Effect of Darcy flux on the release of dissolved organic matter and nitrogen from coal gangue in a coal mine underground reservoir: Column experiments

Abstract Underground reservoirs packed with coal gangue have been widely used to store and purify mine water in the coal mine areas of China; however, minimal information is available about the effects of pollutants released from the packed matrix on the quality of stored water. Column experiments were performed at 25 °C and Darcy fluxes of 0.39, 0.73, and 1.56 cm/h to investigate the effect of Darcy flux on the dynamic changes of dissolved organic matter (DOM) and nitrogen released from the packed coal gangue. Results showed that the values of dissolved organic carbon (DOC), UV254, organic nitrogen (ORG-N), nitrate, and total dissolved solids (TDS) in the effluent decreased rapidly within 3.7 pore volumes (PV) at the three Darcy fluxes and then decreased slowly and tended to become stable during the column experiments. However, the effluent concentrations of NH4+–N, which is the major nitrogen released from the packed matrix, and total nitrogen (TN) decreased and increased alternately, which can be relative to the lithology, mineral, and chemical composition of the studied matrix. The quantities of DOC, NH4+–N, ORG-N, and TN released from the studied matrix decreased with Darcy flux, suggesting a positive effect of hydraulic retention time (HRT) on the resolution of pollutants from the studied matrix. Moreover, the microbial environment got weakened with an increase in Darcy flux, as confirmed by the variation trends of the biological/autochthonous index (BIX) and the fluorescence index (FI). Thus, the evident accumulation of NO2− –N and the highest content of NO3−−N were observed due to incomplete nitrate reduction with a low efficiency at 1.56 cm/h. The findings will be helpful in estimating the variation of water quality during the storage of mine water in a coal mine underground reservoir in China.