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

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

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

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

Matrix Porewater In Crystalline Rocks: Extraction and Analysis

The chemical and isotopic characterization of porewater residing in the inter- and intragranular pore space of the low-permeability rock matrix is an important component with respect to the site characterization and safety assessment of potential host rocks for a radioactive waste disposal. The chemical and isotopic composition of porewater in such low permeability rocks has to be derived by indirect extraction techniques applied to naturally saturated rock material. In most of such indirect extraction techniques – especially in case of rocks of a porosity below about 2 vol.% – the original porewater concentrations are diluted and need to be back-calculated to in-situ concentrations. This requires a well-defined value for the connected porosity – accessible to different solutes under in-situ conditions. The derivation of such porosity values, as well as solute concentrations, is subject to various perturbations during drilling, core sampling, storage and experiments in the laboratory. The present study aims to demonstrate the feasibility of a variety of these techniques to charac-terize porewater and solute transport in crystalline rocks. The methods, which have been de-veloped during multiple porewater studies in crystalline environments, were applied on four core samples from the deep borehole DH-GAP04, drilled in the Kangerlussuaq area, Southwest Greenland, as part of the joint NWMO–Posiva–SKB Greenland Analogue Project (GAP). Potential artefacts that can influence the estimation of in situ porewater chemistry and isotopes, as well as their controls, are described in detail in this report, using specific examples from borehole DH-GAP0

Natural Tracers in recent groundwaters from different Alpine aquifers

Groundwater with underground residence times between days and a few years have been investigated over more than 20 years from 487 remote sites located in different aquifer types in the Alpine belt. Analysis of the data reveals that groundwaters evolved in crystalline, evaporite, carbonate, molasse, and flysch aquifers can be clearly distinguished based on their major and trace element composition and degree of mineralisation. A further subdivision can be made even within one aquifer type based on the trace element compositions, which are characteristic for the lithologic environment. Major and trace element concentrations can be quantitatively described by interaction of the groundwater with the aquifer- specific mineralogy along the flow path. Because all investigated sites show minimal anthropogenic influences, the observed concentration ranges represent the natural background concentrations and can thus serve as a “geo-reference” for recent groundwaters from these five aquifer types. This “geo-reference” is particularly useful for the identification of groundwater contamination. It further shows that drinking water standards can be grossly exceeded for critical elements by purely natural processe

Helium in solubility equilibrium with quartz and porefluids in rocks: A new approach in hydrology

Quartz crystals in sandstones at depths of 1200 m–1400 m below the surface appear to reach a solubility equilibrium with the 4He-concentration in the surrounding pore- or groundwater after some time. A rather high 4Heconcentration of 4.5x10E-3 cc STP 4He/cm3 of water measured in a groundwater sample would for instance maintain a He pressure of 0.47 atm in a related volume. This value is equal within analytical error to the pressure deduced from the measured helium content of the quartz and its internal helium-accessible volume. To determine this volume, quartz crystals of 0.1 to 1 mm were separated from sandstones and exposed to a helium gas pressure of 32 atm at a temperature of 290°C for up to 2 months. By crushing, melting or isothermal heating the helium was then extracted from the helium saturated samples. Avolume on the order of 0.1% of the crystal volume is only accessible to helium atoms but not to argon atoms or water molecules. By monitoring the diffusive loss of He from the crystals at 350°C an effective diffusion constant on the order of 10E-9 cm2/s is estimated. Extrapolation to the temperature of 70°C in the sediments at a depth of 1400 m gives a typical time of about 100 000 years to reach equilibrium between helium in porewaters and the internal He-accessible volume of quartz crystals. In a geologic situation with stagnant pore- or groundwaters in sediments it therefore appears to be possible with this new method to deduce a 4He depth profile for porewaters in impermeable rocks based on their mineral record

Reconstruction of in-situ porosity and porewater compositions of low-permeability crystalline rocks: Magnitude of artefacts induced by drilling and sample recovery

Geological site characterisation programmes typically rely on drill cores for direct information on subsurface rocks. However, porosity, transport properties and porewater composition measured on drill cores can deviate from in-situ values due to two main artefacts caused by drilling and sample recovery: (1) mechanical disruption that increases porosity and (2) contamination of the porewater by drilling fluid. We investigated the effect and magnitude of these perturbations on large drill core samples (12–20 cm long, 5 cmdiameter) of high-grade, granitic gneisses obtained from 350 to 600 m depth in a borehole on Olkiluoto Island (SW Finland). The drilling fluid was traced with sodium–iodide. By combining out-diffusion experiments, gravimetry, UV-microscopy and iodide mass balance calculations, we successfully quantified the magnitudes of the artefacts: 2–6% increase in porosity relative to the bulk connected porosity and 0.9 to 8.9 vol.% contamination by drilling fluid. The spatial distribution of the drilling-induced perturbations was revealed by numerical simulations of 2D diffusion matched to the experimental data. This showed that the rims of the samples have a mechanically disrupted zone 0.04 to 0.22 cm wide, characterised by faster transport properties compared to the undisturbed centre (1.8 to 7.7 times higher pore diffusion coefficient). Chemical contamination was shown to affect an even wider zone in all samples, ranging from 0.15 to 0.60 cm, inwhich iodide enrichmentwas up to 180 mg/kgwater, compared to 0.5 mg/kgwater in the uncontaminated centre. For all samples in the present case study, it turned out that the magnitude of the artefacts caused by drilling and sample recovery is so small that no correction is required for their effects. Therefore, the standard laboratory measurements of porosity, transport properties and porewater composition can be taken as valid in-situ estimates. However, it is clear that the magnitudes strongly depend on site- and drilling-specific factors and therefore our results cannot be transferred simply to other locations. We recommend the approach presented in this study as a route to obtain reliable values in future drilling campaigns aimed at characterising in-situ bedrock properties

Tiefengrundwasser – Vorkommen, Nutzungspotenzial und Schutzwürdigkeit

Tiefengrundwasser findet sich in allen grosstektonischen Einheiten der Schweiz. Die bekannten, teilweise genutzten Vorkommen von Tiefengrundwas-ser und insbesondere die chemische Zusammensetzung dieser Wässer wird für die verschiedenen Einheiten aufgezeigt. Im Hinblick auf mögliche Nutzungskon-flikte werden die gesetzlichen Grundlagen und Massnahmen zum Schutz dieser Ressource diskutiert