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Changes of porosity, permeability, and tortuosity due to physical and geochemical processes are of vital importance for a variety of hydrogeological systems, including passive treatment facilities for contaminated groundwater, engineered barrier systems (EBS), and host rocks for high-level nuclear waste (HLW) repositories. Due to the nonlinear nature and chemical complexity of the problem, in most cases, it is impossible to verify reactive transport codes analytically, and code intercomparisons are the most suitable method to assess code capabilities and model performance. This paper summarizes model intercomparisons for six hypothetical scenarios with generally Electronic supplementary material The online version of this article

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Bentonite clay is considered as a potential buffer and backfill material in subsurface repositories for highlevel nuclear waste. As a result of its low permeability, transport of water and solutes in compacted bentonite is driven primarily by diffusion. Developing models for species transport in bentonite is complicated, because of the interaction of charged species and the negative surface charge of clay mineral surfaces. The effective diffusion coefficient of an ion in bentonite depends on the ion's polarity and valence, on the ionic strength of the solution, and on the bulk dry density of the bentonite. These dependencies need to be understood and incorporated into models if one wants to predict the effectiveness of bentonite as a barrier to radionuclides in a nuclear repository. In this work, P. Alt-Epping ( ) · we present a benchmark problem for reactive transport simulators based on a flow-through experiment carried out on a saturated bentonite core. The measured effluent composition shows the complex interplay of species transport in a charged medium in combination with sorption and mineral precipitation/dissolution reactions. The codes compared in this study are PHREEQC, CrunchFlow, FLOTRAN, and MIN3P. The benchmark problem is divided into four component problems of increasing complexity, leading up to the main problem which addresses the effects of advective and diffusive transport of ions through bentonite with explicit treatment of electrostatic effects. All codes show excellent agreement between results provided that the activity model, Debye-Hückel parameters, and thermodynamic data used in the simulations are consistent. A comparison of results using species-specific diffusion and uniform species diffusion reveals that simulated species concentrations in the effluent differ by less than 8 %, and that these differences vanish as the system approaches steady state.

Interactions between concrete and clays are driven by the strong chemical gradients in pore water and involve mineral reactions in both materials. In the context of a radioactive waste repository, these reactions may influence safetyrelevant clay properties such as swelling pressure, permeability or radionuclide retention. Interfaces between ordinary Portland cement and Opalinus Clay show weaker, but more extensive chemical disturbance compared to a contact between low-pH cement and Opalinus Clay. As a consequence of chemical reactions porosity changes occur at cement-clay interfaces. These changes are stronger and may lead to complete pore clogging in the case of low-pH cements. The prediction of pore clogging by reactive transport simulations is very sensitive to the magnitude of diffusive solute fluxes, cement clinker chemistry, and phase reaction kinetics. For instance, the consideration of anion-depleted porosity in clays substantially influences overall diffusion and pore clogging at interfaces. A new concept of dual porosity modelling approximating Donnan equilibrium is developed and applied to an ordinary Portland cement-Opalinus Clay interface. The model predictions are compared with data from the cement-clay interaction (CI) field experiment in the Mt Terri underground rock laboratory (Switzerland), which represent 5 y of interaction. The main observations such as the decalcification of the cement at the interface, the Mg enrichment in the clay detached from the interface, and the S enrichment in the cement detached from the interface, are qualitatively predicted by the new model approach. The model results reveal multiple coupled processes that create the observed features. The quantitative agreement of modelled and measured data can be improved if uncertainties of key input parameters (tortuosities, reaction kinetics, especially of clay minerals) can be reduced.

Highlights • Natural tracer profiles through clay rock, interpreted by paleo-hydrological modeling, can constrain rates of diffusion over geologic timescales • No combination of imaging, scattering or other approach has yet identified all types of shale porosity and their interconnection • Accurate prediction of solute transport must consider transport and reactions with mineral and organic phases • Carbonate minerals strongly affect pore waterchemistry, porosity evolution, and the retardation of solutes • Interdisciplinary meetings and collaborations can accelerate progress for all communities studying clay rocks

Geothermal systems in amagmatic orogens involve topography-driven infiltration of meteoric water up to 10 km deep into regional-scale faults and exfiltration of the heated water in surface springs. The thermal anomalies along the upflow zones have not been quantified, yet they are key to estimating the geothermal exploitation potential of such systems. Here we quantify the three-dimensional heat anomaly below the orogenic geothermal system at Grimsel Pass, Swiss Alps, where warm springs emanate from an exhumed, fossil hydrothermal zone. We use discharge rates and temperatures of the springs, temperature measurements along a shallow tunnel, and the formation temperature and depth of the fossil system to constrain coupled thermal-hydraulic numerical simulations of the upflow zone. The simulations reveal that upflow rates act as a first-order control on the temperature distribution and that the site is underlain by an ellipsoidal thermal plume enclosing 10 2 -10 3 PJ of anomalous heat per km depth. When the fossil system was active (3.3 Ma), the thermal plume was double its present size, corresponding to a theoretical petrothermal power output of 30-220 MW, with the 120°C threshold for geothermal electricity production situated at less than 2-km depth. We conclude that mountainous orogenic belts without igneous activity and even with only low background geothermal gradients typical of waning orogens are surprisingly promising plays for petrothermal power production. Our study implies exploration should focus on major valley floors because there the hydraulic head gradients and thus upflow rates and heat anomalies reach maximum values.

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