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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 systemic fungicide in water‐saturated homogeneous soil. Chloride and 14C‐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 14C‐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 Kd 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 of 14C‐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.

Diffusion is a key process regarding the redistribution in soils of systemic pesticides applied in seed treatments. We investigated the influence of nonequilibrium sorption on the diffusivity of triticonazole fungicide [(1RS)‐(E)‐5‐(4‐chlorophenyl‐methylen)‐2,2‐dimethyl‐1‐(1H‐1,2,4‐triazol‐1‐ylmethyl)‐cyclopentan‐1‐ol] in loam clay soil from Grignon, France (Typic Eutrochrept). Triticonazole diffusion coefficients were measured in saturated soil columns for time periods ranging from 1 to 25 d. Nonequilibrium sorption was evaluated from measurement of apparent desorption coefficients, and from estimation of apparent partition coefficients from a diffusivity model. Triticonazole availability was also evaluated based on successive aqueous and methanol extractions, and measurement of the nonextractable residues fraction. Triticonazole diffusion in soil was characterized by a strong time‐dependence. Apparent diffusion coefficient decreased from 0.05 cm2 d−1 at 2 d to 0.02 cm2 d−1 at 25 d. Triticonazole apparent sorption coefficient increased with time of diffusion, from 5 L kg−1 at 1 d to 10.5 L kg−1 at 25 d, which was greater than the batch equilibrium value of 4.35 L kg−1. The measured and calculated results were very close, showing that nonequilibrium sorption was the key process responsible for the decrease in triticonazole apparent diffusion rate. The availability of the diffusing residues diminished constantly with time. The water‐extractable fraction corresponding to the more available residues decreased from 31% at 1 d to 17% at 25 d, while a resistant fraction was rapidly formed representing 9% after 25 d. Triticonazole slow desorption behavior was attributed to rate‐limiting intrasorbent diffusion.

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: The sorption of two anionic surfactants and a series of seven nonionic alkylphenolethoxylate surfactants of increasing hydrophilic/lipophilic balance (HLB) in a loamy clay soil was evaluated. The e †ect of low doses of these surfactants on the sorption characteristics of the fungicide triticonazole was investigated. The critical micellar concentration (CMC) of the surfactants in pure water and soilÈwater systems, and surfactant sorption were estimated by surface tension measurements using a batch equilibration technique. Triticonazole sorption, alone and in the presence of low doses of surfactants, was also measured by batch equilibration. CMC of the alkylphenol surfactants increased with their HLB. The sorption of surfactants increased with their lipophilicity. CMC in the soilÈwater systems were considerably higher than in pure water. Sorption of the most lipophilic alkylphenol surfactants at the higher doses signiÐcantly increased triticonazole sorption. Proposed mechanisms are modiÐcations of soil surface properties, and increase of soil organic carbon content. Sorption of the other nonionic and anionic surfactants only resulted in monomeric surfactant concentrations in pore water, and did not a †ect triticonazole sorption.1998 Society ( of Chemical Industry Pestic. Sci., 54, 52È60 (1998)