Reactive transport and stable isotope fractionation of volatilized petroleum hydrocarbons in porous media: Experimental and numerical analysis

Abstract

Diffusion-dominated vapor phase transport is often excluded from quantitative assessments due to the problem of diffusive mixing of concentrations with different isotopic signatures for Compound-specific stable isotope analysis – CSIA interpretation. This thesis presents and verifies a concept for the assessment of biodegradation-induced stable isotope fractionation along a diffusive transport path of VOCs in unsaturated porous media. Reactive transport of vapor-phase VOCs in the subsurface is controlled by a multitude of interrelated factors, such as properties of the chemicals, the presence of microbial communities and the soil properties. The approach used in this thesis is to highlight and interpret the data obtained by multiple experiments and modeling scenarios. To fill the knowledge gap and to improve the understanding of dynamics of vapor-phase VOCs in the unsaturated subsurface environment, various sets of laboratory column experiments were performed. Furthermore, an approximation of Rayleigh-model was derived, which in turn allowed derivation of a stable isotope fractionation factor for diffusion and biodegradation from various depth profiles of vapor-phase VOC concentrations and the associated isotope ratios. The thesis in general consists of: laboratory experiments dealing with vapor transport, phase exchange, biodegradation, CSIA, and numerical model simulations of biodegradation using parameters derived from experiments. Biodegradation is an important removal process of gas phase VOCs in unsaturated porous media. Toluene (non-deuterated and deuterated) has been studied as a model VOC in this work to assess biodegradation by combining data on the concentration gradients in the column and batch reactors and on the associated stable isotope fractionation with numerical modeling. A conservative volatile tracer (MTBE, non-reactive with the given bacterial strain) was used to quantify physical transport mechanisms independently of biological or chemical interactions. Our results show that the presence of bacteria along with sufficient amount of VOCs, moisture, and oxygen lead to total removal of the VOCs. Hence, the unsaturated zone can be an efficient biofilter for VOCs emanating from the groundwater and helps to avoid emissions to the atmosphere even at high gas-phase diffusion rates. However, our findings are based on the key assumptions that there is no governing process for mass transfer except diffusion and the whole reactor setups represents an unsaturated zone with only gas-phase chemicals. Therefore, the results would be misleading if extrapolated to the entire vadose zone. Our results further show that biodegradation can be limited by mass transfer of contaminants to the cells and by toxicity effects due to the combination of chemicals present in the system.