Application of a Double Continuum model to describe reactive transport in disrupted coal mining areas: a case study of the Ibbenbüren Westfield, Germany.

Diego Bedoya Gonzalez1, Thomas Rinder1, Timo Kessler2, Sylke Hilberg1, Maria-Theresia Schafmeister2
1 Department of Geography and Geology, University of Salzburg
2 Institute for Geography and Geology, University of Greifswald

V 9.5 in Bergbau und Grundwasser

24.03.2022, 15:15-15:30, HS 3

Underground hard coal mining operations irreversibly disrupt the pre-existing mechanical equilibrium of the geological media. The employment of high-recovery methods modifies the stress field of the sedimentary sequence, generating movement and failure of the rock layers above and below mined seams. These disruptions also affect the original state of the hydrogeological system, altering flow paths and increasing the surface of rock exposed to air and water. Some of these changes have generated, for example, large volumes of mine water with elevated iron (>200 mg/L) and sulfate (≈4000 mg/L) contents at the Ibbenbüren Westfield coalmine. The present study set up a Double Continuum (DC) model to characterize the water flow and reactive transport of solutes in this area. By using this approach, the mining zone can be treated as a coupled hydrogeological system, where fluid flow and mass transport occur in the porous and fractured media simultaneously.

The studied area is a former mining zone situated on a topographically elevated horst structure composed of Carboniferous rocks. As the groundwater table is maintained above the surrounding areas, the quantity and quality of the discharged water depend on two factors: i) the seasonal fluctuation of infiltrated precipitation, and ii) its interaction with the fractured and matrix continuums. To characterize these processes, the software TOUGHREACT is employed for modeling Darcy-type fluid flow in the variably saturated porous medium. Discretization of the physical medium is made from mineralogical and chemical analyses previously performed by the authors on several core samples, while properties of the water-conducting fracture zones (e.g., growth, density, and permeability) are obtained from extensive literature research.

The modeling results display good agreement with measured mine water discharge. While the fractured continuum reacts readily to constant precipitation events during wet seasons (rapid percolation), water is stored and released over some weeks from matrix blocks after the main recharge event has occurred (slow percolation) Based on the results from the numerical model, the coupled dissolution and precipitation reactions, governing mine drainage chemistry, including extensive oxidation of pyrite, are quantified.  These outcomes add an important aspect to coalmine models, as they can be used to forecast water inflows within mining-disrupted sequence as well as long-term evolution of the mine drainage. Additionally, the DC approach may improve the characterization of the rebound process in post-mining locations.



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