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

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Abstract

In a previous study utilizing fallow field lysimeters of an undisturbed, loamy sand soil treated with 14C-metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide], 2 to 5% of the applied 14C was mobile to soil depths of 56 to 96 cm. The objective of this 120-day study was to determine the temporal distribution of 14C-metolachlor and/or metabolite(s) in soil water from similar field lysimeters and their possible contribution to groundwater contamination. Undisturbed soil column field lysimeters (20.3-cm i.d. × 101-cm long; 16 gauge steel) were driven into a conventionally tilled Dothan loamy sand (fine-loamy, siliceous, thermic Plinthic Kandiudult) and treated with 14C-metolachlor and tritiated water (3H2O) and subjected to natural rainfall or irrigation. Percent recovery of metolachlor and/or metabolite(s) in the soil, as based on 14C measurement, was 62% at 30 days, 63% at 60 days, 51% at 90 days, and 49% at 120 days. Recovery of 3H2O was 36, 24, 6 and 0.25% of the applied for the same time periods. By 30 and 60 days after application (DAA), 3H2O had distributed symmetrically in the soil profile, whereas, a large percentage of the 14C was retained in the upper 24 cm. No 14C and <1% of the applied 3H2O was recovered in leachate the first 30 days. Cumulative recovery of 14C in leachate was <1% of that applied at 60 days, 3% at 90 days, and 7% at 120 days. Cumulative recovery of 3H2O in leachate for the same time periods was 22, 39, and 39% of that applied. The symmetrical breakthrough curve for 3H2O indicated no preferential flow or immobile water, whereas the breakthrough curve for 14C was asymmetrical as a result of the sorption-desorption processes. Peak concentrations of 14C and 3H2O in the leachate occurred at 94 and 63 DAA, respectively. The sorptive tendencies of both radiolabeled species distinguished the magnitude of movement, with 3H2O much more mobile than 14C-metolachlor and/or metabolite(s). Assuming that all 14C 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 matter and high clay content in the subsoil.

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