Field and laboratory study of pesticide leaching in a Motupiko silt loam (Nelson) and in a Waikiwi silt loam (Southland)

Selected pesticides were applied to a Motupiko silt loam located near Wakefield, Nelson, and a Waikiwi silt loam located near Invercargill, Southland, New Zealand, along with bromide as a tracer. Atrazine, bromacil, diazinon, hexazinone, and terbuthylazine were applied to both sites, with procymidone also being applied to the Nelson site, and trifluralin being applied to the Southland site. The persistence and transport behaviour of these compounds were monitored for 15 months using a combination of soil sampling down to a maximum of 1 m and 8 suction cups at each site located between 0.2 and 1.3 m down the profile. The bromide tracer had moved below 0.8 m within 9 months at the Nelson site and below 1.0 m within 12 months at the Southland site. Hexazinone was the most mobile of the pesticides, followed by bromacil at both sites. Diazinon and atrazine dissipated rapidly at both sites, while trifluralin and, to a lesser extent, terbuthylazine and bromacil data indicated some volatilisation losses. The diazinon removal was probably a combination of degradation and volatilisation. There was evidence of some preferential flow at the Southland site, as rainfall was heavy at the time of pesticide application. The K oc values from the sorption isotherms were generally similar to literature values for the Waikiwi soil but were much higher for the Motupiko soil. The exception was diazinon, for which the K oc values were much lower than the literature values for both sites.