Biodegradation of the Organophosphate Insecticide Coumaphos in Highly Contaminated Soils and in Liquid Wastes

Approximately 400000 litres of cattle dip wastes containing approximately 1500 mg litre⁻¹ of the organophosphate insecticide coumaphos are generated yearly along the Mexican border from a USDA program designed to control disease-carrying cattle ticks. Use of unlined evaporation pits for the disposal of these wastes has resulted in highly contaminated soils underlying these sites. Previous work has shown that microbial consortia present in selected dip wastes can be induced to mineralize coumaphos. Our results demonstrate that similar microbial consortia are present in coumaphos-contaminated soils from eight waste sites and that these organisms are capable of mineralizing cou-maphos in these soils using soil slurries to less than 1 mg litre⁻¹ in 7–10 days at 28°C. In addition, our results show that these consortia are able to colonize pea gravel in trickling gravel filters and can be used in these filters to metabolize coumaphos from dip wastes to less than 0·1 mg litre⁻¹ in 7–10 days at 28°C. These simple systems offer potential low cost means to detoxify coumaphos-containing wastes and to bioremediate soils contaminated with this organophosphate compound.