Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 12/19/1996
Publication Date: N/A
Technical Abstract: A small creek draining 5500 ha of cropland consistently loses water to an alluvial aquifer in central Iowa. Infiltration to the alluvial aquifer was estimated by measuring relative head differences, loss of stream discharge, and concentrations of agrichemicals in the creek and shallow groundwater beneath the creek. Base flow measurements made before spring herbicide application showed potential for atrazine and metabolite contamination of the aquifer to be 2 to 3 orders of magnitude greater than leaching beneath a cropped field. Measurements made after herbicide applications to headwater fields showed 3 to 5 orders of magnitude greater infiltration through the creek bed than direct leaching. Nitrate concentrations in the creek were not reflected in concentrations in aquifer water. Slug-tests in flood plain deposits were used to calculate linear vertical groundwater velocities of 0.5 m/day. Velocities of this scale are sufficient to transport conservative contaminants to the aquifer in fewer than 7 days. The hydrologic setting of Walnut Creek is representative of a large number of tributary streams in the Midwest. If the process of tributary stream infiltration is generally applicable, it may answer some of the questions about why herbicides are found more frequently in some groundwater settings.