Predicting the Fate of Nonvolatile Pesticides in the Unsaturated Zone

Abstract

Abstract A simulation model of the fate of nonvolatile pesticides in the unsaturated zone is reported. The model was developed from basic principles of water and solute movement under transient field conditions, and considers water flow and resulting pesticide displacement, plant extraction of water and pesticide (if quantified), and pesticide sorption and degradation during transport. The finite‐differencing procedures were developed to provide substantial flexibility of initial and boundary conditions, allowing consideration of layered soils, multiple rainfall or evapotranspirational cycles, application of chemical in wet or dry form, plant growth, and several bottom boundary conditions. Application of the model to measured field data of aldicarb [2‐methyl‐2‐methylthio)propionaldehyde O ‐(methylcarbanoyl)oxime] movement indicates that the modeling approach can provide reliable and useful estimates of the mass flux of water and nonvolatile pesticide in unsaturated soil. Sensitivity of the model to variability in soil hydraulic properties and aldicarb transformation (degradation) rates is also presented.