Analysis of mining effects on the geochemical evolution of groundwater, Huaibei coalfield, China

Investigations were undertaken at the Xutuan and Renlou coal mines in Huaibei coalfield, Anhui Province, China to determine the effects of mining on the geochemical evolution of groundwater in the area. A total of 77 samples were collected between 1999 and 2017 from Neogene, Permian, and Carboniferous aquifers in two coal mines for hydrogeochemical analysis. The variation of hydrochemical types and the difference of Cl- and TDS suggest that the groundwater in the Neogene aquifer flow from the Xutuan coal mine to the Renlou coal mine. The high concentrations of chloride in groundwater in the Permian aquifer may be associated with recharge from the Neogene aquifer under the effects of mining. Principal component analysis and the results of chemical analysis of water samples were used to explore the water-rock processes in the three aquifers. The results suggest that the main controlling process of the Xutuan coal mine is ion exchange between Na+ and Ca2+, while the principal chemical processes in the Renlou coal mine are likely to be ion exchange and reverse ion exchange between Na+ and Ca2+. Mining will lead to the decline of the Neogene aquifer potentiometric heads and the compaction of aquifer sediments. This, in turn, could lead to the development of fissures caused by mining which could increase the hydraulic connection between the Neogene and the Permian aquifer, and increase the ion-exchange intensity of the Permian aquifer. The inverse geochemical modeling results of groundwater flow-paths in the Neogene and Carboniferous aquifers suggest that reverse ion-exchange process is taking place in these aquifers, which provides additional evidence for geochemical evolution. This combined method of using various lines of evidences from hydrogeology, multivariate statistical analysis, hydrogeochemical research, and geochemical models can explain the hydrogeochemical evolution processes of groundwater at a deeper level.