The chemical and isotopic composition of porewaters in clayrocks records the palaeo-hydrogeological evolution of sedimentary basins. However, porewater extraction is in general demanding and time-consuming. Techniques have been newly developed or adapted in recent years, and progress in analytical methods, in particular the minimisation of sample mass needed for analysis, have opened new opportunities for porewater studies. Depending on their degree of induration, clayrocks span a wide range of porosities. They all have in common a nanometric pore-space architecture, which results in low permeability and the predominance of diffusive solute transport through the formation. The negatively charged clay-mineral surfaces affect both the chemical composition and the binding state of the water molecules in the adjacent pore space. Thus, in a profile across a pore, the composition and the mobility of the porewater vary as a function of the distance from the clay surface. In a simplified way, two water types, namely free, charge-balanced porewater in the central parts of a pore and bound/interlayer water along clay surfaces can be distinguished.Adequate field sampling protocols are needed to preserve core materials from evaporation and oxidation, and to this end well-trained on-site staff is a pre-requisite in order to minimise exposure of the samples to the atmosphere. Porewater extraction from clayrocks requires dedicated methods that are based on different physical principles, including the application of high hydraulic gradients (advective displacement, centrifugation, sampling of in-situ seepages), axial pressure (squeezing), diffusive equilibration (out-diffusion, isotope diffusive exchange, in-situ circulation tests), capillary suction (filter absorption), heating (vacuum distillation), crushing and dilution (aqueous extraction), and outgassing via diffusion (analysis of dissolved noble and reactive gases). Different extraction methods often yield consistent results, but distinct differences may occur if the methods sample different domains of the pore space. The attribution of water extracted by a specific method to a particular porewater reservoir is not always evident, and current research aims at a better understanding of this issue.The suitability of a particular method depends on the properties of the studied clayrock (porosity, degree of cementation, mineralogy) and the parameters of interest (major ions, stable water isotopes, dissolved gases). Each method has its limitations and incurs various kinds of artefacts that may require corrections. It is concluded that there is no single best method to extract the porewater and to analyse the dissolved constituents and isotope composition, and that a method or combination of methods should be selected considering the properties of the studied clayrock.

Over the last decades, several methods have been developed for determining the porewater stable isotope composition (δ2H, δ18O) in low-permeability, argillaceous rocks and pertinent to the acquisition of spatially highly-resolved tracer profiles for investigating subsurface transport processes over large scales of time and space. One of these methods is the so-called isotope diffusive exchange technique (IDE) where the porewater of the rock equilibrates via the vapour phase with a test water of known isotope composition. In this study we aim for 1) identifying and assessing important parameters and artefacts these experiments are sensitive and prone to, respectively, 2) evaluating their impact on the porewater isotope composition derived from such experiments and 3) testing the reproducibility and accuracy of the method. For this, the experimental data and the calculated porewater isotope composition of 752 isotope diffusive exchange experiments, performed on drillcore samples from variable lithologies, were examined under these aspects. The investigations are complemented by comparison between porewater and groundwater isotope values in regions of water-conducting zones and an interlaboratory comparison. Ultimately, this allowed defining a stringent procedure for the evaluation of the experimental data and classifying experiments as ‘reliable’, less reliable’ and ‘failed’. For calculating the porewater isotope composition, a new approach was developed that accounts for sample-scale heterogeneity of the water content. This procedure of data evaluation and processing resulted in smooth isotope profiles with only little scatter across largely different lithologies. The interlaboratory comparison attests the method a very good reproducibility. The comparison with groundwater isotope data reveals slightly enriched δ18O and δ2H signatures by 0.3–0.6 and 1.7–2.7‰ VSMOW, respectively, for some samples investigated by the IDE method. No stringent explanation exists at this stage for these differences, but it must be emphasized that these deviations are small compared to the typical natural variations observed in profiles of these tracers. This demonstrates that porewater isotope data obtained by the IDE method represent the conditions in the in situ porewater reasonably well when strictly following the proposed procedures of the experimental setup, the evaluation of experimental data and the calculation of porewater isotope compositions.

A new technique was developed to analyse chlorine stable isotope ratios of chloride using P urge& T rap- g as c hromatography- c ontinuous f low- i sotope r atio m ass s pectrometry (P&T-GC-CF-IRMS). The chlorine stable isotope analysis using continuous flow technology shows good reproducible results. This technique is much faster than the classical off-line method, needs smaller sample sizes and uses no toxic or dangerous chemicals. Because of its simplicity, this technique is also more cost-effective than other methods. Due to the determination of the δ 37 Cl SMOC value against an external standard (e.g., ISL-354) under the same preparative and chromatographic conditions, the precision and accuracy is comparable to or better than that achieved by any previous methods. This new technique allows more samples to be analysed rapidly and accurately. The P&T-GC-CF-IRMS technique will enhance the application of chlorine stable isotope ratio measurements in more research areas. It will allow the use of this analysis in studies where chlorine content is low and high precision and accuracy were important. The method has been applied in different porewater studies to assess potential crystalline and argillaceous environments for the deposition of radioactive waste.

Sedimentary and crystalline rock formations are currently being intensely investigated in several countries as potential host rocks for deep geological repositories for radioactive wastes. An important aspect of investigations is to characterise and understand the movement and compositions of water in these rocks—both mobile groundwater in aquifers and faults/fractures, and immobile porewater in the matrix of intact rock. Non-reactive solutes, primarily Cl and Br, water isotope ratios, and dissolved helium are ‘natural tracers’ of diffusive transport and exchange between groundwater and porewater. Recent developments of techniques for extracting and analysing porewaters from carefully preserved drill cores are summarised in this paper. It identifies practical challenges and suggests best practice strategies based on experience gained by various national research and repository programs in crystalline and sedimentary rocks.

Dissolved organic matter (DOM) and microorganisms were characterized along the flow path of a geothermal facility that produces water from a deep (2800 m) carbonate rock reservoir for energy provision. A variety of analytical techniques were employed to distinguish between natural and synthetic organic matter, determine the composition of the microbial community, and evaluate the role of microorganisms in the operation of the geothermal site in Bad Blumau, Austria. Ion chromatography (IC), liquid chromatography with organic carbon detection (LC-OCD), and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in negative electrospray ionization (ESI(-)) and positive atmospheric pressure photoionization (APPI(+)) mode were applied to the fluid samples for the purpose of characterizing the composition of DOM and distinguishing natural DOM from a chemical inhibitor used to prevent scaling. The concentrations of dissolved organic carbon (DOC) ranged from 8.5 to 10.4 mg C L-1. The chemical scaling inhibitor contributes approximately 1 mg C L-1 of DOC to the produced fluids. Depending on the applied ionization mode, the FT-ICR-MS results show that between 31 % and 65 % of the macromolecular formulas (150-1000 Da) detected in the fluid samples appear to originate from the inhibitor. However, the DOM is primarily composed of low-molecular-weight acids (LMWA), with acetate being the most prevalent, reaching up to 7.4 mg C L-1. To assess the diversity of the bacterial communities, targeted amplification of the 16S rRNA gene was conducted. The composition of the microbial community exhibited variation along the flow path, with Firmicutes, Proteobacteria, and Thermotogae representing the dominant bacterial phyla. Based on the community composition, metabolic pathways associated with the presence of acetate in the samples were predicted. Microorganisms may produce acetate through diverse fermentation processes, including those involving lysine, pyruvate, and hexitol. Assessing the presence and interaction of organic compounds and microorganisms in geothermal fluids can provide a broader understanding of processes within the geothermal facility. This understanding could be beneficial for efficient operation of a geothermal power plant.

Extreme environments on Earth host a large diversity of microbial life. Bacteria, archaea, and fungi are able to survive under one or several extreme conditions including extreme ranges of temperature, pressure, pH or salinity. Despite extensive research on extremophilic microorganisms, a relatively unexplored frontier within the study of the deep biosphere is the survey of the diversity of microorganisms inhabiting deep geothermal reservoirs used for energy production. These sites offer unique access to investigate life in the deep biosphere. The conditions in these reservoirs are often within the range of the known limits of life, which makes them a suitable habitat for various extremophilic microorganisms. Moreover, microbial-driven processes such as microbially induced scaling or corrosion can decrease the efficacy of geothermal power plant systems. The present review summarizes the current knowledge and uncertainties surrounding microbial life in deep geothermal reservoirs. As the knowledge in deep geothermal fluids is still scarce, the microbial diversity in analogous environments, such as surface geothermal springs, deep-sea hydrothermal vents or deep subsurface environments, is also summarized here. The high diversity of microorganisms inhabiting these analogous environments suggests that deep geothermal fluids may host an unsuspected microbial diversity. Moreover, the challenges associated to the study of microorganisms in geothermal fluids are reviewed. These include notably challenges linked to sampling, DNA extraction from low biomass samples, DNA amplification and sequencing of unknown communities, and biases induced by comparison of the sequences obtained to reference databases. Such biases are even stronger concerning fungi and archaea, as specific databases are less extensive than those for bacteria. A broader knowledge on microorganisms in deep geothermal fluids may not only allow to reduce the negative impact of microbial activity in geothermal power plants, but could also provide new insights into the evolution of microorganisms and their survival in extreme environments.

. Dissolved organic matter and microorganisms were analyzed along the flow path of a geothermal facility in Austria. Various analytical methods were used to characterize and differentiate between natural and synthetic organic matter, characterize the microbial community composition, and determine the implications of microorganisms in an operating a geothermal site. Dissolved organic carbon (DOC) concentrations were in the range of 8.4–10.3 mg C L -1 and typically decreased from the production to the injection side. Carbonate scalings are avoided in the facility by the injection of a chemical scaling inhibitor 5 within the production well at 500 m depth. It was calculated that the inhibitor contributes approximately 1 mg C L -1 DOC to the produced fluids. Ion chromatography (IC), liquid chromatography — organic carbon detection (LC-OCD) and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in negative electrospray ionization (ESI(-)) and positive atmospheric pressure photoionization (APPI(+)) mode were applied to the fluid samples to characterize the dissolved organic matter (DOM) composition and distinguish between the inhibitor and the natural DOM. Depending on the applied ionization 10 mode, FT-ICR-MS results show that between 31 % and

The efficiency and feasibility of geothermal utilisation depends strongly on the characteristics and behaviour of the fluids that transfer heat between the geosphere and the engineered components of a power plant. Chemical and physical processes such as precipitation, corrosion, or degassing are induced by pressure and temperature changes, with potentially serious consequences for power plant operation and project economics. The EU Horizon 2020-funded project REFLECT aims to avoid such problems by collecting high-quality chemical, physical, and microbiological data at extreme salinities, pressures or temperatures and improving the understanding of kinetic processes through laboratory experiments. These data are presented in a European geothermal fluid atlas and implemented in predictive models in order to provide recommendations on how to best operate geothermal systems for a sustainable future.

Groundwaters circulating in Upper Mesozoic carbonates are of great interest for geothermal heat production and storage applications in the Geneva area. This study aims at providing new insights and proposing new interpretations about the mineral-water reactions and the fluid-flow paths mechanisms across the Geneva Basin (GB). Data from previous studies are combined and improved by new ones collected from cold and hot springs and geothermal exploration wells in 2018 and 2020 in the framework of the GEothermies program and HEATSTORE project. Major ions, trace elements, and the isotopes of Oxygen, Hydrogen, Sulfur, Strontium, and Carbo have been analysed and the results show that the sampled waters have a meteoric origin, the carbonate aquifers act as preferential host rocks for geothermal waters, and partial contribution from the Cenozoic sediments can be observed in some samples. The Jura Mountains and the Saleve Ridge are the main catchment areas and an evolution from a pure Ca-HCO3 footprint for the cold springs, to a Na > Ca-HCO3 and a Na-Cl composutions, is observed at the two geothermal wells. The residence time is in the order of a few years for the cold springs and reaches up to 15–20,000 years for the deep wells.

Application of the environmentally friendly scaling inhibitor NC47.1 B in geothermal systems was studied in laboratory and field-scale experiments. Biodegradation was investigated under anaerobic, in situ-like conditions and a mass balance confirmed the almost complete conversion of the polycarboxylate to e.g. acetate, formate, methane and CO2. Much higher concentrations of inhibitor were chosen than applied in situ and rapid degradation was observed in biofilm-inoculated setups: A concentration of 100 mg/L of the inhibitor was degraded below detection limit within 8 d of incubation. Furthermore, the inhibitor was applied at the geothermal plant in Unterhaching, Germany. Monitoring of the microbial community in situ showed an increase in the abundance of Bacteria. Particularly, relatives of the fermenting Caldicellulosiruptor dominated the biocenosis after about six months of continuous inhibitor dosage (5–10 mg/L). However, in long-term laboratory experiments representatives of Caldicellulosiruptor were only detected in traces and the microbial community comprised a broader spectrum of fermentative bacteria. The different composition of the biocenosis in situ and in laboratory experiments is probably caused by the different inhibitor concentrations, temperatures as well as nutrient availability in situ compared to the closed system of the batch experiments.

Summary Within the Horizon 2020 REFLECT project (Redefining geothermal fluid properties at extreme conditions to optimize future geothermal energy extraction), organic compounds in deep fluids from geothermal sites are being characterized and interpreted with regard to numerous site parameters. DOC concentrations and the relative abundance of DOC fractions from fluid samples of two sites located in Austria and Germany were analyzed via Liquid Chromatography - Organic Carbon Detection (LC-OCD). Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS) with Electrospray (ESI) and atmospheric pressure photoionization (APPI) has been applied to characterize DOM on a molecular level and its changes of time and flow path. The Austrian site Bad Blumau is located in Styrian Basin and uses deep fluids from a Palaeozoic carbonate reservoir (104°C). Samples were taken from production and injection side, as well as from the heat central. The Berlin drill site is part of the North German Basin and targets a limestone formation (Muschelkalk) with a reservoir temperature of 32°C. Samples were taken during a lift test over a period of 25 hours. A better understanding of the role of organic compounds in geothermal fluids might help to optimize present and future geothermal energy extraction.

The porosity and pore geometry of rock samples from a coherent granodioritic rock body at the Grimsel Test Site in Switzerland was characterised by different methods using injection techniques. Results from in situ and laboratory techniques are compared by applying innovative in situ resin impregnation techniques as well as rock impregnation and mercury injection under laboratory conditions. In situ resin impregnation of the rock matrix shows an interconnected pore network throughout the rock body, consisting mainly of grain-boundary pores and solution pores in magmatic feldspar, providing an important reservoir for pore water and solutes, accessible by diffusion. Porosity and pore connectivity do not vary as a function of distance to brittle shear zones. In situ porosity was found to be about 0.3 vol.%, which is about half the porosity value that was determined based on rock samples in the laboratory. Samples that were dried and impregnated in the laboratory were affected by artefacts created since core recovery, and thus showed higher porosity values than samples impregnated under in situ conditions. The extrapolation of laboratory measurements to in situ conditions requires great care and may not be feasible in all cases.

The economic and technical efficiency of geothermal plants is often impaired by corrosion, scaling and biological fouling. In Germany, the highly saline fluid of the North German Basin is known to cause severe corrosion. Meanwhile geothermal plants in the southern Molasse Basin, one of the most extensively exploited geothermal regions in Germany, are troubled by carbonate scaling. One possible solution is the employment of a scale inhibitor. A novel scaling inhibitor is evaluated in field- and laboratory tests. This inhibitor consists of a polysaccharide backbone structure and branches of polyacrylic- and maleic acid copolymer. The laboratory tests with different scaling inhibitor concentrations were designed to observe the biodegradation of the scaling inhibitor in an anaerobic environment similar to the conditions found in heat exchangers of geothermal plants. The concentration of inhibitor was quantified by UV/VIS and liquid chromatography (LC). Molecular biological techniques (PCR, DGGE, Microbiome analysis) were used to characterize the biocenosis on metal surfaces and in fluids of the experiments. During the experiment the concentration of inhibitor decreased up to 3 % of the initial concentration. The formation of methane and acetate was observed which indicates a biological degradation by acetoclastic methanogenesis. Hydrogen formation was observed in setups containing steel coupons. This implies that hydrogen is primarily formed by corrosion processes and in tests with active microorganisms hydrogen was consumed completely. Various fermentative bacteria classified as Clostridia and Firmicutes as well as methanogenic archaea were identified. In some experiments sulfate reducing bacteria were found. Those are well known to catalyze corrosion processes. Results of field experiments in a bypass system as well as microbiological monitoring of the inhibitor application in geothermal plant located in the molasse basin will be presented.

A new method to remove hydrogen sulfide from geothermal fluids during well operation was tested in situ at a geothermal site in Vienna (Austria). For this purpose, ferric iron was added either as granulated iron hydroxide or as FeCl 3 solution into a reaction vessel containing the thermal water directly removed from the wells. From the container, the water would be pumped through a particle filter. Physicochemical parameters as well as sulfide were measured constantly over time before and after the filter. It was found that the sulfide was fully removed from the water by both iron additives. While the addition of FeCl 3 led first to the formation of black iron(II) sulfide (FeS), which subsequently oxidized in presence of oxygen to Fe(III) hydroxide, no visible change of the granulated iron hydroxide was observed. The reaction time was longer when using the Fe(III) hydroxide additive as compared to the FeCl 3 (completed in less than 20 min) but could be enhanced by increasing the amount of added particles. In all experiments the pH was constantly rising during the reaction from about 6.3 to 7.5, which was explained by loss of protons due to purging out of the gaseous H 2 S. The redox value, which was measured over time, remained rather constant after addition of granulated iron hydroxide (about −350 mV), but strongly increased from −350 mV to −50 mV after adding the FeCl 3 suggesting a strong electron-consuming reaction. This can be explained by a two-step reaction: first, the Fe(III) was reduced to Fe(II) by oxidation of either sulfide or thiosulfate to sulfate. Afterward, the Fe(II) oxidized again by dissolved oxygen forming orange Fe(III) hydroxides. The application of the investigated method during operation of geothermal wells could prevent H 2 S-induced corrosion and would eliminate the toxic effects of this gas.

Matrix porewater from low permeable Grimsel granodiorite was successfully characterised using indirect methods applied to originally saturated core samples. Core samples were taken from a 17 m long borehole originating from a tunnel of the Grimsel Test Site into the crystalline bedrock intersecting a tectonic shear zone with a water-conducting fracture. Matrix porewater chloride profiles on the meter scale were determined on both sides of the water-conducting fracture. To evaluate transport processes within the bedrock formation, a series of diffusive model calculations were performed, which to fit the porewater data. Boundary and initial conditions were varied according to the geological conditions, whereas other required parameters such as the connected porosity and pore diffusion coefficients were determined by laboratory experiments on the cores and extrapolated to in situ conditions. The main conclusions can be summarized as follows: (1) Chloride porewater profiles at the meter scale can be simulated using diffusive transport models. This provides evidence that diffusive exchange with active fractures occurs over a range of a few meters in the low-permeable crystalline bedrock; (2) the best fit of the diffusion profile was achieved by a model approach, which takes asymmetric initial Cl-concentrations into account. This indicates that prior to the activation of the present water-conducting fracture, the porewater system in the bedrock was already active showing a concentration gradient in chloride; (3) the water-conducting fracture was activated at least between 850 and 1700 years before present, with a best-fit 1200 years before present; and (4) the hydraulic were affected by the construction of the rock laboratory 20 years ago, resulting in a rapid dilution of the fracture groundwater by advection.

We investigate the uppermost 60 cm of sediment in active pockmarks of a deep-water methane seep site from Vestnesa Ridge offshore NW Svalbard. Using video guided core sampling with a remotely operated vehicle we collected push cores directly from bacterial mats within two active pockmarks, Lunde and Lomvi. Pore water analyses show very shallow sulphate methane transition zones and transport-reaction modelling suggests a considerable amount of dissolved methane passing through the sediment water interface due to upwards advection of an aqueous fluid not previously reported from Vestnesa Ridge. In addition, we show that the amount of methane that bypasses the benthic methane filter greatly increases with higher aqueous fluid advection rate. Recent changes in methane flux are evident from lipid biomarker, seep carbonate, and delta C-13-organic carbon profiles in both pockmarks. Hydrocarbons at this cold seep site are supplied both by deep thermogenic sources from below the gas hydrate stability zone but also to a significant degree by microbial methanogenesis which dominates the signature in our shallow sediment cores with delta C-13-CH4 values as low as -77 parts per thousand.

Geothermal plants are often affected by corrosion caused by microbial metabolites such as H2S. In the Bad Blumau (Austria) geothermal system, an increase in microbially produced H2S was observed in the hot (107 °C) and scaling inhibitor-amended saline fluids and in fluids that had cooled down (45 °C). Genetic fingerprinting and quantification revealed the dominance, increasing abundance and diversity of sulfate reducers such as Desulfotomaculum spp. that accompanied the cooling and processing of the geothermal fluids. In addition, a δ34S isotopic signature showed the microbial origin of the H2S that has been produced either chemolithotrophically or chemoorganotrophically. A nitrate addition test in a test pipe as a countermeasure against the microbial H2S formation caused a shift from a biocenosis dominated by bacteria of the phylum Firmicutes to a community of Firmicutes and Proteobacteria. Nitrate supported the growth of nitrate-reducing sulfur-oxidizing Thiobacillus thioparus, which incompletely reduced nitrate to nitrite. The addition of nitrate led to a change in the composition of the sulfate-reducing community. As a result, representatives of nitrate- and nitrite-reducing SRB, such as Desulfovibrio and Desulfonatronum, emerged as additional community members. The interaction of sulfate-reducing bacteria and nitrate-reducing sulfur-oxidizing bacteria (NR-SOB) led to the removal of H2S, but increased the corrosion rate in the test pipe.

81Kr (T1/2 229.000 a) ist ein idealer Datierungstracer für alte Tiefengrundwässer. Die Oberjura-Formation im tiefen Teil des Molassebeckens stellt ein herausragendes Georeservoir für thermale Tiefenwässer (bis 140 °C) dar. Über die genutzten Thermalwässer mit zumeist kaltzeitlicher Bildungscharakteristik (Na-HCO3-Cl-Typ) ist jedoch im Hinblick auf die Neubildungsprozesse, Herkunftsgebiete und Fließdynamik wenig bekannt. Für die Interpretation der Genese und Entwicklung (Ionen- und Isotopenaustausch, Gasflüsse, etc.) fehlen bislang verlässliche Altersinformationen. Erstmals wurden nun neun thermale Tiefenwässer erfolgreich durch 81Kr/85Kr-ATTA-Untersuchungen datiert. Die abgeleiteten Altersinformationen zeigen im westlichen und zentralen Molassebecken vorherrschend eine Bildung während der letzten Kaltzeit (Würm-Glazial), die sehr gut zur subglazialen Bildungshypothese über alpennahe, sehr mächtige Deckschichten hinweg passt. Im Ostteil des Molassebeckens weisen die Tiefenwässer hingegen einheitlich deutlich höhere Alterscharakteristiken (Günz/Mindel Interglazial) bzw. ein langsameres Strömungssystem auf, das allenfalls durch geringe Neubildungsanteile aus den jüngeren alpinen Vergletscherungen beeinflusst ist.

At geothermal plants, process failures often occur due to corrosion and scaling processes. Especially after heat extraction, sulfate reducing bacteria contribute to corrosion processes by producing reduced sulfur compounds. In biofilms containing scales such as iron sulfides, corrosion processes are enhanced. In a mobile bypass system located at the geothermal plant in Neubrandenburg (North German Basin), the influence of biofilm formation on corrosion and scaling was investigated. Short-term heat shocks were successfully tested in the bypass system in order to reduce biofilm formation and thus to diminish corrosion and scaling processes.