Carbon Isotope Fractionation during Diffusion and Biodegradation of Petroleum Hydrocarbons in the Unsaturated Zone: Field Experiment at Værløse Airbase, Denmark, and Modeling

A field experiment was conducted in Denmark in order to evaluate the fate of 13 volatile organic compounds (VOCs) that were buried as an artificial fuel source in the unsaturated zone. Compound-specific isotope analysis showed distinct phases in the ¹³C/¹²C ratio evolution in VOC vapors within 3 m from the source over 114 days. At day 3 and to a lesser extent at day 6, the compounds were depleted in ¹³C by up to −5.7‰ with increasing distance from the source compared to the initial source values. This trend can be explained by faster outward diffusion of the molecules with ¹²C only compared to molecules with a ¹³C. Then, the isotope profile leveled out, and several compounds started to become enriched in ¹³C by up to 9.5‰ with increasing distance from the source, due to preferential removal of the molecules with ¹²C only, through biodegradation. Finally, as the amount of a compound diminished in the source, a ¹³C enrichment was also observed close to the source. The magnitude of isotope fractionation tended to be larger the smaller the mass of the molecule was. This study demonstrates that, in the unsaturated zone, carbon isotope ratios of hydrocarbons are affected by gas-phase diffusion in addition to biodegradation, which was confirmed using a numerical model. Gas-phase diffusion led to shifts in δ¹³C >1‰ during the initial days after the spill, and again during the final stages of source volatilization after >75% of a compound had been removed. In between, diffusion has less of an effect, and thus isotope data can be used as an indicator for hydrocarbon biodegradation.