The objective of this project is to cross-validate carbon dioxide flux and radiocarbon analysis techniques with long-term site assessments to determine chlorinated hydrocarbon degradation rates (the priority, but methods will be applicable to any petroleum-feedstock contaminant) and develop a decision-support tool (DST) for selecting appropriate techniques given site-specific conditions.

This project will cross-validate three independent, although related, methods for determining CO₂ flux coupled with radiocarbon analysis at the same site over the same time-frame. One method involves soil:atmosphere flux, one uses groundwater well headspace CO₂ trapping developed under SERDP project ER-2338, while the last has only recently been described using short-term incubations, similar to biological oxygen demand (BOD) testing. These variations for combining CO₂ flux and radiocarbon all offer the ability to determine the actual contaminant conversion to CO₂ by isotopically tracking the contaminant's carbon backbone. These methods differ by their ease of use, post-deployment sampling/data processing, and ability to resolve temporal changes in contaminant degradation. These differences impact their applicability to individual sites’ characteristics.

Each method will be evaluated relative to REMChlor modeling as a central data repository for the more traditional lines of evidence approach. REMChlor results will be evaluated within the context of all data collected (mature site(s) will be targeted for this proposed effort). A decision support tool (DST) outlining technology applicability will be developed as a transition product for the environmental management community.

With costs far outpacing available resources for site assessment and cleanup, robust tools to evaluate monitored natural attenuation (MNA) and engineered solutions are necessary. Technologies have been developed that use radiocarbon analysis to differentiate old contaminant-related carbon from modern sources not related to contamination. These methodologies target the contaminant carbon backbone and thus are able to accurately measure contaminant degradation over time. These technologies can be deployed using several modes each having different potential suitability based on site characteristics. Well-studied sites may have ample data such that models may provide adequate site management information with minimal CO₂ radiocarbon evidence (e.g. radiocarbon could be used simply to confirm natural attenuation is occurring). Overall, sites with varying characteristics and needs will require varying degrees of coupled flux-radiocarbon evidence based on physical layout, existing data, Record of Decision (ROD) requirements, Remedial Project Manager (RPM) needs, etc. This project seeks to create a DST encompassing these disparate but related characteristics with the end goal to decrease overall uncertainty and increase cost effectiveness and cost avoidance. (Anticipated Project Completion - 2023)