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Comparison of nitrogen, phosphorus, and selected pesticides in the Minnesota, Mississippi, and St. Croix Rivers, Minnesota and Wisconsin, 1971-1994

By Stark, J.R., Fallon, J.D., and Kroening, S.E.

Journal of the Minnesota Academy of Science

The U.S. Geological Survey summarized nitrogen and phosphorus data from 1984-93 from streams and selected pesticide data from 1971094 from streams and selected pesticided data from 1971-94 from streams and streambed sediments from the Upper Mississippi River Basin. Information obtained from data bases maintained by federal, state, and local agencies was analyzed as part of the work for the Upper Mississippi River Basin study of the U.S. Geological Survey's National Water Quality Assessment. The analysis focused on a 19,500 square mile area in the eastern portion of the Upper Mississippi River Basin from Royalton, Minnesota to the outlet of Lake Pepin, the Minnesota River Basin from Jordan, Minnesota to the confluence with the Mississippi River, and the entire drainage basins of the St. Croix, Cannon and Vermillion Rivers. Concentrations of loads of nutrients and concentrations and detection frequencies of selected pesticides in these rivers and in selected tributaries were compared. Water quality data were obtained from the Metropolitan Council Environmental Services, the Minnesota Department of Agriculture, the Minnesota Pollution Control Agency, the U.S. Army Corps of Engineers, the U.S. Geological Survey, and the Wisconsin Department of Natural Resources.

Concentrations of total nitrite plus nitrate nitrogen and total phosphorus generally were greatest in the spring in the Minnesota River. The greatest nitrate and phosphorus concentrations typically occurred during spring and summer. Total nitrite plus nitrate nitrogen in all rivers generally were below the primary drinking water standard of 10 mg/L established by the U.S. Environmental Protection Agency. Total phosphorus concentrations in the Minnesota River generally were above the 0.1 mg/L concentration recommended by the U.S. Environmental Protection Agency. Nitrogen and phosphorus loading to the Mississippi River was predominantly from the Minnesota River during periods of high flow and from sources in the Twin Cities Metropolitan Area during periods of low flow. In the Minnesota River, the majority of the nitrogen load was primarily as total nitrite plus nitrate nitrogen. Streams draining row-crop agricultural areas (the Minnesota and Cannon Rivers) had the most frequent detection of herbicides. Median and maximum concentrations of atrazine, metolachlor, and cyanazine were greatest in July. Detection frequencies of organochlorine insecticides in the Minnesota River were greater in the reach within the Twin Cities Metropolitan Area than on the reach immediately upstream of the area.