ANTHROPOGENIC AND NATURALLY DERIVED CONTAMINANTS AFFECT THE WATER QUALITY OF THE GLACIAL AQUIFER SYSTEM

The glacial aquifer system studied by the U.S. Geological Survey National Water Quality Assessment Program includes all unconsolidated aquifers of glacial origin north of the line of continental glaciation in the United States. The characteristics of the deposits are unique to the individual regions of the glacial aquifer system. The east and west regions, for example are underlain by fractured crystalline rocks and the glacial aquifer system deposits, to varying extents, reflect the geochemistry of those underlying rocks. In the central and west central regions, carbonate rocks and shale are common and geochemistry of the deposits and water quality of the glacial aquifer system reflects those materials.

The most common water quality issues from anthropogenic sources are related to nitrate, pesticides, and chloride. Some concentrations of nitrate in groundwater from wells in the glacial aquifer system were among the highest in the nation, but most concentrations were less than 1 mg/L as N. Groundwater contributed a significant amount of the annual total nitrate load carried by streams. Pesticide concentrations were low, but when pesticides and degradates were detected it was more often found in groundwater pumped from public-supply wells located near rivers draining the Corn Belt. The highest concentrations of chloride in groundwater and streams occur in urban areas where halite is commonly used for roadway deicing and is also common in sewage and animal waste.

The most common water quality contaminants from natural sources include arsenic, uranium, radon, and manganese. Concentrations of arsenic were greatest where groundwater is old, anoxic, and alkaline—generally, these factors increase from east to west and with depth. Concentrations of uranium in groundwater were elevated where uranium was common in glacial deposits and where groundwater had high alkalinity. Concentrations of radon commonly exceeded the lowest proposed human health benchmark (300 pCi/L) but only rarely exceeded a higher proposed human health benchmark (4,000 pCi/L). Manganese concentrations were highest in the west-central region where the benchmark was exceeded in water from 46 percent of 174 public wells and 25 percent of 502 domestic wells.