Models for leaching of pesticides in soils and groundwater

Models are developed which describe leaching of pesticides in the root zone and the intermediate vadose zone, and flushing of residual solute mass in the aquifer. Pollutants' loss pathways in the soil, such as volatilization, crop uptake, and biochemical decay, are emphasized, and the effect of local dispersion and nonequilibrium transport in mobile-immobile phases is analyzed. It is shown that the effect of diffusive transfer on the leached mass fractions is dependent on the volume fraction of the immobile phase, adsorption, the apparent mass-transfer rate coefficient, the first-order decay rate in the immobile zone. Error analysis indicates that complete-mixing models can be: used to simulate leaching fractions of the pollutant when the Peclet number, P-r, is greater than one. However, ignoring the effect of dispersion may underestimate leaching significantly when P-r < 1, such as for volatile compounds. Potential application of the models to the design of groundwater protection zones is investigated, analysis and application results demonstrate the dependence of the size of protective buffer on potential loss pathways in the soil environment, the size of the source area, and aquifer geometric, hydraulic and biochemical properties. Potential use of the models for the management of pesticides is also investigated with implication on hazardous waste land treatment. (C) 2000 Elsevier Science B.V. All rights reserved.