Pesticide sorption and degradation characteristics in New Zealand soils - a synthesis from seven field trials

The transport and fate of selected pesticides was determined at seven field sites located throughout New Zealand. Mixtures of pesticides and bromide (Br) were applied to plots and soil and water samples, collected at 1-3 monthly intervals over the study period, were analysed for residual pesticides and Br. The maximum depth of sampling varied from 0.92 to 2.5 m for the suction cup samplers and 0.8 to 1.0 m for the soil sampling. Inverse modelling was used with three models, GLEAMS, LEACHM, and HYDRUS, to estimate K-oc and half-life values for the applied pesticides. Diazinon and procymidone had much lower K-oc values than the selected literature values (SLVs), indicating they will be much more mobile in New Zealand soils than would be expected from the SLVs. Picloram had similar K-oc values to the SLV at the Te Awa and Twyford 'Scites but was much less mobile in the allophanic Horotiu soil, as was triclopyr. There was a slight effect of allophane on sorption for hexazinone, picloram, and triclopyr for the Horotiu and Kiripaka soils. Atrazine, simazine, and terbuthylazine K-oc values tended to be less than the SLV, whereas bromacil K-oc values were greater than the SLV and hexazinone had similar K-oc values to the SLV. Diazinon, hexazinone, simazine, and terbuthylazine all had similar half-life values to the SLVs. Atrazine was on average c. 4 times less persistent than the SLV, whereas bromacil was slightly less persistent than the SLV. 2,4-D was about twice as persistent as the SLV would indicate whereas picloram, triclopyr, and procymidone were much more persistent than the SLVs, with factors of 4, 5, and 50, respectively, for these pesticides, and were outside the reported range of literature values. These studies have provided a greater knowledge of field-based K-oc and half-life values in New Zealand soils under a range of climatic conditions, and the measured variability in the K-oc and half-life values can be used to carry out a sensitivity analysis of leaching predictions.