Research approach and overview of the thesis 18 

The specific aim of the present PhD thesis was to address the following four research questions: A) Does aqueous phase diffusion and sorption lead to measurable shifts of isotope ratios? B) Are isotope effects due to diffusion and sorption detectable under field conditions? C) Do isotope effects associated with sorption and diffusion impair the identification of reactive processes in aquifer - aquitard systems? and D) Can stable isotope methods provide insight into the impact of reactive processes in aquitards on plume persistence in aquifers? To answer these four research question a research approach consisting of a combination between laboratory, field and modeling methods was applied as outlined in the following paragraph.

Isotope fractionation associated with diffusion and sorption was constrained with laboratory experiment. The mechanism of isotope fractionation during aqueous phase diffusion was investigated in more detail by using molecular dynamic simulation. To evaluate if diffusion- and sorption-induced fractionation also occurs at the field scale, controlled-release experiments were used thanks to the collaboration with the University of Guelph in Canada. Such experiments open the possibility to attribute shifts of isotope ratios to individual or combined processes with a high certainty in contrast to accidental spill sites, where the initial conditions of the contamination source (composition, spill time, volume) are often unknown. Two such controlled release experiments were performed at sites with and without occurrence of (bio)degradation. By investigating these contrasting sites, insight can also be gained as to what extend isotope fractionation associated with sorption and diffusion affects the identification of reactive processes and whether isotope data can be used to demonstrated (bio)degradation in low permeability sediments. Furthermore, it was investigated how (bio)degradation in low permeability sediments affects groundwater quality by using numerical modelling. The described studies are presented in this PhD thesis in the different chapters as follows:

In chapter 2, existing standard diffusion models were reviewed under the aspect if and how a mass dependency of the diffusive transport rate is conceptualized to provide a theoretical background for assessing the magnitude of isotope fractionation due to aqueous phase diffusion.

In chapter 3, the magnitude of diffusion-induced isotope fractionation was quantified on the laboratory scale for two chlorinated hydrocarbons by performing a Rayleigh type diffusion cell experiment. At the field scale, a water-saturated clay core was retrieved from the bottom of a

contaminated site and subsampled as a function of depth to investigate shifts of isotope ratios during downward diffusion into the clay. To compare field and laboratory data, field isotope ratio profiles were simulated using a 1D-diffusion model based on the experimentally determined magnitude of isotope fractionation due to diffusion.

In chapter 4, molecular dynamic (MD) simulations were performed to gain molecular scale insights into the mass dependency of the diffusive transport rate for isotopically distinct chlorinated hydrocarbons. Furthermore, the determined diffusion coefficients from MD simulations were compared with the laboratory determined magnitude of isotope fractionation due to aqueous phase diffusion in chapter 3.

In chapter 5, the goal was to quantify the magnitude of sorption induced isotope fractionation at the laboratory scale and to assess whether sorption induced isotope fractionation is also detectable at the field scale. Moreover, it was assessed how isotope ratios evolve when sorption and diffusion interact. At the laboratory scale a multistep sorption experiment was conducted to quantify sorption induced isotope enrichment factors. At the field scale a long-term field experiment was performed by emplacing two different DNAPL phases into a non-reactive clay unit, where diffusion and sorption occur together. About 15.5 years after emplacement isotope ratio profiles were determined in clay unit to investigate whether isotope shifts with depth can be assigned to diffusion and/or sorption processes. Moreover, to relate laboratory with field observation, determined isotope ratio profiles were compared with a 2D axisymmetrical numerical model, which included laboratory determined enrichment factors for sorption and diffusion.

In chapter 6, the aim was to evaluate whether stable isotope methods can be used to quantify degradation processes in saturated low permeability sediments. For that purpose a second long-term field experiment was conducted by infiltrating 50 liters of a three-component DNAPL mixture into an aquifer overlying an aquitard, in which degradation was expected. Nearly 14.5 years after DNAPL infiltration (5281 days), it was assess whether depth discrete isotope ratio profiles of chlorinated hydrocarbons, which had diffused into the aquitard, can be attributed to reactive processes in the aquitard.

In chapter 7, the effect of chlorinated hydrocarbon degradation in aquitards on plume persistence due to back-diffusion was investigated. Furthermore it was evaluated whether stable isotope methods can be used to track vertically varying aquitard degradation conditions. For that

purpose three different aquitard degradation scenarios were simulated: No-degradation, uniform degradation and non-uniform degradation by adopting a previously described numerical model of an aquifer – aquitard system. In addition an aquifer degradation scenario was simulated and compared with the different aquitard degradation scenarios to assess whether stable isotope methods can also be used to distinguish between degradation activities in the aquifer and the aquitard.

In chapter 8, the general conclusions of this PhD thesis are provided and new research questions are listed, which have been risen during this PhD.