Regional groundwater flow and karst evolution–theoretical approach and example from Switzerland

In regional scale aquifers in the Rhine Valley and Tabular Jura east of Basel (Switzerland), the groundwater circulation was investigated using regional-scale geological and hydraulic 3D models. The main aquifers in the area comprise the Quaternary aquifer of unconsolidated gravel deposits along the River Rhine and its tributaries, as well as the regional scale karst aquifer within the Upper Muschelkalk. Land subsidence, a process likely associated with salt solution mining, indicates further subordinate groundwater bearing segments and complex groundwater interactions along fault zones. In the aquifer systems we investigated, regional-scale groundwater circulation was simulated and visualized in relation to the geological settings. Lithostratigraphic units and fault structures were parameterized and analyzed, including the sensitivity of hydraulic properties and boundaries. Scenario calculations were used to investigate the sensitivity that the aquifer systems had to hydraulic parameter changes during Quaternary aggradation and degradation in the main valley. Those calculations were also done for base-level changes in the Rivers Rhine and Birs. For this purpose, this study considered probable historic base-levels before river regulation occurred, and before river dams and power plants were constructed. We also focused on scenarios considering increased groundwater recharge rates, e.g. due to exceptional long-lasting precipitation, or heavy rainfall events in the catchment area. Our results indicate that increased groundwater recharge rates in the catchment areas during such events (or periods) are associated with orders of magnitude increases of regional inflow into the Upper Muschelkalk karst aquifer. Furthermore, the groundwater fluctuations and groundwater saturated regions within the karst aquifer shift to places where high densities of sinkholes are documented. When the surface water base-levels adapt to probable historic levels, it leads to increased hydraulic gradients (i.e. local lowering of the groundwater level by up to 7 m). Those increased gradients are associated with increased groundwater flow within some aquifer regions that are particularly prone to karst development.

Denitrification potential and occurrence of reactive minerals in the fractured carbonate aquifer of the Upper Muschelkalk, SW-Germany

<p>Fractured or karstified carbonate rocks constitute major drinking water resources all over the world. Nitrate is one of the major threads for drinking water suppliers in regions with intensive agrucultural use. Field scale observations in the Upper Muschelkalk aquifer in the area “Oberes Gäu”, SW-Germany, suggest that denitrification due to oxidation of Fe<sup>(II)</sup> or sulfide-bearing minerals might be a relevant attentuation process at least in cases of extended mean residence times (> 5-40 years).</p><p>To identify reactive minerals within the rock matrix rock samples have been taken from major facies types within the Upper Muschelkalk from outcrops, drillings and carbonate quarries. Samples have been analysed in polished thin sections using transmitted & reflected light microscopy as well as electron microscopy including energy-dispersive elemental analysis.</p><p>Reactive Fe<sup>(II)</sup> and sulfide-bearing minearls are pyrite and marcasite as well as saddle dolomites (Fe-bearing dolomites). Concentrations of these minerals depend on facies types and amount to several weight percent iron. Porosities range from very low values (<1 %) up to 25 %.</p><p>In combination with a hydrogeological characterization, these investigations allowed to delineate reactive zones within the fractured aquifer. Denitrification within these reactive zones depends on the amount and dissolution of these minerals as well as effective diffusion in the pore space.</p>

Predictive DFN modelling for the Upper Muschelkalk aquifer in the northernmost Swiss Molasse Basin based on vertical and horizontal borehole records

<p>The exploration of the underground is a complex, but common task. Structural characterisation of a given sub-surface volume is of interest, for many purposes including deep geological repositories for radioactive waste, exploitation of mineral resources and geothermal energy production. One major challenge in this regard relates to the identification of sub-seismic scale faults and fractures. Discrete fracture network modelling is one possible technique for this purpose. Ideally, it is supported by borehole data. Even so, the results of stochastic models require critical verification to determine resulting uncertainties and model robustness.</p><p>We present a case-study from the village of Schlattingen, located the northernmost Molasse Basin in Switzerland, that is devoted to such a verification aimed at improvement of the DFN modelling workflows. Two boreholes were drilled at this location, a vertical cored borehole reaching into the crystalline basement and a deviated borehole running sub-horizontally for 464 m in the Schinznach Formation (Upper Muschelkalk), a potential geothermal reservoir (Frieg et al. 2015). This borehole layout allows testing the workflow for discrete fracture network modelling from a single borehole and assessment of the the added value of a deviated borehole (and vice versa).</p><p>The modelling workflow used borehole data and outcrop descriptions from a range of locations as input data. The spatial distribution of features was simulated using a Poisson distribution. The aims of the study were to investigate the workflow’s ability to account for the different orientation biases in the two boreholes and develop understanding of spatial variability in fracture orientation and frequency.</p><p>It was found that reasonable consistency in orientation and overall frequency could be achieved using the borehole orientation distributions but that the spatial variability in fracture frequency and clustering of fractures were significant. It was also necessary to critically evaluate the borehole imagery from the deviated borehole.</p><p>Current efforts are focused on better constrain the spatial fracture distribution along the deviated borehole using correlation analysis (Marett et al. 2018, Gale at al. 2018) and assess its influence on the discrete fracture network model. In addition, it is anticipated to integrate observation from nearby outcrops into the modelling strategy.</p><p>References:</p><p>Frieg, B., Grob, H., Hertrich, M., Madritsch, H., Müller, H., Vietor, T., Vogt, T., and Weber, H.P. (2015). Novel Approach for the Extrapolation of the Muschelkalk Aquifer in Switzerland for the CO<sub>2</sub>-free production of vegetables. Proceedings World Geothermal Congress, Melbourne, Australia</p><p>Gale, J. F. W., Ukar, E., & Laubach, S. E. (2018). Gaps in DFN models and how to fill them. 2nd International Discrete Fracture Network Engineering Conference, DFNE 2018.</p><p>Marrett, R., Gale, J. F. W., Gómez, L. A., & Laubach, S. E. (2018). Correlation analysis of fracture arrangement in space. Journal of Structural Geology, 108, 16–33. https://doi.org/10.1016/j.jsg.2017.06.012</p>

Middle-Upper Triassic stratigraphy and structure in the Alt Palància region (eastern Iberian Chain): A multidisciplinary approach

The present study provides new data of the Middle-Upper Triassic successions and their deformation in the eastern Iberian Chain, where contractional tectonics during the Cenozoic disrupted this Mediterranean type of Triassic rocks. The succession, divided into three Muschelkalk units, was studied in the Alt Palància area. In this area, both the lower and upper Muschelkalk consist of two main types of sub-units, those made up of carbonate and those of carbonate-marl alternation. The marked similarity observed between the evaporite units of the middle Muschelkalk and the Keuper humpers their unambiguous discrimination in the field. The integration of geological mapping, stratigraphic logging, palynological dating and gypsum isotope analysis carried out provided that: i) a change in the structural style, facies, and depositional thickness occurs across a SW to NE transect at both sides of the Espina-Espadà Fault, providing evidence for the extensional activity of this major structure; ii) palynological data assign Anisian age to the Röt facies and the lower and the middle Muschelkalk units, and Ladinian to the upper Muschelkalk unit; iii) the δ34SCDT and δ18OSMOW values of gypsum reveal as a useful proxy to discriminate between the middle Muschelkalk (δ34S: 15.6 to 17.8‰) and the Keuper (δ34S: 14 to 15.5‰) units; and iv) the isotopic signature also helps to identify clayey-marly gypsiferous outcrops made up of the two evaporite facies due to tectonic juxtaposition. These results confirm the Mediterranean type of Triassic rocks for the entire Alt Palància and other areas to the NE. This multidisciplinary approach reveals as a robust methodology to study Triassic basins in Iberia and to other geological domains where the carbonate-evaporite successions have been greatly disrupted by tectonism.