Zimmermann Jeremy, Halloran Landon J.S., Hunkeler Daniel (2020), Tracking chlorinated contaminants in the subsurface using compound-specific chlorine isotope analysis: A review of principles, current challenges and applications, in Chemosphere
, 244, 125476-125476.
Murray Alexandra Marie, Ottosen Cecilie B., Maillard Julien, Holliger Christof, Johansen Anders, Brabæk Lærke, Kristensen Inge Lise, Zimmermann Jeremy, Hunkeler Daniel, Broholm Mette M. (2019), Chlorinated ethene plume evolution after source thermal remediation: Determination of degradation rates and mechanisms, in Journal of Contaminant Hydrology
, 227, 103551-103551.
Halloran Landon J.S., Brunner Philip, Hunkeler Daniel (2019), COMPEST, a PEST-COMSOL interface for inverse multiphysics modelling: Development and application to isotopic fractionation of groundwater contaminants, in Computers & Geosciences
, 126, 107-119.
Badin Alice, Braun Fabian, Halloran Landon J. S., Maillard Julien, Hunkeler Daniel (2018), Modelling of C/Cl isotopic behaviour during chloroethene biotic reductive dechlorination: Capabilities and limitations of simplified and comprehensive models, in PLOS ONE
, 13(8), e0202416-e0202416.
Chlorinated hydrocarbons (CHCs) are among the most common groundwater contaminants. CHC are generally released as dense non-aqueous phase liquids (DNAPLs) that accumulate in complex patterns in the subsurface. The persistence of CHC in the subsurface has been explained by the slow dissolution of DNAPLs. More recent research has shown that diffusion into low permeability units and back-diffusion can substantially contribute to contaminant longevity and explain plume persistence even after DNAPL sources have been removed. In low permeability units, geochemical conditions are often more reducing than in the surrounding aquifer hence potentially favouring CHC degradation by abiotic or biotic processes and thus limiting back-diffusion. However, it is also possible that only partial degradation occurs with the formation of more toxic daughter products that back-diffuse into aquifers. The main aim of the project is to improve the current understanding of CHC degradation in low permeability units and its effect on the long term fate of CHCs in aquifer systems. More specifically, the project aims at gaining new insight into the occurrence of transformation processes in low permeability units, the relative importance of abiotic versus biotic transformation, and its effect on contaminant plumes. The project strongly relies on compound-specific isotope analysis (CSIA). The method has the potential to differentiate between transformation mechanisms based on process-specific dual isotope trends and is particularly suited to track slow but yet relevant degradation, which is integrated in the isotope signature over time. The project consists of three parts. In the first part, the additional knowledge required for application of the CSIA approach to low permeability settings is acquired. More information is needed on the effect of sorption on isotope ratios of CHC, as sorption-retarded diffusion is expected to be particularly important in reactive units with a high organic carbon content (WP1). In addition, dual isotope slopes of specific degradation mechanism, especially abiotic transformation by Fe(II)-bearing minerals, need to be better constrained to be able to apply the method for process discrimination (WP2). In part two, detailed multi-isotope profiles (C, Cl, H) are determined for contaminated cores from sites with various geochemical conditions. The possibility to identify transformation processes based on dual isotope trends is explored (WP3). Using core scale numerical models, transformation rates are quantified especially for sites with controlled contaminant releases where the duration of exposure is well constrained. In part three, numerical modeling is employed to evaluate more generally how reactive processes in low permeability units affect the contaminant fate in aquifer systems, how well they can be tracked with CSIA and what type of samples provide most insight (water vs solids). It will also be explored whether time-discrete isotope profiles can be exploited as an archive to reconstruct the history and temporal evolution of reactive processes and whether isotope analysis can distinguish between degradation in the aquifer versus the low permeability units. The project will be carried out by a PhD student (100%) that focuses on WP1, 2 and 3, as well as a postdoc position dedicated to WP4 (50% SNF). The project will be supported by a high-level team with complementary fields of experience. Prof. D. Hunkeler (PI, University of Neuchâtel), a leading expert on isotope methods in contaminant hydrogeology; Prof. B. Parker (Guelph University Canada) a leading expert on contaminant behavior in aquitards; Dr. O. Shouakar-Stash (University of Waterloo, Canada) who has pioneered C und H isotope analysis in CHC; Prof. P. Junier (University of Neuchâtel) a microbial ecologist, and Dr. S. Wirth (University of Neuchâtel), a specialist of geology of lacustrine deposits and techniques to characterize cores from fine-grain, low permeability units.