Herbicide Contamination of Groundwater Beneath Claypan Soils in North‐Central Missouri

Abstract

AbstractThe impact of hydrology and site‐specific soil stratigraphy on herbicide leaching are not well understood for claypan soils. The claypan is a naturally‐occurring argillic soil horizon that limits percolation to groundwater. The primary objective of this study was to determine the extent of nonpoint source herbicide contamination of shallow groundwater beneath claypan soils. The secondary objective was to determine the relative importance of hydrology and land management to observed herbicide contamination of groundwater during a 5‐year period. The study site was the 7250 ha Goodwater Creek Watershed in north‐central Missouri. Three cropped fields with 10 yr of similar management were instrumented with 20 to 25 monitor wells each in 1991. The three fields were treated with different rates of atrazine [2‐chloro‐(4‐ethylamino)‐6‐(isopropylamino)‐s‐triazine] and alachlor [2‐chloro‐2′, 6′‐diethyl‐N‐(methoxymethyl)acetanilide] and the wells were sampled quarterly from 1991 to 1996. Atrazine was detected in 7.2% of samples, with a maximum concentration of 0.12 µg L−1 (limit of detection, LOD = 0.02 µg L−1). Alachlor was detected in 0.4% of samples with a maximum concentration of 0.14 µg L−1 (LOD = 0.11 µg L−1). Site to site variability in hydrology was more important than differences in herbicide application rates, as the field treated with the least atrazine and alachlor had the most frequent detections of atrazine, the atrazine metabolite deethylatrazine [2‐chloro‐(4‐amino)‐6‐(isopropylamino)‐s‐triazine; DEA], and the alachlor metabolite [2‐[(2,6‐diethylphenyl) (methoxymethyl)amino]‐2‐oxoethanesulfonic acid; ESA]. Groundwater recharge occurred primarily during the nongrowing season. Therefore, herbicide and metabolite leaching were limited by the extent of their degradation in the soil during the growing season. The study results demonstrate the importance of hydrologic variability, recharge timing, and chemical properties with respect to the leaching of herbicides and herbicide metabolites.