Anthropogenic processes drive spatiotemporal variability of sulfate in groundwater from a multi-aquifer system: Dilution caused by mine drainage.

The water quality evolution of surface and groundwater caused by mining activities and mine drainage is a grave public concern worldwide. To explore the effect of mine drainage on sulfate evolution, a multi-aquifer system in a typical coal mine in Northwest China was investigated using multi-isotopes (δ34SSO4, δ18OSO4, δD, and δ18Owater) and Positive Matrix Factorization (PMF) model. Before mining, the Jurassic aquifer was dominated by gypsum dissolution, accompanied by cation exchange and bacterial sulfate reduction, and the phreatic aquifers and surface water were dominated by carbonate dissolution. Significant increase in sulfate in phreatic aquifers due to mine drainage during the early stages of coal mining. However, in contrast to common mining activities that result in sulfate contamination from pyrite oxidation, mine drainage in this mining area resulted in accelerated groundwater flow and enhanced hydraulic connections between the phreatic and confined aquifers. Dilution caused by the altered groundwater flow system controlled the evolution of sulphate, leading to different degrees of sulfate decrease in all aquifers and surface water. As the hydrogeochemical characteristic of Jurassic aquifer evolved toward phreatic aquifer, this factor should be considered to avoid misjudgment in determining the source of mine water intrusion. The study reveals the hydrogeochemical evolution induced by mine drainage, which could benefit to the management of groundwater resources in mining areas.

Impact of long-term mining activity on groundwater dynamics in a mining district in Xinjiang coal Mine Base, Northwest China: insight from geochemical fingerprint and machine learning.

Long-term coal mining could lead to a serious of geo-environmental problems. However, less comprehensive identification of factors controlling the groundwater dynamics were involved in previous studies. This study focused on 68 groundwater samples collected before and after mining activities, Self-Organizing Maps (SOM) combining with Principal Component Analysis (PCA) derived that the groundwater samples were classified into five clusters. Clusters 1-5 (C1-C5) represented the groundwater quality affected by different hydrochemical processes, mainly including mineral (carbonate and evaporite) dissolution and cation exchange, which were controlled by the hydrochemical environment at different stages of mining activities. Combining with the time-series data, the Extreme Gradient Boosting Decision Trees (XGBoost) derived that the mine water inflow (feature relative importance of 40.0%) and unit goaf area (feature relative importance of 29.2%) were dominant factors affecting the confined groundwater level, but had less or lagged impact on phreatic groundwater level. This was closely related to the height of the water flow fractured zone and hydraulic connection between aquifers. The results of this study on the coupled evolution of groundwater dynamics could enhance our understanding of the effects of mining on aquifer systems and contribute to the prevention of water hazards in the coalfields.

Hydrochemical and isotopic fingerprints of groundwater origin and evolution in the Urangulan River basin, China's Loess Plateau.

The origin and evolution of groundwater in the Urangulan River basin area under growing concern as its situated in an economically and ecologically crucial area of China. In the present study, a combination of different methods (i.e. self-organizing maps (SOM), piper diagrams, ionic ratios, multiple isotopic analyses and Bayesian isotope mixing model) provided an efficient way for analysing groundwater origin and evolution. The hydrochemical type was found to be Ca-HCO3 in low TDS and Na + K-Cl or Na + K-SO4 in high TDS groundwater. According to the δ2H and δ18Owater values, groundwater in the study area mainly originated from atmospheric precipitation and was influenced by evaporation. In addition, the rock weathering in conjunction with the cation exchange completely dominated the geochemical evolution process. The dual SO42- isotope and Bayesian isotope mixing model showed that gypsum dissolution, fertilizer input and sewage input were the main sources of SO42- in the study area, accounting for an average of 30.2 %, 28.5 %, and 17.3 % of SO42- in the groundwater, respectively. Other than water-rock interactions, human activity (mining and irrigation) distributed throughout the study area in combination with the spatial characteristics was the dominant factor controlling the hydrochemical evolution. The results of this study provided a basis for understanding groundwater origin and evolution while facilitating the effective management and utilization of groundwater.