Abstract
Following tunnel excavation, which is influenced by hydraulic fracturing and geological structure, a series of hydrochemical reactions occur in the karst aquifer, which has a significant impact on groundwater hydrology and the earth process. Based on five sets of 38 samples collected in the Tongzi Tunnel in 2020 and 2021, the main geochemical processes and water quality conditions of the karst aquifer system during tunnel construction were revealed by multivariate statistical analysis and graphical methods. The results showed that water-rock action is the main mechanism controlling groundwater chemistry in the study area; HCO3-, Ca2+, and Mg2+ are associated with the widely distributed carbonate rocks in the study area. SO42- is derived from gypsum and sulfate rocks and special strata, which are another important source of Ca2+. Sodium-containing silicates and reverse cation exchange as the causal mechanisms of Na+ whereas F- is derived from fluorite. According to the mineral saturation index calculations, the dissolution and precipitation of minerals such as alum, gypsum, calcite, dolomite, and salt rock have an important influence on the main chemical components in water. The 38 samples were subjected to cluster analysis, and the results could be classified into seven categories. The representative clusters 1, 3, and 5 were selected for principal component analysis. Clusters 1 and 5 of groundwater represent weathering, dissolution, and ion exchange of carbonate and sulfate rocks and are closely related to the lithologic limestone, limestone intercalated with carbonaceous mudstone, carbonaceous mudstone, and coal-measure strata in the aquifer. Cluster 3 is dominated by upper surface river water and characterizes the geochemistry in natural water bodies dominated by the dissolution of carbonate, sulfate, and salt rocks. Finally, groundwater quality is mostly found in Class IV, with NO3- and F- being the main contaminants in the water.
Highlights
In the karst areas of southwest China, tunneling is the main way to improve transportation due to the special topographical features
The lithology of the aquifer group is dominated by limestone intercalated with carbonaceous mudstone, carbonaceous mudstone, and coal-measure strata representing the weathering and dissolution of sulfate rock and ion exchange
The fifth cluster is dominated by water samples of group V, concentrated between YK43+613 and YK43+730; the lithology of the aquifer group is dominated by limestone and coal-measure strata
Summary
In the karst areas of southwest China, tunneling is the main way to improve transportation due to the special topographical features. Some scholars have studied the hydrogeochemical processes induced by mining activities in the karst areas of Southwest China [15, 16], there are no reports on groundwater chemistry in the karst tunnel fracture zone; the existing research on groundwater related to tunnel mainly focuses on the influence of groundwater level decline and water pressure on the safety of lining structure, lack of understanding of groundwater and surrounding rock matrix water-rock interaction. Through indoor experiments, combined with hydrochemical methods and multivariate statistical methods, the Tongzi Tunnel groundwater chemical characteristics and water quality are evaluated, aiming to understand the water-rock interaction and hydrochemical evolution mechanism caused by tunnel construction
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