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

Abstract

In a previous study utilizing fallow field lysimeters of an undisturbed, loamy sand soil treated with 14 C-metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide], 2 to 5% of the applied 14 C was mobile to soil depths of 56 to 96 cm. The objective of this 120-day study was to determine the temporal distribution of 14 C-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 14 C-metolachlor and tritiated water ( 3 H 2 O) and subjected to natural rainfall or irrigation. Percent recovery of metolachlor and/or metabolite(s) in the soil, as based on 14 C measurement, was 62% at 30 days, 63% at 60 days, 51% at 90 days, and 49% at 120 days. Recovery of 3 H 2 O was 36, 24, 6 and 0.25% of the applied for the same time periods. By 30 and 60 days after application (DAA), 3 H 2 O had distributed symmetrically in the soil profile, whereas, a large percentage of the 14 C was retained in the upper 24 cm. No 14 C and <1% of the applied 3 H 2 O was recovered in leachate the first 30 days. Cumulative recovery of 14 C in leachate was <1% of that applied at 60 days, 3% at 90 days, and 7% at 120 days. Cumulative recovery of 3 H 2 O in leachate for the same time periods was 22, 39, and 39% of that applied. The symmetrical breakthrough curve for 3 H 2 O indicated no preferential flow or immobile water, whereas the breakthrough curve for 14 C was asymmetrical as a result of the sorption-desorption processes. Peak concentrations of 14 C and 3 H 2 O in the leachate occurred at 94 and 63 DAA, respectively. The sorptive tendencies of both radiolabeled species distinguished the magnitude of movement, with 3 H 2 O much more mobile than 14 C-metolachlor and/or metabolite(s). Assuming that all 14 C 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.