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Nutrients and Pesticides in Ground Water in the Upper Mississippi River Basin, Minnesota and Wisconsin, 1978-94

By Andrews, W.J., Stark, J.R., and Fong, A.L.

Published in Proceedings of the 41st Annual Midwest Ground Water Conference, held at Lexington, KY; September 29-October 1, 1996.

The principal purposes of the National Water Quality Assessment (NAWQA) Program are to define ambient water quality on regional and national scales and to evaluate the effects of land use on water quality and aquatic biota. As part of a retrospective analysis of water-quality data for part of the Upper Mississippi River Basin study area of the NAWQA Program, the U.S. Geological Survey collected and summarized analyses of nutrients and pesticides in ground water from data bases maintained by Federal, state, and local agencies. The Upper Mississippi River Basin study area includes all of the Mississippi River basin upstream of Lake Pepin and the St. Croix River Basin. Water-quality data collected from more than 1,500 wells during 1978-94 by the U.S. Geological Survey, the Minnesota Department of Agriculture, the Minnesota Department of Health, the Minnesota Pollution Control Agency, and the Wisconsin Department of Natural Resources were included in this retrospective analysis.

Nitrate-nitrogen concentrations in ground water reflected land uses and hydrogeologic settings of major aquifers. Surficial sand and gravel, buried sand and gravel, and the Prairie du Chien-Jordan aquifers were the aquifers most commonly sampled for nitrate. Water sampled from buried sand and gravel aquifers, which underlie layers of clay or till, had median nitrate concentrations between 0.02 mg/L and 1.0 mg/L, varying somewhat by sampling agency. Median nitrate concentrations in water from surficial sand and gravel aquifers ranged form 0.86 to 6.2 mg/L. Water from the Prairie du Chien-Jordan aquifer had median nitrate concentrations ranging from 0.035 to 5 mg/L. The highest nitrate concentrations, 10 percent of which equalled or exceeded the U.S. Environmental Protection Agency's Maximum Contaminant Level for drinking water of 10 milligrams per liter, were in water from shallower wells--those less than 30 feet deep that were completed in surficial sand and gravel aquifers, and those less than 200 feet deep that were completed in the Prairie du Chien-Jordan aquifer. Water from wells with nitrate concentrations exceeding 3 mg/L were generally restricted to areas of cropland, and mixed cropland and woodland. Dissolved-phosphorus concentrations in ground water were generally highest in urban portions of the study area.

Of 588 wells in the study area sampled for pesticides by Federal and state agencies, atrazine was the most frequently detected pesticide in ground water in the study area. Atrazine was detected in water from as much as 54 percent of wells completed in bedrock aquifers, and in water from as much as 67 percent of wells completed in sand and gravel aquifers. For wells completed in bedrock aquifers, atrazine was most commonly detected in agricultural areas in the eastern part of the study area, where bedrock units are exposed at the land surface or are overlain by thin glacial or fluvial deposits. Atrazine was also the most frequently detected pesticide in water from wells completed in sand and gravel aquifers in the Anoka Sand Plain, which has permeable sandy soils and widespread agriculture. Alachlor, cyanazine, metolachlor, and simazine were detected in less than 5 percent of water samples from wells completed in sand and gravel aquifers.