A simulation model for the fate of pesticide residues in a field soil

The model employed for simulating the fate of pesticides in soil was based on the one dimensional convective-dispersive-reaction equation. The model assumed a constant pore-water velocity (v), linear equilibrium adsorption, and first-order degradation. The adsorption parameter (k_d) was calculated from molecular structure with the help of a first-order molecular connectivity index. On the basis of the results of sensitivity analysis the model was made partially stochastic in order to account for horizontal heterogenity in a field soil. Concentration profiles were obtained by employing the mode, median and mean values of (v, D) while the values of k_d and t_1/2 were kept constants; where D and t_1/2 were the apparent diffusion coefficient and the degradation half-life, respectively. The resulted extremes in concentration profiles were expected to be observed within a relatively small area of the heterogeneous filed soil. A random variable, namely the contaminated depth, was defined so that the soil beneath it would be practically free of pesticide during its entire life-time. The values of (₁, ₂, ₃) under continuous leaching were (0.14, 0.56, 1.00 m), (0.29, 1.50, 2.62 m), and (0.65, 2.25, 4.00m) for DDT, toxaphene and lindane, respectively; where ₁, ₂ and ₃ were values of calculated by employing the mode, median, and mean values of (v, D), respectively. Within a relatively small area the first value would be frequently observed whereas the others would be less pronounced. The depth and shape of the resultant base of the contaminated zone were substantially more sensitive to changes in the surface rates of recharge than to changes in the values of the chemical parameters k_d and t_1/2. The implication of the results for the assesment of pesticide contamination of groundwaters were discussed.