Assessing Initial Conditions for Chloride Transport Across Low- permeability Argillaceous Rocks, Wellenberg, Switzerland

Information about fluid evolution and solute transport in a low-permeability metamorphic rock sequence has been obtained by comparing chloride concentrations and chlorine isotope ratios of pore water, groundwater, and fluid inclusions. The similarity of d37Cl values in fluid inclusions and groundwater suggests a closed-system evolution during the metamorphic overprint, and signatures established at this time appear to form the initial conditions for chloride transport after exhumation of the rock sequence

Porewater chemistry in claystones in the context of radioactive waste disposal

The development of a combined experimental-modelling approach has enabled to constrain the porewater chemistry of different low-permeability clay formations (Boom Clay, Callovo-Oxfordian Fm., Opalinus Clay) foreseen as host rocks for nuclear waste repositories. A variety of methods are available to directly sample porewater or to derive information on the solute concentrations. These include analysis from seepage waters in boreholes, aqueous extractions, high-pressure squeezing, and advective displacement from core samples. Geochemical equilibrium modelling is used for data integration and calculation of internally consistent reference water compositions. The paper provides an overview of current achievements in experimental developments, modelling approaches and open questions

Using 81Kr and Noble Gases to Characterize and Date Groundwater and Brines in the Baltic Artesian Basin on the One-Million-Year Timescale

Analyses for $^{81}$Kr and noble gases on groundwater from the deepest aquifer system of the Baltic Artesian Basin (BAB) were performed to determine groundwater ages and uncover the flow dynamics of the system on a timescale of several hundred thousand years. We find that the system is controlled by mixing of three distinct water masses: Interglacial or recent meteoric water (\delta^{18}\text{O} \approx -10.4\unicode{x2030}) with a poorly evolved chemical and noble gas signature, glacial meltwater (\delta^{18}\text{O} \leq -18\unicode{x2030}) with elevated noble gas concentrations, and an old, high-salinity brine component (\delta^{18}\text{O} \geq -4.5\unicode{x2030}, \geq 90 \text{g Cl}^{-}/\text{L}) with strongly depleted atmospheric noble gas concentrations. The $^{81}$Kr measurements are interpreted within this mixing framework to estimate the age of the end-members. Deconvoluted $^{81}$Kr ages range from 300 ka to 1.3 Ma for interglacial or recent meteoric water and glacial meltwater. For the brine component, ages exceed the dating range of the ATTA 3 instrument of 1.3 Ma. The radiogenic noble gas components $^{4}$He* and $^{40}$Ar* are less conclusive but also support an age of > 1 Ma for the brine. Based on the chemical and noble gas concentrations and the dating results, we conclude that the brine originates from evaporated seawater that has been modified by later water-rock interaction. As the obtained tracer ages cover several glacial cycles, we discuss the impact of the glacial cycles on flow patterns in the studied aquifer system.Comment: Accepted for publication in Geochimica et Cosmochimica Act

Identifying temporally and spatially changing boundary conditions at an aquifer – aquitard interface using helium in porewater

Helium concentrations and 3He/4He isotope ratios of porewater, groundwater and rock were measured on samples collected from a Jurassic sediment sequence at the Mont Terri underground rock laboratory (Northern Switzerland). Porewater He data of rock samples collected from borehole BDB-1 at high spatial resolution across a karstic limestone unit (Passwang Formation) into the underlying claystone sequence (Opalinus Clay, Staffelegg Formation) describe a continuous profile from the water-conducting zone in the limestone into the clay-rich rocks of low permeability. Concentrations of 4He, 3He and their parent nuclides in the rock allow calculating insitu production and accumulation terms. Since the time of sedimentation, 90%–97% of the in-situ produced 4He has been released to the porewater. Today only 2.5% of the maximum possible accumulated 4He is still retained in the porewater while the major part of in-situ produced 4He was removed from the system presumably by porewater–groundwater exchange. The porewater 4He concentrations show a diffusion profile from the aquitard towards the aquifer, reflecting a) a transient state between 4He in-situ production and porewater–groundwater exchange, b) a transient state from previously higher 4He concentrations in the porewater, and c) a spatially variable boundary in the karstic limestone unit. Evolutionary models of porewater 4He concentration profiles in combination with constraints from independent chemical and isotopic tracers allow deciphering a complex palaeo-hydrogeological history of the system over about the last 30 ka. A local excursion from the general profile towards higher 4He concentrations and 3He/4He ratio in a limestone layer in the Opalinus Clay cannot be further constrained in time based on the present sample frequency, but appears to represent a hydrogeological signal

Geochemical evidence for regional and long-term topography-driven groundwater flow in an orogenic crystalline basement (Aar Massif, Switzerland)

Detailed knowledge about the circulation of meteoric water in non-magmatic, orogenic belts is fundamental for assessing the potential of such settings for geothermal power production, as well as their use as potential groundwater resources. To get more general insight into these hydrological processes, we have conducted regional (20 × 10 × 9 km) thermal-hydraulic-chemical (THC) simulations of meteoric water circulation in the orogenic, crystalline basement of the Aar Massif in the Central Alps, Switzerland. Model results were compared to numerous geochemical and isotopic analyses of groundwater discharging into the longest and deepest tunnel of the world, the Gotthard railbase tunnel located within the model domain. Explicitly considering the surface topography in our model was sufficient to reproduce all key characteristics of the tunnel inflows (salinity, temperature, δ18O values, and up- and downward directed flow zones inferred from geochemical constraints). This quantitatively confirms that surface topography operates as the governing control on fluid flow in orogenic crystalline basements with meteoric water infiltration occurring at high altitude and resulting upward directed flow zones along major valleys. Owing to low flow rates below 2 m year−1, computed residence times of the longest flow paths were above 100 k years, confirming that groundwater and/or porewater in orogenic crystalline basements may act as an archive for palaeohydrologic variations. Moreover, simulation results show that down to the lower model boundary at 9 km depth, penetration of meteoric water is not limited by the decrease in permeability with depth that is typically observed in granitic rocks. This suggests that advective fluid transport in orogenic crystalline basements may reach the brittle ductile-transition zone and that infiltrating meteoric water can attain temperatures well above 150 °C. We conclude that orogenic geothermal systems are promising plays for geothermal power production

Characterization of pore water, ion transport and water-rock interaction in claystone by advective displacement experiments

Displacement of preserved pore water from claystones by imposing a hydraulic gradient with an artificial pore water yields early extracts characteristic of the pore water. Long-term tracer breakthrough behavior provides transport properties and anion-accessible porosity, whereas elution of major components are controlled by ion exchange and mineral solubility. A single long-term experiment provides a comprehensive system understanding