Sorption of a Hydrophilic Pesticide : Effects of Soil Water Content

Transport of pesticide to groundwater is governed in part by sorption of the pesticide to soil particles. Sorption may be dependent on soil moisture conditions, but limited data are available from which to elucidate the effect. Our objective was to determine the effect of soil water content on the sorption coefficient of a hydrophilic pesticide. Sorption of dicamba (3,6-dichloro-2-methoxybenzoic acid) was measured in three soils, each at two initial water contents. At low water contents (|0.05 kg kg), sorption coefficients were similar for all three soils, ranging from 0.01 L kg for the loamy sand to 0.07 L kg for the silty clay loam. At higher water contents (0.19–0.24 kg kg), the sorption coefficient for the loamy sand was unchanged, for the silt loam it was doubled, and for the silty clay loam it was increased almost sixfold. Multiple regression analysis revealed a strong linear relationship between the sorption coefficient and the product of soil water content and organic C content (r 2 5 0.86). The number of dicamba sorption sites probably increases with soil organic C content, while the accessibility of these sites appears to increase with soil water content. This may be caused by the decreasing hydrophobicity of soil organic matter with increasing water content. The effects of water content on pesticide sorption require further research and may ultimately have implications for the methods used to determine sorption and for managing pesticide application. IF PESTICIDES LEACH into the groundwater, they have the potential to negatively impact human health and the environment. The leaching risk of a pesticide in soil is characterized primarily by the sorption coefficient (Kd). The sorption coefficient, also called the distribution coefficient, quantifies the distribution of a pesticide between the soil solid phase and the soil solution. Lower sorption coefficients correspond with greater potential for leaching. To correctly assess water quality risks, researchers must understand the factors that influence pesticide sorption coefficients. Currently, the effect of soil water content on pesticide sorption is not well understood. This is a serious gap because pesticides are often applied on or near the soil surface where the water content varies dramatically. Results from several studies have indicated that sorption of some pesticides increases with soil water content. This phenomenon has been demonstrated for the herbicides atrazine [6-chloro-N-ethyl-N9-(1-methylethyl)-1,3,5-triazine-2,4-diamine], linuron [N9-(3,4-dichlorophenyl)-Nmethoxy-N-methylurea], metsulfuron methyl (methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]amino]sulfonyl]benzoate), and the fungicide triadimefon [1-(4-chlorophenoxy)-3,3-dimethyl-1-(1H1,2,4-triazol-1-yl)-2-butanone] (Berglöf et al., 2000a, 2000b, 2003; Rochette and Koskinen, 1996). Additionally, Roy et al. (2000) reported significant water content effects on the sorption of five other fungicides, with the direction and magnitude of the effects depending on the properties of the chemicals and the sorption mechanisms. They found that sorption of the more hydrophilic compounds increasedathigherwater contents.Somepreliminary data have also suggested a positive water content effect on dicamba sorption (Koskinen et al., 2006). The causes of the observed water content effects on pesticide sorption are uncertain. Berglöf et al. (2000a) hypothesized that, at higher water contents, more soil solution is in contact with a larger surface area of soil particles, thereby facilitating greater access of the pesticide to sorption sites. Roy et al. (2000) proposed two possible mechanisms for the water content effect. First, higher water content may facilitate pesticide diffusion to sorption sites. Second, water content may affect the structure of humic substances, leading to more hydrophilic surfaces at higher water contents and thus greater sorption potential for hydrophilic pesticides. The scarcity of studies on the effect of water content on pesticide sorption is due in part to a lack of appropriate experimental methods. Batch methods, which are frequently used to measure sorption coefficients, use soil/solution ratios that are much lower (i.e., much higher water contents) than those that occur in the field. These methods are generally unsuitable for examining the effect of water content on sorption. Roy et al. (2000) used a direct soil solution sampling method to examine the influence of soil water content on sorption. They used water contents of 0.26 and 0.47 kg kg21, which are on the higher end of the range of water contents typically observed near the soil surface, and they indicated the need for new methods of measuring sorption at lower water contents. One possible method for measuring sorption at low water contents is the supercritical fluid extraction (SFE) technique. Several SFE studies have provided evidence that pesticide sorption increases with soil water content (e.g., Berglöf et al., 2003), but interpretation of the data is complicated by the fact that the efficiency of the extraction technique may decrease at higher water contents, resulting in an apparent increase in sorption. The unsaturated transient flowmethod is another possible method for studying sorption at low water contents (Ahmad et al., 2005; Katou et al., 2001). In this technique, soil spiked with pesticide is packed into a horizontal column, which is then attached to a water source. During infiltration, the chemical is redistributed by piston-like displacement of the antecedent solution T.E. Ochsner and W.C. Koskinen, USDA-ARS, Soil and Water Management Research Unit, St. Paul, MN 55108; B.M. Stephens, Dep. of Soil, Water, and Climate, Univ. of Minnesota, St. Paul, MN 55108; R.S. Kookana, CSIRO Land andWater, PMB 2, Glen Osmond, SA 5064, Australia. Received 23 Feb. 2006. *Corresponding author