Assessment of Problems Associated vvith Landfilling or Land Application of Pesticide Waste and Feasibility of Cleanup by Microbiological Degradation

During 1986 soil contaminated with high concentrations of herbicides from drainage of wastewater was excavated and applied to an adjacent field in an effort to stimulate degradation of the pesticides. The field was divided into two plots that were planted with com or soybeans. As a continuation of this earlier study, part of the current project focused on monitoring herbicide residues in the land-applied soil during a second growing season. After two years of monitoring herbicide persistence and biological activity in the plots, it was concluded that prolonged persistence of herbicide residues in soil and crop phytotoxicity are potential problems with land application of herbicide waste. To improve the prospects for use of land application as a remediation tool and the possibility of augmenting biodegradation of pesticide wastes in soil, a series of laboratory experiments were designed to explore microbiological aspects of the persistence of high concentrations of herbicides in soil. Most of the studies focused on the alachlor, which is an acetanilide herbicide used in corn and soybeans. In laboratory experiments, alachlor was not degraded after simulated spills of 1000 ppm or 10,000 ppm. In contrast, 10 ppm and 100 ppm doses of alachlor were partially degraded into water-soluble metabolites Technical-grade alachlor and an emulsifiable concentrate formulation were metabolized similarly. Microbial populations and dehydrogenase activity in soil were reduced upon exposure to 1000 ppm or 10,000 ppm alachlor, and lack of degradation of these high concentrations was attributed to microbial toxicity from alachlor itself rather than additives in its formulation. Amendment of soils with ground corn or soybean stubble enhanced the dissipation of alachlor at a concentration of 100 ppm but not at 1000 ppm. Aged residues of alachlor at a concentration of 100 ppm degraded in soil as fast as or faster than an equal concentration of freshly applied herbicide. Inoculation of soil with a Fusarium sp isolate that could cometabolically degrade alachlor did not enhance the biodegradation of a 100 ppm dose. Future attempts at bioremediation of herbicide wastes in soil should focus on the use of biostimulation with nutrient amendments and extending the range of contaminant concentrations at which biodegradation will rapidly proceed