National Water Quality Assessment Program: Eastern Iowa Basins

Nutrients, pesticides, and pesticide metabolites discharged to the Mississippi River from Eastern Iowa Watersheds

D.J. Schnoebelen, K. D. Becher

Proceedings of the Mississippi Water Resources Conference, April 8, 1999

The intense use of soluble and mobile agricultural chemicals (nutrients and pesticides) in the Midwest poses potential problems of nonpoint source contamination to streams and rivers. Land use in eastern Iowa is 93 percent agriculture, predominantly corn and soybeans. Over 95 percent of the corn and soybeans are treated with fertilizers and pesticides. As part of the U.S. Geological Survey's National Water Quality Assessment Program, water samples were collected from three of the largest watersheds in eastern Iowa that discharge into the Mississippi River. Nutrients, pesticides, and pesticide metabolites concentrations were analyzed for all samples collected. Chemical loads were estimated using a minimum variance unbiased estimator model. A seasonal pattern of concentrations and loads was observed for both nutrients and pesticides. Agricultural-chemical concentrations and loads often peak in the late spring to early summer in conjunction with applications associated with crop planting and subsequent spring runoff. Nutrient concentrations also increased in the late fall to early winter as vegetation goes into dormancy and additional fertilizer is applied to fields. Time trends calculated using long-term (greater than 10-year) historical nutrient concentrations indicate increasing concentrations for one of the three major watersheds. Pesticides concentrations were often 5 to 10 times larger in water samples collected during the spring season than in samples collected during the rest of the year. Pesticide metabolites were detected from 2 to 100 times more frequently than their parent compounds in all samples. Pesticides metabolites were more persistent in stream samples throughout the year compared to their parent compounds. Therefore, including pesticide metabolites in load calculations greatly increases the transport estimates of pesticide compounds over those that just use the parent compounds.

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