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Sand lenses at various spatial scales are recognized to add heterogeneity to glacial sediments. They have high hydraulic conductivities relative to the surrounding till matrix and may affect the advective transport of water and contaminants in clayey till settings. Sand lenses were investigated on till outcrops producing binary images of geological cross-sections capturing the size, shape and distribution of individual features. Sand lenses occur as elongated, anisotropic geobodies that vary in size and extent. Besides, sand lenses show strong non-stationary patterns on section images that hamper subsequent simulation. Transition probability (TP) and multiple-point statistics (MPS) were employed to simulate sand lens heterogeneity. We used one cross-section to parameterize the spatial correlation and a second, parallel section as a reference: it allowed testing the quality of the simulations as a function of the amount of conditioning data under realistic conditions. The performance of the simulations was evaluated on the faithful reproduction of the specific geological structure caused by sand lenses. Multiple-point statistics offer a better reproduction of sand lens geometry. However, two-dimensional training images acquired by outcrop mapping are of limited use to generate three-dimensional realizations with MPS. One can use a technique that consists in splitting the 3D domain into a set of slices in various directions that are sequentially simulated and reassembled into a 3D block. The identification of flow paths through a network of elongated sand lenses and the impact on the equivalent permeability in tills are essential to perform solute transport modeling in the low-permeability sediments.

Three methods for enhanced delivery of in situ remediation amendments in low‐permeability deposits have been tested at a site in Denmark: pneumatic fracturing, direct‐push delivery, and hydraulic fracturing. The testing was carried out at an uncontaminated part of a farm site, previously used for storage of chlorinated solvents, underlain by basal clay till with hydraulic conductivity ranging from 7.1× 10–11 to 3.5 × 10–7 m/s at testing depths 2.5 to 9.5 m b.s. Fluorescent tracers fluorescein and rhodamine WT were delivered. Tests of all three delivery methods have not been carried out at a single site before, and thus, this study provides unique data for comparison of enhanced delivery methods in both the vadose and saturated zone. Results show that pneumatic fracturing with nitrogen gas and propagation pressures of 1 to 9 bar had a distribution radius of less than 2 m, and produced dense networks of tracer‐filled natural fractures above the redox boundary (0 to 3 m b.s.) and widely spaced, discrete, induced, tracer‐filled subhorizontal fractures at depth (>3 m b.s.). Direct‐push delivery at pressures of 8 to 30 bar had a distribution radius of approximately 1 m, distributed tracer primarily in natural fractures above the redox boundary and in discrete, closely spaced (but not merging) induced fractures below the redox boundary. Hydraulic fracturing with a sand‐guar mixture at pressures of 0 to 6 bar produced an elliptical, asymmetrical, bowl‐shaped fracture with a physical radius of approximately 3.5 m at 3 m b.s. The geometry of hydraulic fractures attempted emplaced at 6.5 and 9.5 m b.s. is uncertain, but clearly not horizontal as desired. The direct‐push delivery method is robust and efficient for enhanced delivery at the clay till site in question, which based on thorough geological characterization is deemed a geologically representative basal clay till site.

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Zusammenfassung Zum Ende des Jahres 2018 wurde in Deutschland der Steinkohlenbergbau eingestellt. Damit beginnt auch in den bis zuletzt aktiven Revieren die Phase des Nachbergbaus inklusive einer dauerhaften Wasserhaltung. Diese beinhaltet typischerweise eine teilweise Grubenflutung und die Ableitung von Grubenwasser in nahegelegene Vorfluter. Im Blickpunkt der öffentlichen und wissenschaftlichen Diskussion stehen bergbauliche Schadstoffquellen und deren Risiken für die oberflächennahen Trinkwasserressourcen. Es existieren eine Reihe Kriterien und Kontrollmechanismen, um potenziellen Schadstoffaustrag zu erkennen und zu verhindern. Dazu zählen moderne Monitoringprogramme und Modellanwendungen, welche bestehende Schadstoffprognosen verbessern können. Daneben ist auch eine rechtlich und fachlich begründete Bewertung von Gefahrenszenarios eine entscheidende Größe für nachhaltiges Risikomanagement. Der Grubenwasseranstieg eröffnet jedoch auch Chancen, ehemalige Bergwerke für energetische Nutzungen umzurüsten. Bereits heute werden Heizkraftwerke mit Grubenwasser betrieben und es besteht großes Ausbaupotenzial. Dieser Beitrag diskutiert die Chancen und Risiken von Grubenflutungen in Deutschland und gibt einen Ausblick, welche Entwicklungen und Herausforderungen in Zukunft zu erwarten sind.

Underground hard coal mining usually disrupts the mechanical equilibrium of rock sequences, creating fractures within minor permeable rocks. The present study employs a dual-continuum model to assess how both fractured and porous sandstone media influence the percolation process in postmining setups. To test the approach, the software TOUGH2 was employed to simulate laminar fluid flow in the unsaturated zone of the Ibbenbüren Westfield mining area. Compared to other coal mining districts in Germany, this area is delineated by the topography and local geology, leading to a well-defined hydrogeological framework. Results reveal good agreement between the calculated and measured mine water discharge for the years 2008 and 2017. The constructed model was capable of reproducing the bimodal flow behavior of the adit by coupling a permeable fractured continuum with a low-conductivity rock matrix. While flow from the fractured continuum results in intense discharge events during winter months, the rock matrix determines a smooth discharge limb in summer. The study also evaluates the influence of individual and combined model parameters affecting the simulated curve. A detailed sensitivity analysis displayed the absolute and relative permeability function parameters of both continua among the most susceptible variables. However, a strong a priori knowledge of the value ranges for the matrix continuum helps to reduce the model ambiguity. This allowed for calibration of some of the fractured medium parameters for which sparse or variable data were available. However, the inclusion of the transport component and acquisition of more site-specific data is recommended to reduce their uncertainty.

ZusammenfassungIn einigen alpinen Gemeinden in Österreich sind die lokalen Grundwasserspiegel in den letzten Jahrzehnten auf kritische Niveaus angestiegen. Einerseits werden die Flächenversiegelung, der Wegfall von Retentionsräumen und die lokale Versickerung von Niederschlagswasser als Gründe für diese Entwicklung gesehen. Andererseits unterliegen Grundwasserressourcen dem Klimawandel, der sich örtlich mit variabler Grundwasserneubildung durch extreme Niederschlagsereignisse oder starke Schneeschmelze bemerkbar macht. In diesem Beitrag wird anhand einer Modellierungsstudie die Sensitivität eines lokalen, oberflächennahen Grundwasserleiters in Bezug auf naturräumliche, klimatische und anthropogene Entwicklungen analysiert. Es zeigt sich, dass eine unkontrollierte Interaktion von Oberflächengewässern maßgeblich und langfristig in den Grundwasserhaushalt eingreifen kann. Dies gilt insbesondere, wenn die Transferrate von Oberflächengewässern durch hydraulische Maßnahmen (Drainagen, Dichtwände, Sohlabdichtungen) oder natürliche Phänomene wie Hochwasser verändert wird. Die Studie verfolgt das Ziel, multiple Einflussfaktoren auf alpines Grundwassermanagement zu untersuchen und im Hinblick auf mögliche zukünftige Entwicklungen zu bewerten.

We tested the suitability of the multiple interactive continua approach (MINC) to simulate reactive mass transport in a disturbed post-mining coal zone. To the authors’ knowledge, this approach has not been employed in such mining settings despite its relative success in other environmental fields. To this end, TOUGHREACT software was used to set up a MINC model of the unsaturated overburden of the Ibbenbüren Westfield. With it, we examined and evaluated water–rock interactions in both the fractured and porous continua as the main driver of elevated hydrogen, iron, sulfate, and chloride concentrations in the coal mine groundwater. Long and seasonal geochemical signatures were obtained by formulating and applying a five-stage modelling process that depicts the mining history of the area. The simulation results agree well with the concentrations and discharge trends measured in the mine drainage. Oxygen and meteoric water flow through the fractured continuum, leading to a high and steady release of hydrogen, iron, and sulfate ions derived from pyrite oxidation in the matrix continua closest to the fractures. Likewise, high chloride concentrations resulted from the mixing and gradual release of relatively immobile solutes in the matrix as they interacted with percolating water in the fracture. In both cases, the use of a multiple continua approach was essential to resolve sharp gradients for advection and faster kinetic reactions, while reducing the model’s dependence on block size for diffusive transport at the fracture–matrix interface. The model further allows for the calculation and analysis of solute exchange and transport in the unsaturated overburden resulting from rebound and imbibition processes, something pioneering when compared to other models in the field.