Leaching of Terbuthylazine and Bromacil Through Field Soils

A field experiment was designed to provide data on the effect of soil heterogeneity on the distribution of herbicides following leaching by irrigation and rain water. Terbuthylazine and bromacil, two nonconservative herbicides, together with CaBr₂, a conservative chemical, were used in the reported experiment. The experimental field consisted of a noncultivated 175-m² plot in which 20 observation points were randomly selected. A hundred and ten centimeters of irrigation and rainwater were applied and the field was periodically sampled for chemical distributions. The spatial variability of the field was determined by measuring the K_s (saturated conductivity) and α (Gardner parameter). Auxiliary laboratory experiments were performed to define the adsorption-desorption of the chemicals studied in these field soils. Results on the adsorption-desorption of terbuthylazine and bromacil and on the redistribution of these chemicals in the field to a depth of 120 cm during leaching are shown. Bromacil leached to a greater extent than terbuthylazine. The differences among the concentrations of herbicides in the various cores studied may be explained in terms of properties of the chemicals and soil spatial variability. The residual concentrations of terbuthylazine and bromacil were also determined to a depth of 400 cm after the leaching of 110 cm of water. In some of the cores, two zones showing a relatively high concentration of terbuthylazine and bromacil were observed at depths of 40–60 and 200–300 cm, respectively. This redistribution pattern of the herbicides could be explained by the preferential flow of the solute in the cores studied.