Effects of groundwater level decline on shallow soil water in grassland open-pit mining areas: Water vapor migration and water sources estimation

Study region The Chenqi Coalfield in the Hulunbuir grassland, a typical semi-arid grassland open-pit mining area in Inner Mongolia, China, where mining has caused severe groundwater level decline. Study focus This study adopted in situ monitoring and hydrogen-oxygen stable isotope (δD and δ¹⁸O) analysis to clarify the response of shallow (0–100 cm) soil water–vapor–heat dynamics to increasing groundwater level depth (GLD) and to quantify liquid and vapor water sources in the unsaturated zone. New hydrologic insights from the region (1) An integrated isotope-thermodynamic equilibrium method was developed to quantify groundwater contributions to both liquid and vapor water in shallow soils. (2) With rising GLD, the minimum relative humidity (RH) in the surface layer decreased from 67.8% to 20.5%, weakening groundwater's temperature regulation capacity and expanding the active temperature layer from 40 cm to 100 cm depth. (3) Deeper GLDs reduced the frequency and depth range of condensation-favorable conditions, with potential implications for root-zone water availability. (4) When GLD < 4 m, groundwater-derived liquid water averaged 227.13 ± 14.95 kg/m³ and vapor water averaged 0.76 ± 0.06 g/m³ , approximately twice the values under deeper GLD conditions. The results provide a site-specific hydrological threshold and a transferable methodological framework for groundwater management and ecological restoration in grassland mining areas.