Submitted to: American Water Resources Association Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 10/23/1997
Publication Date: N/A
Technical Abstract: Walnut Creek is a tributary to the South Skunk River in central Iowa draining 5500 ha of cropland to which herbicides and nitrogen fertilizer are applied annually. The Creek consistently loses water along a 1500 meter reach to an alluvial aquifer underlying the South Skunk River flood plain. Leakage 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. Measurements made during base-flow conditions before spring herbicide application showed a 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 watershed fields showed 3 to 5 orders of magnitude greater infiltration through the creek bed than direct leaching estimates. Nitrate concentrations found 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. It is possible that traditional in-field leaching models may not be useful to explain herbicide contamination in alluvial aquifer settings.