Characteristics of δD and δ18O of Reclaimed Mine Soil Water Profile and Its Source Water Bodies in a Coal Mining Subsidence Area with High Groundwater Level—A Case Study from the Longdong Coal Mining Subsidence Area in Jiangsu Province, China

Coal mining, as one of the key drivers of land degradation worldwide, caused land subsidence problems. In this study, we conducted experimental research to explore the reclaimed mine soil (RMS) water dynamics and its sources in relation to reclaimed land use types using stable water isotopes in the Longdong coal mining area with high groundwater level in east China. We collected water samples seven times in 2017 from all of these water bodies (precipitation, surface waters (river water and water from subsidence pits (WSP)), groundwater and soil water). Our main findings are three fold: (1) the values of slope and intercept of the local meteoric water line of Craig (LMWL) of precipitation for the study area are higher than the global meteoric water line of Craig (GMWL) because of the humid monsoon climate zoon, and the values of δD and δ18O of surface waters and soil water and groundwater deviated from LMWL to some extent with a range of 5–30%, and the D and 18O of precipitation and the surface waters have higher seasonal variation than groundwater; (2) the values of δD and δ18O of RMS for the whole soil profile (0–100 cm) are lower than that of precipitation and have obvious seasonal variations and great fluctuation in the topsoil (0–30/40 cm) and decrease at depth (30/40–70 cm) and stable in deep soil layers (below 70 cm deep); (3) the RMS with forest and crop enhanced water infiltration capacity and soil water mixing strength compared with the waste RMS, so establishment of forest and crops should be encouraged in the RMS; (4) the main sources of topsoil (0–30 cm for crop and 0–40 cm for forest) of RMS are precipitation through infiltration, the main supply for deep soil water (below 70 cm deep) is groundwater, and the soil water for the middle deep soil layers (30/40–70 cm) is mainly from mixing sources of precipitation, groundwater, and river water through pant root water absorbing and groundwater upshifting.