Modeling pesticide transport in an irrigated field with variable water application and hydraulic conductivity

The impact of spatially varying hydraulic properties on chemical transport in the unsaturated zone has been extensively studied. However, few studies have investigated the impact of variable water application on chemical transport. This paper uses field data and a modeling study to evaluate the relative importance of intrinsic and extrinsic sources of variability on pesticide transport through a field soil. Saturated hydraulic conductivity represented the source of intrinsic variability and water application was studied as a source of extrinsic variability. A conceptual model of water flow and solute transport was developed based on results from soil cores, shallow groundwater samples, and dye studies. One important result of the field study was the detection of low levels of the applied pesticide metolachlor ([2-chloro-N-(2-ethyl-6-methylphenyl)-N- (2-methoxy-1-methyethyl) acetamide]) in groundwater samples collected at 2.6 m below ground surface after an average application of only 0.097 m of water. Dye studies indicated gravity-driven fingered flow occurred in the subsoil, which may explain the early detection of pesticide in the shallow groundwater. Monte Carlo simulations revealed that the variability of water application had a greater impact on pesticide transport than the variability of saturated hydraulic conductivity at this site. Therefore, improving sprinkler uniformity represents an important best management practice (BMP) for reducing groundwater contamination by agricultural chemicals under the conditions studied