DESCRIPTION OF SIMAZINE TRANSPORT WITH RATE-LIMITED, 2-STAGE, LINEAR AND NONLINEAR SORPTION

This study was conducted to reconcile an apparent inconsistency betwee n the simazine laboratory sorption isotherm data and the field lysimet er transport experiment reported by Poletika et al. (this issue). In t his investigation, linear and nonlinear one- and two-stage simazine so rption models were fitted to the sorption and desorption isotherm labo ratory data to obtain parameter estimates for use in the transport mod el. Once obtained, the calibrated sorption model was combined with the parameterized outflow concentration record from a mobile Br tracer to represent rate-limited sorption and transport of the simazine added s imultaneously with the Br. The calibrated model did an excellent job o f representing the final simazine profile in the soil, particularly wi th the nonlinear model. This is in contrast to a single-stage adsorpti on model tested by Poletika et al. (this issue), which reached poor ag reement with the field profile when laboratory-measured sorption param eters were used. The results demonstrate the compatibility of field an d laboratory experiments on pesticide movement and also indicate that sorption isotherms may require substantially longer to reach equilibri um than is customarily allowed in current protocols.