A FIELD-STUDY OF LEACHING AND DEGRADATION OF 9 PESTICIDES IN A SANDY SOIL

A field study was conducted on a coastal sandy soil (Karrakata sand) of Western Australia from July to December 1990, to measure the leaching and degradation rates of chlorpyriphos, chlorthal dimethyl, fenamiphos, linuron, metalaxyl, metribuzin, prometryne, propyzamide and simazine. Commercial grade products were sprayed in dilute solution form at recommended rates to the surface of held plots. Irrigation was applied daily which together with rainfall gave effective rainfall equivalent to at least 120% of potential evaporation. Pesticide residue concentrations in soil samples taken at 5 cm increments down to 50 cm, at different days after application, were determined by high performance liquid chromatography (HPLC). Mean leaching depths (MLDs) were calculated by weighting the percentage residues remaining at different depths in the soil profile. The MLDs for the 5 month period followed the order: chlorpyriphos and chlorthal dimethyl (<5 cm) < linuron and simazine (6) < propyzamide (8) and prometryne (9) < metribuzin (12) < metalaxyl (18) < fenamiphos and metabolites (28). This generally corresponded inversely with the sorption coefficients (K-oc). The degradation rates (i.e. decreases with time of total residue concentration in the sampled profile) could mostly be described by first-order regressions (R(2) = 0.59-0.95). Calculated half-lives showed the order: metribuzin (27 days), simazine (28) < fenamiphos (43), chlorthal dimethyl (45) < prometryne (58), propyzamide (59) < metalaxyl (70) < chlorpyriphos (81) < fenamiphos plus metabolites (98) < linuron (219). The potential for causing ground water contamination, as indicated by the ratio of half-life to K-oc followed the order: chlorthal dimethyl, chlorpyriphos < metribuzin, prometryne < simazine, propyzamide < linuron < metalaxyl < fenamiphos and metabolites.