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.