The mining industry, especially coal mining, may severely affect the spatial and temporal evolution of groundwater. To provide a potential model outlining the hydrogeochemical evolution of groundwater as influenced by coal mining, a multi-layer groundwater system in a coal mining area was investigated. A total of 76 groundwater samples were collected between 1964 and 2018 for hydrogeochemical analysis from boreholes, underground tunnels, and surface pits at Xinglongzhuang Coal Mine in China. Hierarchical cluster analysis and principal component analysis were used to classify four significant groundwater zones and two principal components. The results showed that mining may have caused the rock strata to sink, obstructing the path linking the Permian and lower Quaternary aquifers, and resulting in intensified ion exchange in the lower Quaternary aquifer. The concentrations of Na⁺ and Ca²⁺ in the groundwater from the lower Quaternary aquifer changed over the studied period (~54 years). The gradual increase of Na⁺ and decrease of Ca²⁺ concentration verified the action of cation exchange. The four main aquifers had relatively independent hydrogeochemistry, suggesting that the hydraulic conductivity of the natural faults is weak and the sealing properties of hydrogeological drill holes in the coal field are good. Analysis of 26 heavy metals in the mine drainage water showed that there is only minor pollution after the water has been simply treated. These data analysis methods and results could be useful in areas with long-term mining to interpret the evolution of groundwater and to promote groundwater quality management.

Accurate estimation of irrigation return flow plays an important role in the effective management of groundwater, especially in arid and semiarid irrigation regions. However, there is a lack of sufficient research to clarify hydrological process dynamics associated with irrigation return flow. In this study, first, a two-dimensional/three-dimensional model, HYDRUS-2D/3D, was adopted to analyze two different irrigation types in the Delingha Depression, which is located at the northeastern margin of the Qaidam Basin, China. Then, a 3D saturated flow model was established. This study determined the effect of agricultural water application on the dynamics of irrigation return flow. A large difference in the irrigation return-flow coefficient (IRFC) was seen during the growing season; an IRFC of 0.3 was obtained using flood irrigation, whereas ditch irrigation resulted in an IRFC of only 0.1. The lag time of recharge was approximately 150 days. It was necessary to consider the lag time for the 3D numerical model to obtain satisfactory results. Flood irrigation led to a groundwater recharge rate of 90 mm/year. These results indicate that the lag time should be considered when groundwater recharge is estimated or modeled.