Assessment of the Effectiveness of Composting for the Reduction of Toxicity and Mutagenicity of Explosive-Contaminated Soil

Abstract

Composting is being developed as an economical method for remediating explosive-contaminated soils and has been found to reduce the concentrations of target contaminants such as 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). However, whether environmental safety is improved by composting can be determined only by assessing the effects of the treated material on living organisms. In this study two bioassays, the Mutatox assay and the earthworm acute toxicity test, were used to evaluate the effectiveness of a pilot-scale composting demonstration in reducing environmental hazard. Explosive-contaminated soil was collected from a military installation and amended for composting in two adiabatic reactors. The unamended soil was lethal to all exposed earthworms, as were both amended replicates, prior to composting. Serial dilutions of the finished composts with artificial soil had earthworm 14-day LC50values of 35.7 and 100% finished compost:artificial soil. Extracts of the initial materials were also toxic to bacteria in the Mutatox assay. Dilutions of those extracts to sublethal concentrations revealed a low level of mutagenicity. Extras of the finished composts indicated reduced bacterial toxicity, but the mutagenicity was markedly increased by composting. The reduction in lethality reflected the attenuation of explosives caused by composting, as indicated by chemical analysis. However, the increased mutagenicity was a result that would not have been indicated by chemical analysis alone and is inferred to be the result of the formation of mutagenic metabolites of explosives during composting and their incomplete degradation.