MODELING PESTICIDE TRANSPORT IN AN IRRIGATED FIELD WITH VARIABLE WATER APPLICATION AND HYDRAULIC CONDUCTIVITY

The impact of spatially varying hydraulic properties on chemical trans port in the unsaturated zone has been extensively studied. However, fe w studies have investigated the impact of variable water application o n 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 variabilit y and water application was studied as a source of extrinsic variabili ty. A conceptual model of water now and solute transport was developed based on results from soil cores, shallow groundwater samples, and dy e studies. One important result of the field study was the detection o f low levels of the applied pesticide metolachlor 2-ethyl-6-methylphen yl)-N-(2-methoxy-1-methyethyl) acetamide]) in groundwater samples coll ected at 2.6 m below ground surface after an average application of on ly 0.097 m of water. Dye studies indicated gravity-driven fingered now occurred in the subsoil, which may explain the early detection of pes ticide in the shallow groundwater. Monte Carlo simulations revealed th at the variability of water application had a greater impact on pestic ide 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 con tamination by agricultural chemicals under the conditions studied.