Genotoxicity Assessment of Wood-Preserving Waste-Contaminated Soil Undergoing Bioremediation

ABSTRACT

Bioremediation represents one of the most cost-effective technologies for treatment of petroleum hydrocarbons in contaminated surface soils. A major concern for regulatory agencies when evaluating bioremediation is how to determine acceptable levels for residual organics in soil. Although guidelines have been developed for some organics in soil, limited information is available to define acceptable levels of the metabolites of biological degradation. The products of oxidative degradation are likely to be more water soluble and may also be more toxic. The purpose of the current study was to monitor changes in compound concentration and genotoxicity in soils undergoing bioremediation. The site selected for this study was a former wood-preserving site in the northwestern United States. Soil samples were collected over a 4-year period from two 6075-m^sup 2^ land treatment units. Conditions for biodegradation were enhanced by the addition of water and nutrients, as well as by frequent tilling to add oxygen. Due to the location of the facility, the temperature was conducive to a more rapid rate of biodegradation for approximately 6 months per year. Soil samples were collected using a grid system and solvent extracted. Polycyclic aromatic hydrocarbons were quantified in soil extracts using gas chromatography-mass spectrometry (GC/MS), and genotoxicity measured using the Salmonella/microsome assay. After 2 years of treatment, concentrations of total and carcinogenic polycyclic aromatic hydrocarbons (PAHs) were reduced to approximately 10% the concentration in the untreated soil. However, the mean weighted activity of the untreated soil was 293 net revertants per g soil, whereas the extracts of soil collected after 2 years induced a mean weighted activity of 325 net revertants per g soil. Thus, although biodegradation clearly reduced the concentration of total and carcinogenic PAHs in the surface soils, the results from the genotoxicity bioassay indicate that there was a lag in the reduction of mutagenicity in treated soils.

KEYWORDS biodegradation, genotoxicity, polycyclic aromatic hydrocarbons, land treatment

INTRODUCTION

An estimated 275 million tons of hazardous wastes are produced in the United States each year (Ziegler, 1993). Currendy, there are approximately 40,000 sites on the Environmental Protection Agency's inventory of uncontrolled hazardous waste sites. The remediation of these sites is mandated by the federal Superfund statute. As of 1997, 1405 of these sites had been placed on the National Priority List (NPL) (U.S. Environmental Protection Agency [USEPA], 2000a). Determining the adverse health effect of exposures to hazardous substances, especially from environmental sources, poses a challenge for both health and regulatory agencies. Evaluating the health hazard associated with media that have been treated using bioremediation is even more challenging. Although the concentration of parent compounds may be significantly reduced, information regarding the genotoxicity of the metabolites of degradation is often extremely limited.

Based on 1980's demographic data, the National Research Council cites an EPA estimate that approximately 41 million people were living within a 4-mile radius of the 1134 NPL sites at that time (Johnson and DeRosa, 1997). Residence near a site does not constitute an exposure to hazardous substances unless contaminants migrate off-site. Risk assessment of the residual contaminants is a complex issue because population exposures likely include low dose exposures to chemical mixtures over long periods of time (Vrijheid, 2000).

Information is needed to determine whether the measurement of chemical endpoints alone accurately reflects the genotoxic risk posed by exposure to a mixture of chemicals. One of the most difficult questions considered during the remediation of contaminated waste sites is "how clean is clean?" Although biological degradation may be effective at reducing contaminant concentrations, little is known about the toxicity or genotoxicity of the products of degradation.

Previous studies have demonstrated that highmolecular-weight polycycli aromatic …