Field experiments in transport of pesticides in tropical soils in Hawaii

Past incidences of ground water contamination from nematicides and herbicides has focused on a progressive approach to pesticide registration in Hawaii. The state of Hawaii, in the early 1990's made an assessment of the pesticides used in Hawaii and their potential to leach into ground water. Since there is increased pressure to bring new agricultural chemicals to Hawaii, the state and the University of Hawaii worked together to evaluate five EPA-registered chemicals for their leachability in Hawaii soils. Three herbicides (s-metolachlor, sulfometuron methyl, and imazaquin), a fungicide (trifloxystrobin), and an insecticide (imidacloprid) were tested for their leachability in various tropical soils. In addition, the herbicide atrazine and bromide ion were used as two reference chemicals in the study. All chemicals were applied to tilled soils at label rates for agriculture or insecticidal use of these chemicals, except for bromide applied at the rate of 21.9 kg Br ^-/ha. Five test sites at differing topographic, climatic and soil conditions were selected on three Hawaiian islands: three on Oahu, one on Maui and one on Kauai. Each site had four test plots; two for herbicides and two for insecticide/fungicide. No crops were grown and the soil surface was covered with a thin layer of mulch to prevent weed growth and decrease water evaporation from the soil surface. An irrigation system was set up at each site to apply water at close to potential evapotranspiration rate to induce leaching. Loose soil samples were collected on the day after pesticide application and in one to four week intervals for four months. Soil hydraulic and climatic data from each site were collected during the same period. Soil samples collected in the field to a depth of 2.1 m were analyzed for the content of the pesticides and bromide, yielding concentration profiles for elapsed times of 1 day to 16 weeks. Prior to pesticide application, soil profiles were also characterized for pH, organic carbon content and basic soil nutrients: P, K, Mg, Ca. Undisturbed soil core samples were taken and sets of water-retention curves were developed in the laboratory. Field tests for unsaturated and saturated hydraulic conductivity using tension disc infiltrometer were carried out at all sites. To duplicate the complexity of processes in the field, laboratory tests on the sorption and degradation of the chemicals were carried out for all 5 sites and 6 pesticides using appropriate analytical methods. Selected degradation products were tested for their competitive sorption to soils. The physico-chemical properties of soils and pesticides, evaluated in this study, will be used to determine whether these compounds will be permitted for unrestricted use, or with some restrictions. This information will be used by the state of Hawaii to classify this group of chemicals for intra-state sale and use. Copyright ASCE 2005.