Field study of pesticide leaching in an allophanic soil in New Zealand. 2: Comparison of simulations from four leaching models

Seven pesticides were applied to an allophanic silt loam along with a bromide tracer and their concentrations in soil and water monitored over a 2- year period. Inverse modelling was carried out using GLEAMS, LEACHM, and HYDRUS- 2D to derive field- based mobility and degradation parameters. Hexazinone and procymidone were more mobile and more persistent than most literature values would suggest, whereas picloram and triclopyr were much less mobile but more persistent. The greater mobility for hexazinone, a weak base, and the reduced mobility of picloram and triclopyr, weak acids, are consistent with the effects of allophane. Mobility values for 2,4- D, atrazine, and terbuthylazine could not be determined with confidence from experimental results, but both atrazine and terbuthylazine appeared less persistent, and 2,4- D more persistent than literature values. A fourth model, SPASMO, currently under development, was used as an independent test of the optimised parameters. It performed well for the soil water concentrations but tended to overestimate the observed soil concentrations using the derived parameters. HYDRUS- 2D simulations of bromide and hexazinone concentrations in the groundwater gave a good fit to observed data from 3 monitoring wells following a large recharge pulse.