Multi-scale transport and thermally-enhanced bioremediation of chlorinated ethenes in subsurface porous media

The accurate description of multi-scale natural transport and temperature-dependent bioremediation of chlorinated ethenes is of fundamental importance to understand the interplay between non-reactive and bioreactive processes in heterogeneous groundwater systems. The study of transport phenomena at various scales leads to a mechanistic understanding of underlying physical mass transfer processes, while process-based modeling of bioremediation activities sheds light on the complex coupling of fluid flow, heat transport, and reactive solute transport during thermallyenhanced bioremediation of chlorinated ethenes.

The overall aim of this PhD project is to enhance the mechanistic understanding of natural contaminant transport and thermally-enhanced bioremediation of chlorinated ethenes in subsurface porous media. Accordingly, this thesis presents research on non-reactive and bioreactive mass transfer processes, which occur during the transport of chlorinated ethenes in heterogeneous aquifer systems. To this end, detailed experimental investigations at laboratory and pilot scale were combined with high-resolution process-based modeling in three studies. This combination results in a fundamental understanding of the key principles of non-reactive and bioreactive transport processes both at multiple scales as well as considering various hydraulic, thermal, and operational conditions in an in situ bioremediation setup.

In a first study, the effect of diffusive-dispersive isotope fractionation on carbon and chlorine stable isotopes of chlorinated ethenes was investigated by employing a quasi-two-dimensional flow-through setup; the results were quantitatively
interpreted using a detailed numerical model. This model was used to explore the physical effects of diffusion and dispersion at the pore scale and larger field scale. It was demonstrated that diffusion and dispersion lead to significant isotope fractionation (–6‰ for carbon; –10‰ for chlorine) in a pore-scale domain not only at low flow velocities, but also at high flow velocities of 10 m/day. These effects were experimentally validated in bench-scale laboratory setups, where strong isotope fractionation was determined at the fringes of a cis-DCE contaminant plume. Diffusive-dispersive isotope fractionation also manifests at the larger field scale; this was shown by results at field scale, where transport in a heterogeneous field-scale cross section with aquifer characteristics representative of sandy aquifer sediments was investigated. The results of this multi-scale investigation highlighted the key role of incomplete mixing for diffusive-dispersive isotope fractionation at various scales. Incomplete mixing significantly influenced the isotope pattern of carbon and chlorine stable isotopes of non-degrading chlorinated ethene plumes in subsurface porous media. However, bioreactive processes often occur in contaminated aquifers and temperature changes play an important role for remediation interventions.

Therefore, in the second work, a process-based modeling framework was developed to assess and quantitatively interpret the results of a pilot-scale remediation intervention, which combined Aquifer Thermal Energy Storage (ATES) and in situ bioremediation (ISB) of chlorinated ethenes. The model accounted for the complex coupling of fluid flow, heat transport, solute transport, and non-isothermal bioreactive transport processes. Chlorinated ethenes were sequentially transformed through microbial reductive dechlorination. Amendments, such as lactate and specialized degraders were injected into the aquifer. A dipole recirculation system was operated by abstracting cold groundwater, heating it to 21 °C, and subsequent reinjection into the contaminated aquifer. The outcomes of this experimental and model-based investigation highlighted the benefits of combining ATES with ISB and showed the enhancement of contaminant mass reduction and more complete reductive dechlorination compared to a natural attenuation scenario.

Finally, in a third study, data from microcosms was combined with reactive transport scenario simulations, to explore the effectiveness of the ATES-ISB approach under various hydraulic and thermal conditions in a heterogeneous aquifer. Scenario modeling was carried out using the developed numerical simulator with input data from temperature-dependent biodegradation microcosm studies. Furthermore, the range for the optimal injection temperature was detected at 20 °C to 30°C, where the indigenous bacteria grow and can carry out reductive dechlorination under optimal biogeochemical conditions.

In conclusion, the research performed in this PhD project has improved the understanding of multi-scale natural transport of chlorinated ethenes, as well as providing a mechanistic understanding of the complex interplay between physical
and biogeochemical processes during thermally-enhanced bioremediation by characterizing: (i) the effects of diffusive-dispersive isotope fractionation on the transport of chlorinated ethenes in non-degrading contaminant plumes; (ii) the role of temperature on microbial reductive dechlorination in bioreactive remediation interventions in heterogeneous groundwater systems; and (iii) the influence of hydraulic and thermal conditions in contaminated aquifers on the efficiency of fullscale ATES-ISB systems.