Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 27, 2000
Publication Date: N/A
Interpretive Summary: Assessment of the effects of herbicide usage in crop production on water quality in the Midwest requires an integrated understanding of two closely related factors. First, how does water move across, into, and down through a cultivated field. Second, how do these processes regulate the distribution of herbicide chemicals. Through extensive field instrumentation and intensive sampling, we have characterized the legacy of long-term herbicide usage in ridge-till corn on water quality. Data shows that within the characteristic topography of the Loess Hills, the degree to which herbicide chemicals move through the soil profile to contaminate groundwater is largely a function of landscape position. Under prevailing weather patterns of southwestern Iowa, herbicides can move rapidly from the surface to shallow groundwater and in shallow groundwater be carried to the edge of the field. Awareness of this kind of information should lead to weed-control strategies that prevent or minimize contamination of water resources by helping farmers to manage their pesticide usage more efficiently.
Technical Abstract: The environmental fate and movement of herbicides widely used for weed control in field corn is assessed for a deep loess soil in southwestern Iowa. Beginning in the early 1980's, the herbicide based weed control program emphasized the application of atrazine, cyanazine, and metolachlor for both broadleaf and grass control. Between 1992-1995, concentrations of atrazine, deethylatrazine, deisopropylatrazine, cyanazine and metolachlor were measured in rainwater, both shallow and deep unsaturated-zone water, and well water. Results show that the frequency of herbicide detections and range and distribution of occurrences are dependent upon both landscape position and temporal inputs of recharge water from rainfall. Generally, deisopropylatrazine was observed more frequently and in higher mean concentration in well water than deethylatrazine, while the deethylatrazine was observed more frequently than deisopropylatrazine in the unsaturated-zone ground water. A chromatographic analogy is suggested to explain the occurrence patterns observed for both parent herbicide and degradates within the unsaturated-zone water. Analysis of rainwater samples collected during this time also revealed low concentrations of atrazine, cyanazine and metolachlor, with the timing of detects indicative of non-local transport. Results show that the deep loess soil conducts both water and agricultural chemicals relatively rapidly and as such represents a production system that is vulnerable to contamination of shallow groundwater by herbicide-derived chemicals. Results also illustrate the importance of including all herbicide degradates in impact assessment studies.