Conceptualization and finite element groundwater flow modeling of a flooded underground mine reservoir in the Asturian Coal Basin, Spain

Secure workings in underground coal mines usually require the lowering of water levels via pumping. After mine closure, pump systems are stopped and the water level recovers in a process known as groundwater rebound. In the Asturian Coal Basin, Spain, the closure and flooding of underground coal mines carries associated environmental impacts that ought to be assessed. Among them, those related with groundwater flow and pollutant transport are of main interest. To evaluate the environmental risks during and after mine closure the construction of numerical flow models is suggested, which rely on an appropriate conceptualization of the mine-hydrogeological system. Underground mines of this region are characterized by very steep coal seams due to a folded and faulted geological structure. The resulting geometry translates into a complex network of hundreds of kilometers of mine tunnels and galleries. This is a challenge for the construction of numerical models in issues like geometrical problems, assignation of hydraulic parameters, coupling of physical laws and high computational cost. This paper presents the conceptual and numerical model of two linked mines of the region, Mosquitera and Pumarabule, that recently ended the flooding stage, to be used in assessment of post-closure environmental risks. To implement the geometry of the underground mine, a methodological approach to translate the mining information to the numerical simulator has been developed, that retains the hydraulic behavior of the key underground mine elements. A methodological approach for coupled simulation of mine conduits and porous media flow has been explicitly adapted. The results, in terms of the velocity distribution field inside the mine and possible outflow sites, are useful to provide a basis for later extension of transport models and assessment of hydro chemical impacts on the area.