Identification of the effects of shallow-buried mining on the hydrochemical evolution of phreatic groundwater in arid and semi-arid regions: A case study of the Ten Tributaries Basin

Revealing the evolution of phreatic water hydrochemistry under natural processes and mining activities in shallowly buried mining areas of arid and semi-arid regions is key to identifying the impacts of mining on groundwater. Taking the Ten Tributaries Basin in the upper Yellow River as the study area, this study combined ion ratios, stable isotope tracing, and the Chemical Mass Balance (CMB) model to reveal and quantify the effects of coal mining (recharge area) and mirabilite mining (discharge area) on phreatic water chemistry. Results show that mining activities are the key anthropogenic factor driving the spatial differentiation of phreatic water chemistry, with influence intensity exhibiting significant spatial heterogeneity. In undisturbed areas, natural dissolution processes contribute more than 80% of the hydrochemical composition, dominated by carbonate dissolution. However, in mining-affected areas, groundwater chemistry deviates from natural evolutionary pathways, characterized by enhanced dissolved-ion input and more complex ionic compositions. In recharge areas, coal mining mainly promotes carbonate dissolution and vadose-zone disturbance, increasing TDS by factors of 1.96 and 1.88, respectively, relative to natural conditions. In discharge areas, mirabilite mining is dominated by evaporite dissolution and deep saline-water mixing, leading to TDS increases by the factors of 4.41 and 3.24, respectively. These mining effects are superimposed on the pathway-controlled groundwater flow system, resulting in distinct spatial differentiation of groundwater hydrochemistry. The improved CMB model effectively quantifies the impacts of mining disturbances on groundwater chemistry. The results provide scientific support for groundwater resource management and ecological protection in shallowly buried mining areas of arid and semi-arid regions.