Transport of triticonazole in homogeneous soil columns: influence of nonequilibrium sorption

Nonequilibrium sorption of pesticides is frequently reported to greatly affect their transport and dissipation in soil. This study was aimed at evaluating the performances of equilibrium and two site-two region nonequilibrium convective-dispersive models for describing the sorption and decay characteristics during transport of triticonazole systematic fungicide in water-saturated homogeneous soil. Chloride and (14)C-triticonazole column displacement experiments were carried out in a loamy clay soil under steady-state water flow at high pore water velocities. The symmetrical breakthrough curves (BTC) obtained with the conservative tracer showed no significant physical nonequilibrium and were used to estimate a dispersivity of 0.06 cm. Compared with chloride, the (14)C-triticonazole BTC was strongly asymmetrical and shifted to the right, with a broad, extended tailing characteristic of sorption nonequilibrium. Chemical analysis of the soil after elution showed that bound residues were rapidly formed during transport. These bound residues were accounted for as decayed in the models. The two-site model correctly described the first part of the tailing, with an estimated partition coefficient K(d) of 1.5 L kg(-1) for instantaneous sorption, and it predicted high values in the range of 58 d(-1), and 7 d(-1) for the sorption and decay first-order rates, respectively. However, the model failed to describe the slower, extended release (14)C-triticonazole. Nonequilibrium sorption and formation of bound residues of triticonazole were attributed to the rate-limiting diffusive process. It was thus concluded that use of a single first-order rate constant for description and prediction of both nonequilibrium sorption and dissipation of triticonazole in soil is not appropriate.