Groundwater Circulation Mechanism of the Upstream Area of Beiniuchuan River Using Isotope–Hydrochemical Tracer

In order to achieve the rational development and utilization of underground water resources in the Dongsheng mining area under coal mining conditions, we selected the upstream area of Beiniuchuan River as a typical region. Through field investigations, sampling tests, and the application of hydrochemical and isotope techniques, we traced the groundwater circulation mechanism in the Dongsheng mining area. The results indicate that the majority of the Quaternary alluvial and Salawusu Formation groundwater is of the HCO₃-Ca type, with a TDS content below 300 mg/L. However, in some areas, the hydrochemical type becomes complex due to anthropogenic contamination. The shallow-buried Yan’an Formation groundwater is either of the HCO₃-Ca·Mg type or the HCO₃·SO₄-Ca·Mg type, with TDS content ranging from 200 to 750 mg/L. The Yan’an Formation at depths greater than 40 m exhibits complex water chemistry, with a TDS content higher than 500 mg/L, and it belongs to the Cl-Na type, with TDS around 700 mg/L. The hydrogen and oxygen isotope results indicate that the local groundwater is primarily recharged via atmospheric precipitation. The ³H and ¹⁴C results show that the Quaternary alluvial and shallow-buried Yan’an Formation groundwater has a fast turnover rate, while the deep-buried Yan’an Formation and Yan’chang Formation groundwater have a slower turnover rate. The regional groundwater circulation can be generalized into three flow systems: shallow, intermediate, and deep. Under the influence of coal mining activities, the water circulation conditions in the study area have undergone significant changes. The sealing integrity of the Yan’an Formation has been compromised, and precipitation and shallow groundwater have enhanced the vertical infiltration capacity of the formation, increasing the proportion of groundwater participating in the intermediate flow system. As a result, the river runoff mainly dependent on the discharge from the shallow flow system has drastically decreased.