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) o f Western Australia from July to December 1990, to measure the leachin g and degradation rates of chlorpyriphos, chlorthal dimethyl, fenamiph os, linuron, metalaxyl, metribuzin, prometryne, propyzamide and simazi ne. 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 concentr ations in soil samples taken at 5 cm increments down to 50 cm, at diff erent days after application, were determined by high performance liqu id 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 inv ersely 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 day s), simazine (28) < fenamiphos (43), chlorthal dimethyl (45) < prometr yne (58), propyzamide (59) < metalaxyl (70) < chlorpyriphos (81) < fen amiphos plus metabolites (98) < linuron (219). The potential for causi ng ground water contamination, as indicated by the ratio of half-life to K-oc followed the order: chlorthal dimethyl, chlorpyriphos < metrib uzin, prometryne < simazine, propyzamide < linuron < metalaxyl < fenam iphos and metabolites.