Simulation of picloram, atrazine, and simazine transport through two New Zealand soils using LEACHM

Picloram, atrazine, simazine, and bromide were applied to 2 sites, a Twyford fine sandy loam and a Te Awa silt loam, located in Hawkes Bay, New Zealand, and monitored at approximately monthly intervals using suction cup samplers for 1200 and 800 days, respectively. Simulations using LEACHM, in combination with the PEST nonlinear optimisation package, were used to estimate soil sorption coefficients and degradation rates. Six different methods were used to estimate Campbell's a and b coefficients for the water retention curve, and results indicated that solute transport simulations were relatively insensitive to the estimation method. There were indications that preferential flow had occurred at both sites. The Koc values for picloram at both sites were greater than the `best available' value given by Wauchope et al. (1992), indicating less mobility, but the degradation rates were much lower at both sites, particularly in the subsoil. The mobility for atrazine and simazine at the Te Awa site was slightly greater than that indicated from the best available values. The degradation rates were similar to the best available values in the topsoil but the degradation rate for atrazine decreased in the subsoil. The estimates from LEACHM were also compared with estimates using the GLEAMS model.