U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Method for Estimating the Activity of Carbofuran-Degrading Microorganisms in Environmental Samples

Abstract

A method was developed to determine the presence and activity of carbofuran degrading organisms in aqueous field samples. Carbofuran is a broad spectrum systemic insecticide used on rice in the Sacramento Valley of California. Following the release of treated rice irrigation water, concentrations of carbofuran can be detected in agricultural drains and in the Sacramento River. Microbial populations influence the degradation rate of carbofuran in soils, but little is known about the importance of these organisms in the degradation of carbofuran in aqueous systems. The method described uses laboratory-scale microcosms containing an environmental culture and carbon-14 (¹⁴C) ring-labeled carbofuran to evaluate the ability of microorganisms to degrade carbofuran. Degradation is indicated by the evolution of ¹⁴C-carbofuran-7-phenol and carbon-14-carbon dioxide (¹⁴C-CO₂). Ethyl acetate was used to extract the ¹⁴C ring-labeled compounds from aqueous solution. ¹⁴C-carbofuran was resolved from ¹⁴C-carbofuran-7-phenol by means of thin layer chromatography, and the ¹⁴C quantified using a radioisotope plate reader. Sodium hydroxide base traps were used to capture the ¹⁴C-CO₂ evolved from the cultures and quantified using a scintillation counter. The method was tested using three different aqueous cultures. Overall recoveries of ¹⁴C were 66.8 percent plus or minus 6.3 percent at 95-percent confidence interval (n=24).