Source of mine water during coal mining in the contiguous area of Inner Mongolia and Shaanxi

The region where Inner Mongolia and Shaanxi converge is pivotal for the underpinning of national energy security and is also central to the ecological conservation and sustainable development of the Yellow River Basin. Intensive coal extraction in this zone potentially poses a substantial threat to ecological conservation, notably by undermining the integrity of the shallow Quaternary groundwater environment. Accurate assessment of the leakage from various aquifers within the overburden is a complex task, thereby complicating the development of scientifically sound and targeted strategies for water conservation during coal mining. Our research leverages the principle of isotopic mass balance, following the analysis of environmental isotopes (Deuterium and Oxygen-18) present in the groundwater of the aforementioned border region, to quantify the relative contributions of different mine water sources. Findings indicate that the isotopic signatures in the aquifer waters are influenced by a multitude of factors including topography, stratigraphic configuration, and the nature of groundwater storage. The direct sources of groundwater in the Quaternary aquifer of the study area are atmospheric precipitation and surface water, characterized by rapid circulation and renewal, replenished by modern water with tritium-rich features. The deuterium (D) and oxygen-18 (¹⁸O) values in the groundwater are close to those of atmospheric precipitation and surface water. In the deep-buried Cretaceous aquifer, the hydraulic connection with the Quaternary is tight, and the cycle and renewal process is prolonged, resulting in a decrease in environmental isotope values. The deuterium values in the Cretaceous groundwater range from –80.2‰ to –75.6‰, and the oxygen-18 values range from –10.6‰ to –8.7‰. The groundwater in the Jurassic aquifer, which is controlled by vertical recharge, exhibits a slower cycle and renewal rate, with gradually decreasing δD and δ¹⁸O values. The shallow and moderately deep-buried strata have a long depositional history and better cementation, affected by later tectonic movements, and are in direct contact with the Quaternary or Neogene, receiving replenishment from Quaternary groundwater, thus having relatively higher δD and δ¹⁸O values. In contrast, the Jurassic strata in the deep-buried area, with its substantial thickness and overlying thick layers of Quaternary and Cretaceous strata, have poor groundwater replenishment, a long groundwater flow path, and are relatively closed and stagnant, resulting in relatively lower δD and δ¹⁸O values. The application of the D-value, in conjunction with the binary mixing model, facilitates the quantitative assessment of the contribution of Quaternary waters in shallow-buried coal mine waters. In the case of the SGT and HLG coal mines, the proportion of Quaternary waters is typically less than 20%. In contrast, the Quaternary water content in the BLT and LSJ coal mines varies between 28.0% and 57.0%. Notably, the Quaternary water proportion in the YBJ coal mine approaches 80%, indicating a significantly higher contribution. For the middle and deep-buried coal mine waters, the Quaternary water content is generally less than 20%. However, it is noteworthy that in the HLW and SS coal mines, which have been developed earlier and feature 'skylights' of the Puding Formation red soil layer in the roof, the Quaternary water content has reached 37.05% and 26.24%, respectively. This suggests that under specific geological conditions, the proportion of Quaternary waters may be substantially elevated. In the deep-buried mine waters, the contribution of Cretaceous water is approximately around 30%. IsoSource modeling has calculated that the contribution rate of Quaternary water ranges from 7.6% to 9.3%, the contribution rate of Cretaceous water is between 12.0% and 17.1%, and the contribution rate of the Upper Jurassic water is from 74.9% to 80.4%. Moreover, the contributions of various sources to the mine waters are similar across different mining areas. By using the D content value in groundwater and a binary mixed model, it was calculated that the proportion of Quaternary water in shallow buried mine water was less than 20% for SGT and HLG coal mines, 28.0% to 57.0% for BLT and LSJ coal mines, and nearly 80% for YBJ coal mines; The proportion of Quaternary water in the mine water in the middle and deep buried areas was generally less than 20%. The proportion of Quaternary water in HLW and SS coal mines, which were developed earlier and had a "skylight" in the red soil layer of the Baode Formation on the roof, reached 37.05% and 26.24% respectively. The proportion of Cretaceous water in deep buried mine water was about 30%. The IsoSource model calculates that the contribution rates of Quaternary water were between 7.6% and 9.3%, Cretaceous water was between 12.0% and 17.1%, and the contribution rates of upper Jurassic water were between 74.9% and 80.4%. The contribution rates of various sources of mine water in different mining areas were similar. This study accurately identified the proportion of water sources in various aquifers of mine water, which was of great significance for ecological environment protection and green and sustainable development of coal resources in the contiguous area of Inner Mongolia and Shaanxi. Accurately identifying the source proportions of water from various aquifers in mine water is of significant importance for the conservation of water resources and ecological protection in the coal mining areas bordering Inner Mongolia and Shaanxi. This study aims to provide a precise delineation of these proportions, thereby contributing to sustainable mining practices and the preservation of the ecological environment in the region.