Carbon Isotope Fractionation During Volatilization of Petroleum Hydrocarbons and Diffusion Across a Porous Medium: A Column Experiment

Abstract

The study focuses on the effect of volatilization, diffusion, and biodegradation on the isotope evolution of volatile organic compounds (VOCs) in a 1.06 m long column filled with alluvial sand. A liquid mixture of 10 VOCs was placed at one end of the column, and measurements of VOC vapor concentrations and compound-specific isotope ratios (delta(13)C) were performed at the source and along the column. Initially, the compounds became depleted in 13C by up to -4.8% per hundred along the column axis, until at 26 h, uniform isotope profiles were observed for most compounds, which is expected for steady-state diffusion. Subsequently, several compounds (n-pentane, benzene, n-hexane) became enriched in 13C throughout the column. For the same compounds, a significant decrease in the source vapor concentration and a gradual enrichment of 13C by up to 5.3% per hundred at the source over a period of 336 h was observed. This trend can be explained by a larger diffusive mass flux for molecules with light isotopes compared to those with a heavy isotope, which leads to a depletion of light isotopes in the source. The isotope evolution of the source followed closely a Rayleigh trend and the obtained isotope enrichment factor corresponded well to the ratio between the diffusion coefficients for heavy and light molecules as expected based on theory. In contrastto diffusion, biodegradation had generally only a small effect on the isotope profiles, which is expected because in a diffusion-controlled system the isotope shift per decrease of mass flux is smaller than in an advection-controlled system. These findings open interesting perspectives for monitoring source depletion with isotope and have implications for assessing biodegradation and source variability in the unsaturated zone based on isotopes.