Field study of pesticide leaching in an allophanic soil in New Zealand. 1: Experimental results

Seven pesticides, 2,4-D, atrazine, hexazinone, picloram, procymidone, terbuthylazine, and triclopyr, were applied to a Horotiu soil (allophanic loamy silt) located near Hamilton, New Zealand, along with 2 tracer compounds, bromide and deuterated water. Their movement and persistence was monitored for about 2 years using soil sampling down to a maximum of 1 m and 9 suction cups located between 0.2 and 2.5 m down the profile. There was rapid leaching of the tracers as well as hexazinone, picloram, and, to a lesser extent, triclopyr. Procymidone was much less mobile but was very persistent. Atrazine, terbuthylazine, and 2,4-D showed low mobility and persistence at the study site. Bromide and hexazinone were observed in the underlying groundwater following winter recharge and an intensive irrigation period at the end of the study. There was slight adsorption of bromide by the allophane in the soil. The combination of both soil sampling and suction cups gave a more complete description of the leaching process and the distribution of the compound through the profile than either technique by itself. This combination is worthwhile and gives complementary information providing the strengths and limitations of both techniques are appreciated.