Quartz sandy soils from Simcoe, Ontario, Canada and North Carolina had sorption properties for chlorothalonil that were nearly the same. For labile surface sorption kinetics, the Simcoe soil gave a pseudo first-order rate constant of k{sub}(s1) = (7.4 ± 0.7) × 10{sup}(-2) days{sup}(-1). At equilibrium, the labile surface sorption capacity θ{sub}c of Simcoe soil for chlorothalonil was 23.8 × 10{sup}(-6) (mol/g). The sorption properties of the 4-hydroxy derivative of chlorothalonil were different in two important respects. They were larger by an order of magnitude, and they were substantially different for the two soils. Sorption by the Simcoe soil was too fast for kinetics measurements by the on-line HPLC micro extraction method, but for the North Carolina soil k{sub}(s1) = (1.15 ± 0.01) days{sup}(-1) was recorded. For the Simcoe and North Carolina soils, respectively, θ{sub}c > 200 (μmol/g) and θ{sub}c ≈ 113 (μmol/ g). Two conclusions can be drawn. First, the replacement of the Cl by OH on the 4 position of chlorothalonil makes the sorption effects much greater. Second, the stronger interactions are associated with a greater sensitivity to small differences in the chemical compositions of the soils. Subtle soil properties causing significant effects might include small amounts and physical structures of organic matter and metal oxides. This implies that, for predictive computer models, mechanism parameters will have to be correlated in two dimensions: chemical structure, and the composition and amounts of chemical materials in soils.
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