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Technical Report

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A theoretical and experimental study of the transport of pesticides was conducted in several Arkansas soils with metribuzin, a herbicide. In a field study, chloride and metribuzin were applied to a Captina silt loam under maximum leaching conditions and their redistribution was compared with that of soil water. Metribuzin was found in significantly detectable quantities to a depth of 61 cm; the largest concentrations were detected in the surface 23 cm and particularly in the 0-5 cm increment. Two days after application 72.6 and 33.6% could be detected in the vegetation and no-vegetation plots. The metribuzin half life was 7.88 and 5.13 days in the no-vegetation and vegetation plots, respectively. Chloride was found throughout the profile. Metribuzin and chloride generally were observed to move in the same direction as soil water, but at a considerably slower rate. Persistence of metribuzin within the soil was influenced greatly by microbial degradation. The laboratory studies centered on further quantifying the transport and adsorption-desorption parameters of metribuzin under controlled environmental conditions. Diffusion coefficients of 14C - metribuzin, 36CI, and 3HOH were shown to be influenced by soil type, soil water content, and soil temperature. The magnitude of the diffusivities were in the order 3HOH>^^Cl>^^C-metribuzin; however, the ratios varied. The rates of adsorption of metribuzin were found to be dependent on shaking time and soil type. For the most part linear adsorption isotherms were observed. Desorption rates were found to be influenced by solution concentration, shaking time and soil type. It was concluded from these studies that the potential polluting effects of metribuzin leaching through the soil and subsequently moving into the water table or underground streams are minimal. Metribuzin will redistribute within the soil profile, but will be degraded by microorganisms before it becomes a potential pollution hazard.

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