Ground-water quality in the Red River of the North Basin, Minnesota and North Dakota, 1991-95

Surveys of water quality in surficial, buried glacial, and Cretaceous aquifers in the Red River of the North Basin during 1991-95 showed that some major-ion, nutrient, pesticide, and radioactive-element concentrations differed by physiographic area and differed among these aquifer types. Waters in surficial aquifers in the Drift Prairie (west) and Lake Plain (central) physiographic areas were similar to each other but significantly higher than those in the Moraine (east) area in dissolved solids, sodium, potassium, sulfate, fluoride, silica, and uranium concentrations. Radium, iron, nitrate, and nitrite concentrations were also significantly different among these areas. Pesticides were detected in 12 percent of waters in surficial aquifers in the Drift Prairie area, 20 percent of those in the Lake Plain area, and 52 percent of those in the Moraine area. Triazines and bentazon accounted for 98 percent of summed pesticide concentrations in waters in surficial aquifers. Waters in buried glacial aquifers in the central one-third of the basin had significantly higher concentrations of dissolved solids, sodium, potassium, chloride, fluoride, and iron than did waters in surficial aquifers. No pesticides were detected in five samples from buried glacial aquifers or six samples from Cretaceous aquifers. Waters in all sampled aquifers had a calcium-magnesium ratio of about 1.75 ± 0.75 across the basin regardless of anionic composition.

Agricultural land use and soil texture can explain pesticide distributions; soil texture best explains nutrient distributions in waters in surficial aquifers. Confining beds protect waters in buried glacial aquifers from land use effects, resulting in no or low concentrations of nutrients and pesticides. Upward movement of bedrock waters high in dissolved solids concentration can increase concentrations in waters in buried glacial and, to a lesser degree, waters in surficial aquifers in the Lake Plain and Drift Prairie areas. Waters in surficial aquifers exceeded the U.S. Environmental Protection Agency (USEPA) maximum contaminant level in drinking water for nitrate in the Drift Prairie (27 percent) and Moraine (8 percent) areas. Their limited areal extent and susceptibility to contamination restrict the usefulness of surficial aquifers as a drinking water source. Waters in buried glacial aquifers exceeded USEPA health advisories for dissolved solids, sodium, and manganese. Sixty-six percent of waters in surficial aquifers also exceeded the Health Advisory for manganese.