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Analytical models were developed that simulate stable isotope ratios ofvolatile organic compounds (VOCs) near a point source contamination in theunsaturated zone. The models describe diffusive transport of VOCs,biodegradation and source ageing. The mass transport is governed by Fick's lawfor diffusion, and the equation for reactive transport of VOCs in the soil gasphase was solved for different source geometries and for different boundaryconditions. Model results were compared to experimental data from aone-dimensional laboratory column and a radial-symmetric field experiment, andthe comparison yielded a satisfying agreement. The model results clearlyillustrate the significant isotope fractionation by gas-phase diffusion undertransient state conditions. This leads to an initial depletion of heavyisotopes with increasing distance from the source. The isotope evolution of thesource is governed by the combined effects of isotope fractionation due tovaporization, diffusion and biodegradation. The net effect can lead to anenrichment or depletion of the heavy isotope in the remaining organic phasedepending on the compound and element considered. Finally, the isotopeevolution of molecules migrating away from the source and undergoingdegradation is governed by a combined degradation and diffusion isotope effect.This suggests that in the unsaturated zone, the interpretation ofbiodegradation based on isotope data must always be based on a model combininggas-phase diffusion and degradation.

Analytical modelling of stable isotope fractionation of volatile organic compounds in the unsaturated zone