Estimation of Granular Carbofuran Dissolution Rates in Soil

Abstract

Losses of efficacy are due to rates of microbial degradation which exceed rates of granular leaching/dissolution, resulting in carbofuran concentrations below the lethal threshold needed for rootworm control. Previous studies have documented enhanced rates of carbofuran biodegradation and concentrations required for corn rootworm control; however, little information is available on rates of release of carbofuran from granules. Rates of carbofuran leaching from granules as a function of rainfall intensity/infiltration rate, and of granular dissolution as a function of time, were estimated using a rain simulation device. Rates of leaching (1.5 µg (mg of granule)−1 (cm of rainfall)−1) were positively correlated with rainfall/infiltration, and rates of granular dissolution were linear with time (0.28 µg (mg of granule)−1 h−1). Adjusted for field conditions, ca. 50 h of rainfall would be required to leach carbofuran from granules or ca. 11 days (at field capacity) for dissolution of granules (assuming linear rates). Soil solution concentrations would be ca. 16 µg mL−1, or 4 µg (g of bulk soil)−1 (at field capacity and a partition coefficient of 0.1). Rates of spherical carbofuran diffusion from a theoretical granule were calculated. High carbofuran concentrations in the vicinity of granules may result in localized high population densities of carbofuran-degrading microorganisms. In combination with previous studies, these data suggest that losses of efficacy may potentially result either from the complete leaching/dissolution of granules and biodegradation before larvae hatch or from rates of biodegradation which exceed rates of granular leaching/dissolution after larvae hatch. © 1993, American Chemical Society. All rights reserved.