Water Is An Important Factor That Affects Sorption

Water is an important factor that affects sorption because when there is excess water or when water drains through soil from sources at the surface, adsorbed pesticide molecules can become unattached, or desorbed, and wash away to a new location (Rao, 1999). Just as degradation has a measure in the half-life of a compound, sorption also has a measure in the partition coefficient (Koc), which is a ratio of pesticide concentration in a soil bound state versus dissolved in soil-water (Rao, 1999). In fact, â€Å"the solubility of a pesticide and its sorption on soil are inversely related; [in other words], increased solubility results in less sorption† (Rao, 1999). Both sorption and degradation are effected by soil type and pH, both of which†¦show more content†¦Volatilization is a means of major pesticide loss and its rate of loss can often exceeds that of degradation, runoff, or leaching (van der Werf, 1996). For example, in an Oregon study, soil samples 64km from any a griculture were found to have DDT residues, and in Saskatchewan, Canada, 20% of 2, 4-D iso-octyl ester volatilized in 24 hours (Pimentel, 1995). Once in the atmosphere, pesticide residues can spread anywhere, even Antarctica (Pimentel, 1995). Large amounts of pesticides and organic compounds can be transported around the globe in the atmosphere, for example, â€Å"in the atmosphere of the Netherlands, the amounts of BHC, DDT, and heptachlor were reported to be 4600, 1064, and 190 pg/m 3, respectively† (Pimentel, 1995). Topography and Geology Topography and geology affect pesticide movement in as much as directing or transporting pesticides. Topography is very obvious, for example, if a field is right next to a river, it is much more likely for runoff to get into the river system, or if a field is in a low area not very far above the water table, pesticides are very likely to leach into the groundwater. On the other hand, a field could be high and dry and very far from any river or groundwater, but if it is hilly that can pose its own problems. As T. Y. Tong and Chen point out, surface runoff is an important source of non-point pollution (Tong and Chen, 2002). Though it was pointed out by van der Werf that volatilization is the biggest source of