Comparison of Pesticide Transport Processes in Three Tile-Drained Field Soils Using HYDRUS-2D

The purpose of this study was to assess the transport of the herbicide bentazone [3-(1-methylethyl)-1H-2,1,3-benzothiadiazin-4(3H)one 2,2-dioxide] in three contrasting tile-drained cultivated field soils subject to otherwise similar experimental conditions. Observed drain discharge rates and chemical concentrations in the drainage water reflected the different transport processes at the three sites in the same area of northeastern France: a sandy loam site (Villey), a silt loam site (Bouzule-1), and a silty clay site (Bouzule-2). The sandy loam site showed very little tile drainage (240 m ha) during the 100-d study in the spring of 2002, as well as low chemical losses in the drainage water (0.16% [v/v] of the applied amount). While little drainage was observed also for the silty clay soil (175 m ha), observed pesticide losses were considerably larger (1.25% of the applied amount). The silt loam soil, in comparison, showed much more drainage (521 m ha) and the highest chemical loads in the drainage water (2.7% of the applied amount). Numerical simulations of drain discharge with the HYDRUS-2D variably saturated flow and solute transport model compared well with the observed data for the relatively homogeneous sandy loam (Villey) and the silt loam (Bouzule-1) soils. The saturated hydraulic conductivity of the bottom layer in both cases was key to correctly predicting the drainage fluxes. Accurate predictions of the silty clay field data (Bouzule-2) could be obtained only when the soil hydraulic functions were modified to account for preferential flow through drying cracks near the soil surface. Chemical concentrations could be better described using a dual-porosity (mobile–immobile water type) transport model for all three soils, including the sandy loam. Results indicate that water and pesticide transport in soils is governed by site-specific processes. Optimal use of the HYDRUS-2D flow and transport model allowed a reasonable description of the field-scale pesticide processes using only a limited number of adjustable parameters. APPLICATION of pesticides to cultivated fields often leads to their unintentional leaching through the vadose zone toward underlying and down-gradient water resources. Since excessive leaching compromises soil and water quality, and indirectly human health (Garcia, 2003; Wong et al., 2003; Miersma et al., 2003), many studies have been performed to assess the fate and transport of a variety of pesticides in different soils and for different soil management practices. Experiments focusing on the governing transport processes are often performed using repacked soil columns in the laboratory (Davidson and Chang, 1972; van Genuchten et al., 1977; Gamerdinger et al., 1990; Kookana et al., 1993; Guo and Wagenet, 1999). While useful, such experiments are generally not representative of field conditions and often underestimate the leaching potential of chemicals in the field because of preferential flow or other processes (Flury, 1996). Field experiments provide a more realistic assessment of pesticide transport processes. Tile-drained agricultural fields in particular have been shown to be attractive for studying field-scale flow and solute transport processes. Many studies have demonstrated the merit of such systems for studying the transport of tracers (Vanderborght et al., 2002; Fox et al., 2004; Gerke and Köhne, 2004), N (Mohanty et al., 1998; de Vos et al., 2000; de Vos, 2001), and pesticides (Abbaspour et al., 2001; Kohler et al., 2001; Zehe and Flühler, 2001; Jaynes et al., 2001; Novak et al., 2003; Larsbo and Jarvis, 2005). Tile drainage systems, in essence, function as large undisturbed field-scale lysimeters (Gerke and Köhne, 2004; Kung et al., 2000) since they integrate the sitespecific flow and transport processes within the full transport domain that contributes to drainage outflow. Optimally interpreting tile drainage response (e.g., drain discharge rates and solute concentrations) is still a challenge because of field-scale spatial and temporal variability and the related problems of preferential flow (Southwick et al., 1995; van Genuchten et al., 1999; Larsson et al., 1999; Hendrickx and Flury, 2001; Kohler et al., 2003). During the past two decades, several models have been developed to simulate water flow and solute transport in the vadose zone. These models range from relatively simplistic approaches to more complex, physically based, dual-porosity, dual-permeability, and multiregion models (Jarvis, 1994; Banton et al., 1995; Abbaspour et al., 2001; Kohler et al., 2003; Pruess, 2004; Larsbo et al., 2005). A review of various approaches to simulate preferential, nonequilibrium flow and transport was recently given by Šimůnek et al. (2003). Although the more sophisticated of these models seem to be able to describe the outflow patterns from tile-drained fields, this may be partly due to the large number of parameters that are often fitted to the observed field data (e.g., Haws et al., 2005). The resulting parameter estimation process by itself can be quite problematic because of problems of parameter uniqueness, identifiability, and stability (e.g., Hopmans and Šimůnek, 1997). One challenge when simulating the outflow response from tile drains at a given site is that different conceptual formulations sometimes will lead to equally acceptable Boivin and M.Th. van Genuchten, USDA-ARS, George E. Brown, Jr., Salinity Lab., 450 West Big Springs Rd., Riverside, CA 92507. Jirka Šimůnek, Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521. A. Boivin and M. Schiavon, Laboratoire Sols et Environnement, UMR INPL-ENSAIA/INRA, 2 Avenue de la Forêt de Haye, B.P. 172, 54505 Vandoeuvre-lès-Nancy Cedex, France. Received 22 July 2005. *Corresponding author ([email protected]). Published in Vadose Zone Journal 5:838–849 (2006).