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 loca l dispersion and nonequilibrium transport in mobile-immobile phases is anal yzed. It is shown that the effect of diffusive transfer on the leached mass fractions is dependent on the volume fraction of the immobile phase, adsor ption, the apparent mass-transfer rate coefficient, the first-order decay r ate in the immobile zone. Error analysis indicates that complete-mixing mod els can be: used to simulate leaching fractions of the pollutant when the P eclet number, P-r, is greater than one. However, ignoring the effect of dis persion may underestimate leaching significantly when P-r < 1, such as for volatile compounds. Potential application of the models to the design of gr oundwater protection zones is investigated, analysis and application result s demonstrate the dependence of the size of protective buffer on potential loss pathways in the soil environment, the size of the source area, and aqu ifer geometric, hydraulic and biochemical properties. Potential use of the models for the management of pesticides is also investigated with implicati on on hazardous waste land treatment. (C) 2000 Elsevier Science B.V. All ri ghts reserved.