Applying the Rayleigh Approach for Stable Isotope-Based Analysis of VOC Biodegradation in Diffusion-Dominated Systems

Compound-specific stable isotope analysis (CSIA) has become an established tool for assessing biodegradation in the subsurface. Diffusion-dominated vapor phase transport thereby is often excluded from quantitative assessments due to the problem of diffusive mixing of concentrations with different isotopic signatures for CSIA interpretation. In soils and other unsaturated porous media volatile organic compounds (VOCs) however, are mainly transported via gas-phase diffusion and may thus prohibit a CSIA-based quantitative assessment of the fate of VOCs. The present study presents and verifies a concept for the assessment of biodegradation-induced stable isotope fractionation along a diffusive transport path of VOCs in unsaturated porous media. For this purpose data from batch and column toluene biodegradation experiments in unsaturated porous media were combined with numerical reactive transport simulations; both addressing changes of concentration and stable isotope fractionation of toluene. The numerical simulations are in good agreement with the experiment data, and our results show that the presented analytically derived assessment concept allows using the slope of the Rayleigh plot to obtain reasonable estimates of effective in situ fractionation factors in spite of diffusion-dominated transport. This enlarges the application range of CSIA and provides a mean for a better understanding of VOC fate in the unsaturated subsurface.