In the deep loess hills along the Missouri River valley of southwestern Iowa, field studies are underway to determine the impact of continuous corn production on both surface water and groundwater quality. The landscape is characterized by gently sloping ridges, steep side slopes, and well-defined alluvial valleys often with incised channels that usually terminate at an active gully head. The impact of this highly-erodible silt-loam landscape on both on-site and off-site water quality is determined by sampling several hydrogeologic compartments. Surface water quality is evaluated by analyzing field runoff for nitrate-N, atrazine and metolachlor. A comparison among four field-sized watersheds under different tillage practices reveals different responses to an early Spring runoff event. In each case, the long-term impact resulting from more than 20 years of nitrogen fertilization and more than 15 years of atrazine and metolachlor application is assessed. Beneath the snow cover of March 1993, a diurnal freeze-thaw cycle was observed to cause displacement of both nitrate-N (as the legacy of fertilizer usage) and parent herbicides in surface runoff generated by melting snowcover. During the five days of repetitive events, the nitrate-N and herbicides exhibited different displacement patterns, perhaps as a result of their fundamentally different chemical properties.

Dr. T. R. Steinheimer
USDA-ARS, National Soil Tilth Laboratory
2150 Pammel Drive
Ames, Iowa, 50011, USA

Phone: (515) 294-2952
Fax: (515) 294-8125
E-mail: steinheimer@nstl.gov