IMPLICATION OF SPATIAL VARIABILITY OF ORGANIC-CARBON ON PREDICTING PESTICIDE MOBILITY IN SOIL

In this study, the depth distribution of organic carbon (OC) content i n a sandy soil was characterized for a small agricultural plot (1.5 ha ). The means and the distributions for measured OC were used as sorpti on-related parameters in a solute transport model to predict the movem ent of the herbicide atrazine in the unsaturated zone. Simulations wer e performed in order to assess the impact of OC variability in the eva luation of the herbicide concentration reaching the water table. The f requency distributions for soil OC at various depth intervals followed typical normal laws. The value for the coefficient of Variation was l ower at the first depth interval than at deeper soil intervals, possib ly as a result of soil tillage practices. In predicting pesticide tran sport, the deterministic approach was shown to be sensitive to the var iability of the soil sorption coefficient controlled by OC content. Ho wever a stochastic approach showed that in actual field situations, th e in situ variability of soil parameters that control the soil-water f low could have a greater influence than the sorption-related parameter such as OC content. In this case, the field characterization of the h orizontal distribution of OC may not improve the representativeness of the simulation results.