Groundwater level modeling during the coal mines´ decommissioning

Problem Statement and Purpose. Abandoned or non-operating coal mines that were closed due to flooding pose environmental hazards, primarily due to the risk of groundwater contamination, subsidence, and waterlogging of surrounding areas resulting from flooding and rebounding groundwater levels. Estimating the risk of flooding with the help of the numerical hydrogeological modelling for areas inhabited by coal mines that were built at the beginning of the last century is challenging, since information on operational parameters (such as mine workings’ features, pumping rates, etc.), has not been preserved to date or is limited. A study examined the capabilities of groundwater flow numerical modelling in predicting groundwater rebound and its accuracy in areas surrounding old, closed coal mines. Data and Method. As pilot study areas, the Saxony coal mine region (Germany) and the Central Donbass (Ukraine) coal mines group area were established. Both pilots integrate more than 20 minefields of former interconnected coal mines with more than 100 years of mining history and a similar excavation method. In this study, the numerical modelling software packages were used as a tool for the numerical groundwater flow simulations and predictions in a three-dimensional groundwater system of coal mines areas. All of them contain a three-dimensional (3D), finite-element groundwater flow code. Additionally, adjustments in the form of calculation decisions (such as sensitivity analysis and multivariate simulations on a block-simplified model) were developed and implemented to address hydraulic system parameter uncertainties and the insufficiency of historical mining data (lack of pumping rates information, number of mining horizons, mine workings features, and geometry). Result and discussion. This study demonstrates the conceptual approach and numerical groundwater flow evaluation for the condition in which pumping isn’t maintained and no dewatering measures are provided in the area to prevent flooding at the critical level. The developed numerical hydrogeological model covers all common steps of precise and complete numerical simulations, including transient conditions simulations, that reconstruct the full range of mining stages during the mines' life. The special methodology of calibration was adjusted for this purpose – the cycle of steady-state, transient, and prediction simulations of the groundwater level until the lowest possible RMSEs between observed and simulated groundwater levels in the model were achieved.