Study of multi-aquifer groundwater interaction in a coal mining area in China using stable isotopes and major-ion chemical data

Investigations have been undertaken to determine the interactions between the main aquifers in a coal mining area near the Taihang Mountains in China to determine the sources of water. Environmental isotopes (¹⁸O, ²H, ³H, ³⁴S) and water chemistry ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO ₃ ⁻ , SO ₄ ²⁻ , Cl⁻) were used as tracers. Furthermore, batch sampling and testing were conducted on mountain spring water and karst water in the aquifers of the Taiyuan Group, Fengfeng Group, and Majiagou Group on the seatearth of the coal mining area, via field observations and laboratory experimental analysis. The δ¹⁸O and δ²H values of the mountain spring water and karst water in the coal mining area showed a common distribution with the local meteoric water line EL1 of karst water in the Majiagou Group but significantly deviated from the surface water evaporation line (EL2) in the coal mining area, when combined with the distribution of karst water level values. Thus, karst water in the coal mining area is mainly supplied by groundwater from mountainous areas. Furthermore, the concentration of sulfate ions increased dramatically in the groundwater flow of karst water from mountain spring water to karst water in the Fengfeng Group and Majiagou Group of the coal mining area. When equivalent concentrations of (Ca²⁺ + Mg²⁺)/HCO ₃ ⁻ and SO ₄ ²⁻ /HCO ₃ ⁻ reached their peak, the chemical type of groundwater gradually evolved from Ca–Mg–HCO₃ to Ca–Mg–SO₄–HCO₃. In addition, significant positive correlation was found between the δ³⁴S and SO ₄ ²⁻ values of the water samples, indicating that gypsum is involved in groundwater lixiviation. In contrast, the relationships of Ca/Na versus Mg/Na, (Na + HCO₃) versus total dissolved solids (TDS), and Na⁺ versus Cl⁻ revealed that TDS and salinity accumulate from the mountain spring water and karst water in the Taiyuan Group of the coal mining area. Furthermore, the chemical type of groundwater gradually evolved from Ca–Mg–HCO₃ to Na–HCO₃, as revealed by a Piper trilinear diagram.