National Water Quality Assessment Program: Eastern Iowa Basins

Changing stream water quality during the last decade in two Midwestern agricultural watersheds

Stephen J. Kalkhoff, USGS, Iowa City, Ia
Mark D. Tomer, USDA-ARS, Ames, Ia
Jessica D. Garrett, USGS, Iowa City, Ia
James D. Fallon, USGS, Mounds View, Mn
David C. Lampe, USGS, Indianapolis, In

Program abstracts for Iowa Department of Natural Resources 10-year Water Monitoring Conference & IOWATER Open Forum April 9-10, 2010 Scheman Center, Iowa State University, Ames, IA

The water quality of two agricultural watersheds was monitored from 1996 through 2007 to evaluate changes due to differing land-use practices. Both the South Fork Iowa River watershed in Iowa and the Little Cobb River watershed in Minnesota are located in Des Moines Lobe of the Wisconsinan glacier topographic setting. Fertile soils developed on glacial till have allowed for development of intensive row crop agriculture. More than 85 percent of the watersheds are used for row-crop agriculture. In contrast to the similar intensity of row crop agriculture in the two watersheds, intensity of animal production in the South Fork Iowa River is significantly greater than in the Little Cobb River.

Nitrate concentrations from 1996 through 2007 were significantly greater in the South Fork Iowa River than in the Little Cobb River. There were no significant differences in phosphorus concentrations in the two streams. In addition to being larger, nitrate concentrations tended to increase over time in the South Fork Iowa River while those in the Little Cobb River were relatively constant. Chloride, which is indicative of human impacts from fertilizers, animal and human waste, and road deicing, was present in significantly larger concentrations in the South Fork Iowa River than in the Little Cobb River. As with nitrate, chloride concentrations increased over time in the South Fork Iowa River while those in the Little Cobb River remained relatively constant.

Larger stream discharge and greater nitrate concentrations in the South Fork Iowa River resulted in substantially larger nitrogen and phosphorus load or mass. During some years, almost twice as much nitrogen and more than five times as much phosphorus were transported from the South Fork Iowa River watershed than from the Little Cobb River watershed. During 1997 and 1998, more than 6,000,000 pounds per year of nitrogen and about 375,000 pounds per year of phosphorus were transported from the South Fork Iowa River watershed. In this same period, about 2,000,000 pounds per year of nitrogen and almost 41,000 pounds per year of phosphorus were transported from the Little Cobb River.


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