Submitted to: Clean Water Clean Environment 21st Century Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: March 1, 1996
Publication Date: N/A
Technical Abstract: Walnut Creek watershed is a 12,700 acre watershed located south of Ames, IA. The watershed is located on the Des Moines Lobe landform region and is characterized by gently rolling topography, prairie potholes that are not well-drained, an extensive tile drain network, and intensive row crop production. Over 85% of the watershed is in a corn-soybean rotation. Fertilizer and herbicide use in Walnut Creek is typical of the corn belt. The watershed has been monitored since 1990 for movement and fate of atrazine, alachlor, metribuzin, metolachlor, and nitrate-N. Loss of the materials has been measured in tile drains from individual fields, subbasins within the watershed, and from the entire watershed. Shallow piezometers were installed to measure movement of nitrate-N and herbicides into the shallow ground water. One detection of atrazine in the shallow ground water below 8 feet has been found in over 1700 samples. Nitrate-N decreases rapidly with depth, and below 30 feet it is less than 2 ppm. Losses of nitrate-N are larger in the surface water because the tile drains intercept the water at the bottom of the root zone and concentrations are between 15 and 20 ppm. These losses can total to over 50 percent of the applied fertilizer within the watershed, and in 1993 nearly 150% of the amount of applied N was lost. Atrazine is most frequently detected in surface water, and annual losses can reach 7% of the applied total. Herbicides will be easier to manage than nitrate-N because a change in farming practice can reduce potential movement. Nitrogen will be more difficult because it is present in large amounts of these organic rich soils. To affect water quality will require management of the overall watershed to achieve water quality goals.