TEMPORAL DISTRIBUTION OF C-14 IN SOIL-WATER FROM FIELD LYSIMETERS TREATED WITH C-14-METOLACHLOR

In a previous study utilizing fallow field lysimeters of an undisturbe d, loamy sand soil treated with C-14-metolachlor -ethyl-6-methylphenyl )-N-(2-methoxy-1-methylethyl) acetanlide], 2 to 5% of the applied C-14 was mobile to soil depths of 56 to 96 cm. The objective of this 120-d ay study was to determine the temporal distribution of C-14- metolachl or and/or metabolite(s) in soil water from similar field lysimeters an d their possible contribution to groundwater contamination. Undisturbe d soil column field lysimeters (20.3-cm. i.d. X 101-cm long; 16 gauge steel) were driven into a conventionally tilled Dothan loamy sand (fin e-loamy, siliceous, thermic Plinthic Kandiudult) and treated with C-14 -metolachlor and tritiated water ((H2O)-H-3) and subjected to natural rainfall or irrigation. Percent recovery of metolachlor and/or metabol ite(s) in the soil, as based on C-14 measurement, was 62% at 30 days, 63% at 60 days, 51% at 90 days, and 49% at 120 days. Recovery of (H2O) -H-3 was 36, 24, 6 and 0.25% of the applied for the same time periods. By 30 and 60 days after application (DAA), (H2O)-H-3 had distributed symmetrically in the soil profile, whereas, a large percentage of the C-14 was retained in the upper 24 cm. No C-14 and <1% of the applied ( H2O)-H-3 was recovered in leachate the first 30 days. Cumulative recov ery of C-14 in leachate was <1% of that applied at 60 days, 3% at 90 d ays, and 7% at 120 days. Cumulative recovery of (H2O)-H-3 in leachate for the same time periods was 22, 39, and 39% of that applied. The sym metrical breakthrough curve for (H2O)-H-3 indicated no preferential no w or immobile water, whereas the breakthrough curve for C-14 was asymm etrical as a result of the sorption-desorption processes. Peak concent rations of C-14 and (H2O)-H-3 in the leachate occurred at 94 and 63 DA A, respectively. The sorptive tendencies of both radiolabeled species distinguished the magnitude of movement, with (H2O)-H-3 much more mobi le than C-14-metolachlor and/or metabolite(s). Assuming that all C-14 in leachate was parent, average metolachlor concentrations in leachate were less than the National Health Advisory level, which may indicate that metolachlor should be considered a low risk chemical because of its potential to contaminate groundwater in soils with low organic mat ter and high clay content in the subsoil. (Soil Science 1998;163:872-8 82).