Modelling the evolution of water quality in abandoned mines of the Lorraine Iron Basin

Abstract

Mining operations disrupt the local groundwater balance both during and after mining. The galleries excavated during mining often require dewatering and thus restructure the groundwater flow. Then, once mining and dewatering have ceased, the mines flood and the groundwater attempts to attain a new chemical and dynamic equilibrium. Mine flooding in the Lorraine Iron Basin (France) over the past 20 years has resulted in a degradation of the groundwater quality, as indicated by high concentrations of SO4, Ca, Na, Mg, K, Mn and B, making it unsuitable for human consumption. Before new resource management schemes can be implemented, we must predict the long-term evolution of the groundwater’s discharge and quality. Few spatially distributed data being available, a global approach has been adopted that consists in representing the iron basin as a network of chemical reactors. Flow rates through the network are calculated by processing available data records with an inverse rainfall-discharge model. Each reactor includes a specific chemical kinetic model for the rocks of the Lorraine Iron Basin (LIB), the kinetic constants having been determined from experimental studies. This approach was applied to the South Basin of the LIB, abandoned in 1995. The model can predict the concentration of major elements (SO4, Na, Ca, Mg and HCO3) in the groundwater at its overflow and gives an order of magnitude for elements such as K with lower concentrations. The global approach developed to model the evolution of groundwater quality when mines are flooded would seem to be well suited to the LIB case, probably due to the homogeneity in the distribution of the various types of mining methods at the mine reservoir scale.