Combining Molecular Tools and Compound‐Specific Isotope Analysis for Natural Attenuation Assessment
Assessing natural attenuation potential, progress and degradation rates is critical for decision making regarding the feasibility of monitored natural attenuation (MNA) remedies for sites with chlorinated solvents, petroleum hydrocarbons, 1,4-dioxane, and other contaminants. While numerous tests are employed in evaluating natural attenuation, the combination of compound-specific isotope analysis (CSIA) and molecular tools can be particularly useful. CSIA indicates the extent of degradation and quantifies bond breaking processes over non-degradative losses such as sorption. CSIA also has the potential for estimating long-term degradation rates. Molecular tools, including quantitative polymerase chain reaction (qPCR) tests and next generation sequencing (NGS) that quantify key biodegradative microorganisms, functional genes and whole microbial communities, provide insight into whether observed degradation can be explained by biotic processes. CSIA and molecular tools are tertiary lines of evidence in MNA protocols, and provide compelling data to answer the questions (1) Has degradation occurred and to what extent? and (2) Which microbes are responsible for any observed biodegradation?
This presentation will provide examples of where CSIA and molecular genetic data were collected from several sites undergoing MNA assessments. The CSIA data (mainly for carbon: 13C/12C as δ13C, was used to determine if contaminant degradation processes were evident, or absent, based on enrichment of the heavy isotope in the remaining undegraded contaminant fraction. Corresponding molecular genetic data, including qPCR was used to determine if biodegradative microbes were present, and at quantities able to meaningfully impact contaminant degradation. In several cases, CSIA and molecular genetic testing data indicated natural attenuation processes were occurring and clarified complex biotic and abiotic remediation processes. At a site in Alaska, trichloroethene to cis-1,2-dichloroethene (cDCE) degradation was conclusively confirmed by CSIA but further cDCE degradation by reductive or oxidative processes was inconclusive. By correlating the extent of 13C-enrichment in cDCE with the proportion of dechlorinators as determined by molecular tools the case for natural attenuation of cDCE was more compelling. At and industrial site, downgradient detections of 1,2-DCA and PCE were attributed to production of these compounds, from breakdown of 1,1,2-TCA and pentachloroethane respectively, and not transport of 1,2-DCA and PCE from the source zone. At a site in Denmark a complex combination of reductive dechlorination abiotic degradation by iron sulfides and aerobic bioremediation was indicated by CSIA and molecular data.
Overall, the combination of CSIA and molecular tools allows conclusions to be drawn on natural attenuation processes that are not easily arrived at by using other analytical methods.