Modelling the Spatial and Temporal Trends of Carbon Isotope Fractionation of VOC Mixtures in the Unsaturated Zone

The transport of volatile organic contaminant (VOC) mixtures in the unsaturated zone is typically controlled by a combination of transport (i.e., advection, dispersion, and diffusion) phenomena, phase change (i.e., volatilization, dissolution) and reaction processes (i.e., sorption, biodegradation). As revealed from field and laboratory studies, biodegradation is probably the most important attenuation mechanism of VOCs in the unsaturated zone since it transforms organic contaminants to harmless products. However, in field applications, it is usually difficult, time consuming, or expensive to distinguish and quantify biodegradation among the different natural phenomena and processes, such as diffusion, dispersion, volatilization and sorption, without the aid of numerical modeling. Recent experimental studies showed large variations in space and time in the 13C/12C ratio for many organic compounds and suggested the possibility of using carbon isotope fractionation as a tool to assess biodegradation of organic contaminants in the unsaturated zone. The processes that were indicated to likely have the largest effect on the observed isotope ratio variations are (i) diffusion, which results in molecules with lighter isotopes to diffuse slightly faster than molecules with heavier isotopes, and (ii) biodegradation as a consequence of a preferential breakdown of the chemical bonds of the lighter isotopes. This study used numerical modeling of VOC mixture transport in the unsaturated zone to quantify the contribution of diffusion and biodegradation to carbon isotope fractionation of individual compounds, and to evaluate the use of 13C/12C ratio as a possible tool to assess biodegradation of organic contaminants. The numerical model incorporated transport of carbon isotope fractions of selected organic compounds and used data from a controlled fuel source emplacement field experiment and parameters from laboratory-scale studies. Costeffective modelling of the transport and fate of volatile organic contaminant mixtures was realized with the use of the recently developed constituent averaging technique [1]. Results indicated that (i) both diffusion and biodegradation lead to enrichment in 13C compared to the initial isotope ratio in locations within and close to the contaminant source, (ii) biodegradation lead to enrichment in 13C while diffusion resulted in smaller 13C/12C ratio in locations further away from the source, and (iii) the above effect of both processes on the isotope ratio was increased as a function of time. Modelling results showed that the use of carbon isotope fractionation at the field represents a promising tool for assessing and possibly quantifying biodegradation of organic contaminants in the unsaturated zone.