Porewater Geochemistry, Method Comparison and Opalinus Clay – Passwang Formation Interface Study at the Mont Terri URL

Waber, H. Niklaus; Rufer, Daniel (2017). Porewater Geochemistry, Method Comparison and Opalinus Clay – Passwang Formation Interface Study at the Mont Terri URL (NWMO Technical Reports). Toronto, Canada: Nuclear Waste Management Organization, NWMO

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The present study focuses on the geochemical characterisation of porewater solutes in the Opalinus Clay at the Mont Terri URL. The investigations were carried out within the Mont Terri Project DB-A Experiment (Deep inclined borehole across the Opalinus Clay) conducted by an international consortium. At the Mont Terri URL, borehole BDB-1 is the first borehole that crosscuts the Opalinus Clay in its entire thickness. Borehole BDB-1 cuts across the Jurassic sediment sequence of low-permeability rock, from the Hauptrogenstein, across the Passwang Formation (Fm) and the Opalinus Clay, and into the rocks of the Staffelegg Formation (Fm). This allowed, for the first time, collection of porewater samples at a high spatial frequency. Furthermore, all samples experienced the same history in drilling, sampling and laboratory treatment. This facilitates identification of artefacts induced by indirect porewater characterisation techniques and allows improved interpretation of the data in terms of porewater evolution as a function of space and time. In addition, groundwater could be collected from a water-conducting zone in the Passwang Fm at 58.6m BHL, but was not encountered in the lithologies in the footwall of the Opalinus Clay.

The different natural tracers in the porewater (Cl–, δ37Cl, Br–, δ18O, δ2H, He, 3He/4He, Ar) all describe well-defined concentration profiles from the Staffelegg Fm across the Opalinus Clay into the Passwang Fm. The concentration profiles of all tracers indicate diffusion as the dominant solute transport process across the Opalinus Clay. These findings are in accordance with previous work conducted at the Mont Terri URL (Pearson et al., 2003; Mazurek et al., 2009, 2011). In the rocks of the Passwang Fm, the tracer concentrations display more complex profiles that are, at least, partly due to the poor knowledge about anion-accessible porosity in the low clay-content rocks. Chemical compounds and noble gas concentrations indicate local minima at locations closer to the Opalinus Clay than the present-day water-conducting zone. These local minima are also observed in isotope and ion-ion ratios, independent of any porosity value, and are interpreted to have acted at some time in the past as boundary conditions for the solute exchange between the Opalinus Clay and the Passwang Fm. Quantification of the 4He concentration profile suggest that these old boundaries may have been active until a few tens to a hundred thousand of years ago.

Ion-ion ratios in aqueous extract solutions reveal similarly well-defined profiles across the Opalinus Clay into the Passwang Fm. Ratios of Br/Cl and SO4/Cl are below and above, respectively, those of modern seawater. Consistent with all natural porewater tracers, these ratios are best explained by long-term exchange between porewater in the Opalinus Clay with porewater or groundwater in the Triassic evaporite sequences underlying the Opalinus Clay. This contrasts previous interpretations, which assumed residual seawater as the main origin of solutes in porewater of the Opalinus Clay (e.g. Pearson and Waber, 2001; Pearson et al., 2003; Mazurek and de Haller, 2017). It is, however, not in conflict with the most recent history of tracer profile evolution over the last few millions of years (e.g. Mazurek et al., 2009, 2011).

The present data, combined with geochemical modelling, indicate that the SO42– concentrations obtained by aqueous extraction are compatible with the geochemical properties of the Opalinus Clay rocks (such as the cation exchange properties and mineral equilibria) when compared to SO42– concentrations obtained by high-pressure squeezing and water accumulated over long time periods from boreholes, where potential for oxidation exists prior to analysis. It is concluded, that the SO42– concentration obtained from aqueous extraction serves as a suitable proxy for the in-situ porewater SO42– concentration. For future modelling of the porewater composition of the Opalinus Clay at Mont Terri, it is recommended to use the SO4/Cl ratio obtained in cautiously prepared aqueous extract solutions instead of the seawater SO4/Cl ratio or fixation of the SO42– concentration by mineral solubility controls.

Item Type:	Report (Report)
Division/Institute:	08 Faculty of Science > Institute of Geological Sciences 08 Faculty of Science > Institute of Geological Sciences > Applied Rock-Water-Interaction
UniBE Contributor:	Waber, Niklaus, Rufer, Daniel
Subjects:	500 Science > 550 Earth sciences & geology
Series:	NWMO Technical Reports
Publisher:	Nuclear Waste Management Organization, NWMO
Language:	English
Submitter:	Niklaus Waber
Date Deposited:	06 Dec 2017 13:05
Last Modified:	05 Dec 2022 15:08
Additional Information:	NWMO-TR-2017-10
URI:	https://boris.unibe.ch/id/eprint/106899