When does back-diffusion from low-permeability porous media into aquifers create isotope fractionation?

At many historically polluted sites with chlorinated solvents, dissolved compounds have diffused into aquitards by aqueous diffusion. Remediation of the residual solvents in aquifers is possible, but after source removal, back diffusion of the dissolved compounds from aquitards can sustain groundwater contamination and requires prolonged post-remediation monitoring. The objective of this work is to evaluate the applicability of compound-specific stable isotope analysis (CSIA) as a tool for monitoring processes at sites impacted by back diffusion. Diffusion experiments in agar gels were performed with the three chlorinated solvents cis-1,2-dichloroethene (CIS), trichloroethene (TCE) and chlorobenzene (CB). No significant isotope fractionation was observed for any of the compounds during back diffusion from agar gels. The isotope fractionation during sorption to either Montmorillonite, Kaolinite and Amberlite XAD-2 yielded small inverse isotope effects for CB on Montmorillonite, and for CB and TCE on Amberlite XAD-2. Analytical reactive transport equations were developed to simulate the carbon isotope fractionation of compounds during forward and back diffusion in aquitards. The model identifies the conditions under which measurable isotope shifts in TCE, used as a model compound, are generated diffusing forward into a clay aquifer for 45 years and then diffusing back for 100 years. The model indicated that back diffusion cancels small isotope effects that occurred during forward diffusion in absence of degradation. The model also showed that aquitards can only contribute to isotope fractionation when fractionating degradation reactions are present in the aquitard (e.g., abiotic reductions). The results of this study provide an improved scientific basis for applying CSIA for post-remediation process monitoring in setting with back diffusion from contaminated aquitards.