Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: April 1, 1996
Publication Date: N/A
Technical Abstract: Walnut Creek watershed is typical of hydrologic conditions in central Iowa. Four components of the hydrologic system were studied: groundwater; artificial subsurface (tile) drainage; runoff; and the interaction between the creek with the alluvial aquifer associated with the Skunk River. Groundwater beneath most of the watershed occurs in till. Water samples collected between 1991 and 1994 from piezometers throughout the watershed showed consistent nitrate infiltration to 1-3 m, but minimal herbicide leaching. Nitrate concentrations exceeded 10 mg/L in only 10% of the samples taken below 3 m deep. Tile drainage removes groundwater at a depth of 1.5-2 m throughout much of the watershed. Flow in the creek is dominated by tile flow except immediately following storms. Concentrations of nitrate in tile flow were consistently near 10 mg/L. Atrazine was the most consistently found herbicide, but when detected in tile water was or near the quantitation limit of 0.2 mg/L. Runoff is a minor contributor to streamflow except in the relatively small area with high relief and during periods of extreme rainfall. Where the creek flows across the Skunk River alluvium, it consistently loses water and herbicides to the underlying alluvial aquifer. Atrazine loss through the creek bed provides potential for flux of atrazine at a rate of 2-3 orders of magnitude greater than estimates of field leaching of atrazine.