Leveraging Temporal CSIA Groundwater Data and Automated Workflows to Evaluate Degradation Behavior at DNAPL Sites

This study presents a methodology for evaluating natural and enhanced degradation behavior at complex dense non‐aqueous phase liquid (DNAPL) sites via automated processing of compound‐specific isotope analysis (CSIA) groundwater datasets developed over several years of sampling. The method utilizes CSIA datasets from two Australian sites, the Orica Botany Bay Facility in Matraville, New South Wales, and a former chemical manufacturing facility in Victoria, to gain qualitative and semi‐quantitative insights into natural and enhanced attenuation processes. An automated workflow was developed to evaluate CSIA data trends to assess degradation mechanisms and rates, providing insight into contaminant attenuation progress across large, complex sites. In this study, isotopic enrichment factors were estimated based on temporal groundwater concentration and CSIA data at individual well locations, and those enrichment factors were evaluated alongside corresponding geological, microbial, and geochemical data to identify areas where attenuation plays a significant role in contaminant mass reduction. Interpretation of the CSIA data was ground‐truthed by comparing to enrichment factors in the academic literature, assessing other evidence supporting degradation activity, and considering aspects of the site conceptual model that could affect isotopic behavior. By applying an automated workflow to CSIA datasets, the findings of this study demonstrate a valuable standardized approach to gain useful knowledge on the contribution of monitored natural attenuation (MNA) and enhanced biodegradation to contaminant mass reduction at complex sites. The study also illustrates some complexities associated with DNAPL sites (e.g., multiple sources of mass, multiple degradation mechanisms) that need to be considered when interpreting CSIA data.