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Water-Quality Assessment of Part of the Upper Mississippi River Basin, Minnesota and Wisconsin--Pesticides in streams, streambed sediment, and ground water, 1974-94: U.S. Geological Survey Water-Resources Investigations Report 97-4141

By Fallon, J.D., Fong, A.L., and Andrews, W.J.

Available data on pesticides in streams, streambed sediment, and ground water from Federal, state, and local agencies are reviewed for part of the Upper Mississippi River Basin study unit of the National Water-Quality Assessment Program. The analysis focuses on a smaller study area encompassing 19,500 square miles that includes the Upper Mississippi River Basin from Lake Pepin upstream to sampling stations on the Mississippi River near Royalton, Minnesota and the Minnesota River near Jordan, Minnesota, and the entire drainage basins of the St. Croix, Vermillion, and Cannon Rivers. Assessment is generally restricted to two groups of pesticides--the most frequently detected herbicides and organochlorine insecticides-- although pesticides rarely or never detected are noted.

Herbicides, including alachlor, atrazine, cyanazine, or metolachlor, were detected in every stream sampled except the Kettle River. Streams draining row-crop areas had the most herbicide detections. Atrazine was the most widely detected herbicide with detections in all streams sampled except the Kettle River. Concentrations of atrazine, metolachlor, and cyanazine were greatest in July and detectable most of the year at very low (parts-per-trillion) concentrations. The herbicides EPTC and trifluralin were never detected although they were used in amounts equal to or greater than those detected, reflecting the fact that some herbicides are less persistent than others. A small urban stream draining part of the Lake Harriet watershed in Minneapolis, Minnesota contained substantial concentrations of pesticides as well. Eighty- five percent of runoff events sampled in this entirely urbanized watershed had detections of herbicides commonly used for residential purposes and 43 percent of the events had detections of alachlor, atrazine, cyanazine, or metolachlor--herbicides used predominantly for agriculture. Pesticide concentrations in urban runoff remained well above detection limits throughout the summer, indicating repeated applications of pesticides.

Selected organochlorine insecticides, banned since the 1970's, still were detected in recent streambed-sediment samples. Three insecticides, 4,4'-DDT, heptachlor, and lindane, and their metabolites account for almost two-thirds of the organochlorine insecticides detected. Organochlorine insecticides were detected more frequently in streambed sediment than in stream water. Detections in both phases were most frequent within or downstream of the Twin Cities metropolitan area, indicating that most of these insecticides originated from the Twin Cities metropolitan area.

The most frequently detected herbicides in ground water were the same as those frequently detected in streams. Most detections were found in the surficial sand and gravel aquifers underlying agricultural areas, including the Anoka Sand Plain and Bonanza Valley. Atrazine, deethylatrazine, and deisopropylatrazine were detected most frequently. Detection frequencies of atrazine were extremely variable among the various agencies, ranging from 0 to 66.7 percent, probably as a result of different sampling purposes, well locations, and detection levels. Atrazine and atrazine metabolites were the only pesticides detected in bedrock aquifers, with detections found mainly in the agriculture-dominated southeastern part of the study area where bedrock commonly outcrops near the surface. Thus, most detections of herbicides in ground water were found in environmental settings where ground water is vulnerable to contamination.

Atrazine was the only pesticide that equaled or exceeded a maximum contaminant level (of 3.0 mg/L) for drinking water. Two stream samples from a small urban watershed in Minneapolis had atrazine concentrations of 3.6 and 3.8 mg/L, and one ground-water sample had a concentration of 3.0 mg/L. Trace concentrations (less than 0.06 mg/L) of the organochlorine insecticides chlordane, dieldrin, endrin, and heptachlor exceeded chronic freshwater-quality criteria in stream samples from the Mississippi, Minnesota, St. Croix, and Vemillion Rivers in 1981 and 1990.

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