Analytical modeling of effects of application frequency on pesticide concentrations in wells

Pesticide transport from the soil surface to the ground-water table was modeled by means of an analytical solution of the convection-dispersion equation, assuming steady water flow, a linear sorption isotherm and first-order transformation kinetics. Pesticide transport in the saturated zone to a well was described by means of an analytical solution of the conservation equation for a cylindrical flow system, assuming steady flow, no dispersion, linear sorption, and first-order transformation. We derived an analytical solution for this coupled unsaturated-saturated system. We limited our analysis to systems that are difficult to protect with a protection zone. The strongest fluctuations in the pesticide concentration in the well were calculated for the lowest frequency of application, shortest characteristic travel time in the saturated zone and shortest half-life in the saturated zone, A simplified model, ignoring the variation of travel times in the unsaturated zone, resulted in concentration fluctuations in the well, The peak concentrations differed less than two times from those calculated with the analytical coupled unsaturated-saturated model, except for cases with the combination of a short characteristic travel time and a short half-life in the saturated zone.