Groundwater Dating with Noble Gases - from Groundwater-Surface Water Interaction and Contaminant Transport to Paleohydrogeology

Gerber, Christoph (2017). Groundwater Dating with Noble Gases - from Groundwater-Surface Water Interaction and Contaminant Transport to Paleohydrogeology (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Groundwater dating provides essential information for an improved understanding of groundwater systems, which, in turn, is crucial for the protection and the sustainable management of groundwater resources. Information on the residence times of groundwater can be obtained, for example, from measurements of natural environmental tracers such as noble gas radioisotopes. If the atmospheric input history of an environmental tracer is known, measuring the tracer concentration in the groundwater provides an estimate of when the water sample was in contact with the atmosphere for the last time.
In this thesis, multiple environmental tracers were combined to investigate physical processes in groundwater such as advection, mixing of different water masses, degassing and gas stripping, and the role of the unsaturated zone. These processes occur on timescales ranging from days to hundred thousands of years and on spatial scales from a few meters to hundreds of kilometers. Since many tracers are only applicable on certain timescales, a range of tracers was employed to contribute to resolving the challenges presented in the following.
For certain timescales and applications, there still is a lack of suitable established tracers. This is especially true for dating on timescales from a few weeks to months, which is of relevance for studying groundwater-surface water (GW-SW) exchange. This gap could be filled with 37Ar, which is applied to quantify GW-SW exchange for the first time in this thesis. Furthermore, to complement the analytical capabilities of the laboratory, a preparation line for radiocarbon analysis on CH₄-rich gas extracted from groundwater samples was set up and tested. Radiocarbon dating of CH4 was then used to constrain the origin and genesis of CH4 in the Red River Delta, Vietnam.
For the relatively new tracer 81Kr, there is some uncertainty as to what the exact atmospheric concentration is. For dating purposes, it is sufficient to know the 81Kr/Kr ratio of groundwater relative to the atmospheric ratio. However, an absolute calibration of the 81Kr measurements of the ATTA system would not only enable a more accurate determination of the absolute atmospheric concentration of 81Kr, but would also constrain parameters related to the cosmogenic production of 81Kr in the atmosphere. To this end, a mixing system was designed which is capable of diluting a 81Kr-enriched sample of Kr (for which the ratio is measurable by normal mass spectrometry) by a factor of 106 with a precision of better than 5%.
Multi-tracer studies are a valuable tool to obtain a detailed picture of residence time distributions and assess the relative importance of the saturated and unsaturated zone, but quantifying the uncertainty of estimated parameters of groundwater flow and transport can be quite challenging. Here, a recently published method for determination of the joint probability distribution of estimated parameters was applied to assess the future evolution of nitrate concentrations in a nitrate-contaminated aquifer with a thick unsaturated zone based on natural environmental tracers.
Depending on the environmental and hydrogeochemical conditions, groundwater may lose some of the dissolved gases. Such degassing complicates or even impedes the use and interpretation of many environmental tracers. One exception are the noble gas isotopes 37Ar, 39Ar, 85Kr, and 81Kr, which are insensitive to degassing and thus the best tracers in aquifers where a significant portion of the gas has been lost, for example by ebullition caused by CH4. In this thesis, the aquifer system of the Red River Delta, Vietnam, was studied, where high organic carbon contents and reducing conditions lead to a CH4-driven degassing. Groundwater dating was employed to better understand the dynamics and mechanisms of arsenic release to the groundwater and saltwater intrusion near the coast.
Increasingly, it is being recognized that most hydrological and hydrogeological systems are non-stationary, for example due to a changing climate. Dating of groundwater in the Baltic Artesian Basin with 81Kr revealed that the oldest groundwater must have remained in the aquifer over several glacial cycles. When deconvoluting a mix of different end-members to obtain the age of the individual end-members, the different noble gas concentrations of the end members due to changing climatic conditions or degassing have to be taken into account. Finally, a conceptual model is developed to demonstrate how several glacial cycles may have affected groundwater flow.

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