MICROSCALE VARIABILITY OF ATRAZINE AND CHLORIDE LEACHING UNDER FIELD CONDITIONS

Pesticide leaching experiments using widely spaced sampling sites may not adequately characterize chemical leaching behavior such as nonunif orm flow between sampling points. We conducted this study to determine the three-dimensional variability of atrazine and chloride movement w ithin a small volume of soil (2,700 cm(3)) under field conditions, A 1 -m(2) area of Williamson silt loam (coarse-silty, mixed, mesic, Typic Fragiochrept) was sprayed uniformly with atrazine (1.1 kg ai/ha) and c hloride (80 kg/ha). We used the Leaching Estimation and Chemistry Mode l (LEACHM) to simulate chemical movement, After 6.5 cm of rainfall dur ing a 29-d period, we sampled 36 squares (5 by 5 cm) in the central 30 - by 30-cm portion of the treated area at six depth increments (0 to 2 , 2 to 5, to 5, 5 to 10, 10 to 15, 15 to 21, and 21 to 30 cm) and dete rmined atrazine and Cl- concentrations. We recovered 26% of the applie d atrazine and 138% of the applied chloride. Low atrazine recovery may have been due to leaching beyond 30 cm and/or degradation while exces s chloride recovery is attributed to high background concentrations, C oefficients of variation (CVs) for atrazine significantly increased wi th depth and ranged from 26 to 353%, while CVs for Cl- were independen t of depth and ranged from 32 to 66%. Derived atrazine concentration i sograms illustrated highly nonuniform herbicide transport. Although LE ACHM overestimated atrazine movement in the upper 15 cm, it was fairly accurate in the lower 15 cm. The overall trend in Cl- flow was adequa tely predicted, even though the predicted Cl- concentrations were unde restimated. LEACHM could not accurately predict nonuniform flow or the variability in solute concentrations between points, However, its pre diction of the atrazine center oi. mass (about 4.7 cm) agreed well wit h the derived isograms. These findings demonstrate that localized noni deal solute transport may be missed in larger sampling schemes and in simulation models.