Field Study of Napropamide Movement Through Unsaturated Soil

A pulse of chloride and napropamide, a moderately absorbed herbicide, was applied to a 1.44-ha loamy sand field and was leached for 2 weeks with 23 cm net applied water. Thirty-six soil cores were taken on a 6 × 6 grid with a 12-m spacing to a depth of 3 m and were analyzed in 0.1-m increments for chloride concentration. Nineteen of the cores were also analyzed for napropamide concentration. Prior to the field leaching experiment, 36 undisturbed soil columns taken on a 6 × 6 grid l m adjacent to the field samples were removed and taken to the laboratory for controlled leaching experiments. Subsamples of soil at each site were used to determine adsorption distribution coefficients by the batch equilibrium method. Distribution coefficients were also calculated from the column leaching experiments by fitting the chloride and napropamide breakthrough curves to the solution of the convection dispersion equation. Although these two methods produced similar field average absorption coefficient values and variances, they were uncorrelated. Both the field cores and the laboratory leaching experiments showed evidence of some napropamide movement without apparent adsorption. This was particularly evident in the field experiments, where 73% of the chemical was found above 10 cm but trace concentrations reached depths down to 180 cm.