  Analytical models were developed that simulate stable isotope ratios of
volatile organic compounds (VOCs) near a point source contamination in the
unsaturated zone. The models describe diffusive transport of VOCs,
biodegradation and source ageing. The mass transport is governed by Fick's law
for diffusion, and the equation for reactive transport of VOCs in the soil gas
phase was solved for different source geometries and for different boundary
conditions. Model results were compared to experimental data from a
one-dimensional laboratory column and a radial-symmetric field experiment, and
the comparison yielded a satisfying agreement. The model results clearly
illustrate the significant isotope fractionation by gas-phase diffusion under
transient state conditions. This leads to an initial depletion of heavy
isotopes with increasing distance from the source. The isotope evolution of the
source is governed by the combined effects of isotope fractionation due to
vaporization, diffusion and biodegradation. The net effect can lead to an
enrichment or depletion of the heavy isotope in the remaining organic phase
depending on the compound and element considered. Finally, the isotope
evolution of molecules migrating away from the source and undergoing
degradation is governed by a combined degradation and diffusion isotope effect.
This suggests that in the unsaturated zone, the interpretation of
biodegradation based on isotope data must always be based on a model combining
gas-phase diffusion and degradation.
