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Helium migration from water into natural quartz crystals - a new dating technique for very old pore- and groundwaters

English title Helium migration from water into natural quartz crystals - a new dating technique for very old pore- and groundwaters
Applicant Waber Niklaus
Number 110712
Funding scheme SCOPES
Research institution Klima- und Umweltphysik Physikalisches Institut Universität Bern
Institution of higher education University of Berne - BE
Main discipline Hydrology, Limnology, Glaciology
Start/End 01.10.2005 - 31.12.2008
Approved amount 72'000.00
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All Disciplines (3)

Discipline
Hydrology, Limnology, Glaciology
Geochemistry
Mineralogy

Keywords (4)

Helium; Hydrogeology; groundwater residence time; pore water

Lay Summary (English)

Lead
Lay summary
Successful groundwater management requires the quantification of groundwater flow and the residence time of groundwater in the subsurface since the time of infiltration. In groundwater system with groundwater residence times up to a few tens of thousand of years several natural radioactive tracers can be applied for groundwater dating. However, no such tools exist for deep groundwater systems and pore water in aquitards with very slow flow rates or stagnant water and with the residence times that can exceed one million of years. Deep-seated groundwater systems in large sedimentary basins ((e.g. Molasse Basin, CH, D; Paris Basin, F; Oman Basin, Arabian Peninsula; Great Artesian Basin, Australia) become increasingly important as groundwater resources while aquitard systems are currently investigated as potential host rocks for the deep disposal of toxic and radioactive wastes. The quantification of such old ground- and pore water residence times is therefore essential for future groundwater resources management and the safety assessment of underground disposal of toxic and radioactive wastes.In principle, radiogenic helium is a tracer which is able to date very slow processes that last several millions of years in groundwater systems.The two isotopes of helium (3He, 4He) are produced in the minerals of a rock via natural radioactive decay of U and Th series (4He) and associated nuclear reactions (3He). The energy produced during the decay is large enough that He is released from the mineral to the surrounding pore- and groundwater. Under knowledge of the helium inventory of a water rock system, the relation between local and external source, and the removal rate from the system the residence time of the pore- or groundwater can be derived. Previous results support the feasibility of this approach and further suggest that the helium content in certain minerals (e.g quartz) might act as an archive for helium and thus the water residence time. This latter aspect is especially important within the framework of underground waste disposal because such sites are preferably located in aquitards with completely stagnant pore water. Although acting as migration paths for contaminants, this pore water cannot be sampled by conventional groundwater sampling techniques and has to be characterised indirectly based on the rock material.The present project focuses on the systematic of helium in mineral (mainlyquartz) as a tool for dating water with residence times in the order of millions of years. It aims to experimentally derive the diffusion parameters of He in quartz, to characterise the residence sites of He in the mineral, and to derive the theoretical basis (model) of the exchange between helium and the mineral. Due to the low concentrations a very high analytical resolution is required to accomplish this task. The results obtained will be tested and modelled on a regional scale in a case study with well known boundary conditions (Swiss Molasse Basin). It is aspired to develop the analytical and modelling techniques to such a degree that they can easily be applied and transferred to other sites of interest.
Direct link to Lay Summary Last update: 21.02.2013

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