At least one contaminant was found at levels of human-health concern in about one third of untreated ground water 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 percent 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 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.

"The USGS puts scientific findings and trends on water quality into the public domain so that citizens, water managers, and public officials can decide on appropriate and effective actions to address current or emerging issues that may be cause for concern now or in the future," says USGS Director Marcia McNutt. "Both the quality of our water and scientific understanding of what is useful to monitor evolve over time, hence the need for the USGS to constantly update our water quality reports."

"The U.S. Environmental Protection Agency (EPA) regulates public wells, and elevated concentrations of contaminants are reduced or removed from the water before people drink it," says John Wilson, USGS hydrologist and author of the study. "This study examined contaminants that pose human-health concerns, including some that are not regulated, and findings can help water utility managers make decisions about future monitoring and treatment needs."

Human-health benchmarks used to evaluate the significance of contaminant concentrations in raw water samples included EPA Maximum Contaminant Levels (MCLs) and USGS Health-Based Screening Levels (HBSLs) for unregulated contaminants, developed by USGS in collaboration with the 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.

Major findings include:

Water quality of the aquifer system can vary greatly between areas where the aquifers are shallow and deep. Natural contaminants such as major ions, trace elements and radium tend to occur at higher concentrations in deeper areas. Human-caused contaminants, such as pesticides, volatile organic compounds and nitrate, are detected more often in shallow areas.

  • Radon-222 and radium were most frequently measured at concentrations greater than human-health benchmarks, but geographic distributions were related to different depths of the aquifer system. Radon levels exceeded the proposed MCL of 300 picocuries per liter in 43 percent of 140 wells, of which 90 percent were in regionally shallow areas. Radium levels exceeded the MCL of 5 picocuries per liter in 40 percent of 88 wells, of which 89 percent were in regionally deeper areas.

  • The trace elements strontium, manganese and barium exceeded a human-health benchmark in at least one sample. Strontium levels exceeded the HBSL in nine percent of 107 wells, and manganese levels exceeded the HBSL in four percent of 154 wells. Barium levels exceeded the MCL in one of 136 wells sampled by NAWQA. Concentrations of strontium were significantly higher in regionally deeper areas, and concentrations of manganese and barium were significantly higher in shallow areas.

  • Nitrate was detected at a concentration greater than one milligram per liter (mg/L) in 21 percent of the wells sampled by NAWQA. Concentrations of nitrate greater than 1 mg/L were assumed to be influenced by human activity. All but one of the wells were in regionally shallow areas, indicating that the shallower areas of the aquifer system are more susceptible to manmade contaminants. Nitrate levels exceeded the MCL of 10 mg/L in approximately four percent of 154 wells sampled by NAWQA.

  • Nine different pesticides were detected in wells sampled by NAWQA from 2002 through 2007, but usually at concentrations significantly below human-health benchmarks. No pesticide concentration exceeded a human-health benchmark. Water samples were analyzed for as many as 83 pesticides, but atrazine and its degradate deethylatrazine accounted for 67 percent of all pesticide detections. Eighty-six percent of wells with a pesticide detection were in shallow areas.