Metabolite Analysis as Direct Proof of Biodegradation: Experience from Monitored Natural Attenuation (MNA) Projects

Abstract

Signature metabolites provide direct geochemical indication that in-situ biodegradation of released organic compounds (e.g., oil and its refined products) is occurring. Experience shows that monitored natural attenuation site conditions are often more complex than in theory and often require a more profound comprehension of the governing natural attenuation processes. Frequently, there is lack of direct proof that contaminant degradation (mainly through biodegradation) is occurring. Advanced tools are emerging that aim to provide answers about whether contaminants of concern are actually (bio)degraded and to the extent. Signature metabolite analysis provides direct proof of mineral oil hydrocarbon biodegradation and is among these advanced tools. Yet, during the previous 15 years, metabolite analysis has only been used sporadically in research projects. The target metabolites consist of aromatic acids such as benzoates and benzylsuccinates and uniquely indicate in-situ biodegradation of individual contaminants of concern. Three case studies have been summarized to share practical experience with signature metabolite analysis for contaminants that include benzene, toluene, ethyl benzene, and xylenes; trimethylbenzenes; and polycyclic aromatic hydrocarbons. The summarized case study involving jet fuel-contamination was the first reported field study in which aromatic acid homologs were formed by microbial metabolism of C4 through C7 benzene. Signature metabolite analysis can be used to improve understanding of natural attenuation processes to close data gaps with respect to the general degradation mechanisms. Direct evidence for biodegradation (e.g., metabolite identity and concentration; microbial identity and quantity; daughter product degradation ratios, stable carbon isotope ratios) facilitates remediation planning and management; provides information useful to scientists and engineers that must determine the mechanisms that produced observed environmental conditions; and provides information for stakeholders involved in environmental cleanup litigation and cost appropriation.