Submitted to: Journal of Environmental Monitoring
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 27, 2000
Publication Date: N/A
Interpretive Summary: Assessment of the effects of farm chemicals used 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 the soil volume in 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 the long-term usage of weed killers used by farmers to control weeds in corn, also known as herbicides, on water quality. We have discovered that within the characteristic topography of the Loess Hills, the degree to which herbicide chemicals move to contaminate groundwater is largely a function of landscape position. Loess Hills topography is such that total relief across a 100-acre field may be as great as 50-60 ft. Under prevailing weather patterns of southwestern Iowa, herbicides move rapidly from the surface to shallow groundwater and in shallow groundwater may be carried to the edge of the field. Awareness of this kind of information should lead to weed-control strategies which 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 corn are 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 (ATR) or cyanazine (CYN) and metolachlor (MET) for both broadleaf and grass control. Between 1992- 1995, concentrations of ATR, desethylatrazine (DEA), desisopropylatrazine (DIA), CYN and MET were measured in rainwater, both shallow and deep vadose-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, DIA was observed more frequently and in higher mean concentration in well water than DEA, while the DEA was observed more frequently than the DIA in the vadose-zone groundwater. A chromatographic analogy is suggested to explain the occurrence patterns observed for both parent herbicide and degradation products within the unsaturated-zone water. Analysis of rainwater samples collected during this time also revealed low concentrations of ATR, CYN and MET, with the timing of the detections 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 which is vulnerable to contamination of shallow groundwater by herbicide-derived chemicals. Results also illustrate the importance of including major herbicide degradation products in water resource impact assessment studies.