Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 13, 1999
Publication Date: N/A
Interpretive Summary: Understanding which soil factors relate to the presence of nitrate and herbicides in shallow groundwater may be useful to reduce or avoid future contamination of aquifers. Well samples from a previous study of shallow aquifers in the Midwest were used to relate soil factors to these chemicals in water samples. It was found that clay content in soils, their bulk density, seasonally high water table, and the presence of somewhat poorly drained soils correlated with nitrate concentrations in shallow groundwater sampled throughout the Midwest. It was found that atrazine and its breakdown products correlated to soil organic matter content, the silt content, available water holding capacity, seasonally high water table, soil bulk density, and the presence of poorly drained soils. Further, it was found that the relations of soil factors to nitrate contamination were substantially different from those related to atrazine contamination. Consequently, soil types that are conducive to nitrate contamination may not allow atrazine contamination. When these soil data are combined with land use and hydrogeologic information from a previous study, it may be possible to identify areas that are specifically vulnerable to groundwater contamination by nitrate and selected herbicides. This would allow management of land in these areas by scientists, producers, or others to reduce shallow groundwater contamination.
Technical Abstract: Soil variables were correlated to concentrations of nitrate, atrazine, and atrazine residue (atrazine + deethylatrazine + deisopropylatrazine) in unconsolidated aquifers in the Midwest. Soil maps were digitized and 96 variables were aggregated within 2 km of each of 99 wells. Soil factors relating to water flux and landscape were consistently the most significant tvariables among all agrichemicals examined. Soil factors that affected th movement of water were directly related to the occurrence and concentrations of agrichemicals in groundwater. Substantial differences, however, exist among the soil variables that correlate with nitrate and the herbicide compounds. For example, organic matter is a significant variable for atrazine and atrazine residue, but not for nitrate. These statistical differences support the hypothesis that different processes affect the transformation, adsorption, or transport of nitrate and herbicides. A multi-variate analysis determined that soils explained 19.0% of the variability in nitrate, 33.4% variability in atrazine, and 28.6% variability in atrazine residue. These results document that although soils do affect the transport of agrichemicals to groundwater, they provide insufficient information to consistently predict the occurrence of atrazine or nitrate.