Hydrochemical characteristics of abandoned coal mines derived acid mine drainage in a typical karst basin (Wuma river basin, Guizhou China)

The hydrochemical characteristics of acid mine drainage (AMD) were investigated in Wuma River Basin, China. AMD was sampled from nine closed coal mine (CCM) sites to study the temporal and spatial evolution of pH, dissolved oxygen (DO), electrical conductivity (ED), total hardness (THR), total dissolved salt (TDS), and trace elements. The surface water (river) and groundwater surrounding mine sites were sampled to evaluate the potential pollution derived from AMDs. The TDS content of AMD was higher than that of surface water and groundwater. The dominant factors influencing TDS were the pH, temperature, and wet or dry season (which played a role in controlling microbial activity), HCO3− balance, and REDOX during the evolutionary process. The hydrochemical type of AMD was dependent on the evolutionary stage. From observations, most AMDs were in the form of the SO42--Ca2+•Mg2+ type that was characterized by a low pH, low [HCO3−], high [SO42−], and high [Fe]. In addition, the AMD samples were undergoing stage I and II processes, in which SO42− and trace elements were generated. The surface water and groundwater were primarily classified as the HCO3−-Ca2+•Mg2+ type, which accounted for their self-cleaning capacity, as indicated by the high [HCO3−]. The surface water and groundwater could be affected by the surrounding AMD depending on the geographical location. The surface water and groundwater sites that were located downstream of subsurface and surface runoff were obviously affected by AMD. After being polluted by AMD, surface water and groundwater contained higher levels of trace elements and emerged as the HCO3−•SO42--Ca2+•Mg2+ type. •Temporal and spatial variations of AMDs in Wuma River basin was investigated.•Hydro-chemical types of AMD depend on evolution stages.•Most of the AMDs were in SO42--Ca2+.•Mg2+ type with low pH/[HCO3−] and high [SO42−]/[Fe]•Surface water and groundwater were affected by surrounding AMDs.