Genetic mechanism of karst water revealed by hydrochemistry and isotopes in Pingliang, Gansu province

Karst water is one of the significant water resources in the Pingliang region. Investigating the formation mechanism of karst water is crucial for the sustainable utilization of karst water resources, the protection of water sources, and the formulation of environmental protection measures in the area. It provides a scientific basis and theoretical support for promoting the sustainable development of regional water resources and the health of the water environment. The hydrochemical composition of karst water can reveal the processes of recharge, runoff, and discharge, while isotopic tracing methods help indicate the scope and dynamic changes of the karst water system. To uncover the genetic mechanism of karst water in Pingliang City, we comprehensively analyzed the hydrochemical characteristics and sources of karst water in Pingliang City using methods such as Piper trilinear diagrams, mathematical approaches, and ion ratio coefficients. The research results indicate that the groundwater in the region can be mainly divided into two types: HCO₃-Ca and HCO₃·SO₄-Na, with Ca²⁺ and Na⁺ as the primary cations and ${\rm{HCO}}_3^{-}$ as the primary anion. The pH value ranges from 6.72 to 6.9, exhibiting weakly alkaline hydrochemical characteristics overall. The total dissolved solids (TDS) value of groundwater ranges from 158.01 mg·L⁻¹ to 1519.40 mg·L⁻¹. Spearman correlation analysis shows that the hydrochemistry of karst water in the study area is primarily controlled by the dissolution of minerals such as dolomite and evaporites. TDS is highly positively correlated with the contents of Na⁺, Mg²⁺, Cl⁻, ${\rm{SO}}_4^{2-}$, and ${\rm{HCO}}_3^{-}$. Additionally, the content of Mg²⁺ is significantly positively correlated with the contents of the main anions Cl⁻, ${\rm{SO}}_4^{2-}$, and ${\rm{HCO}}_3^{-}$ (r > 0.7, p < 0.01), and the content of Na⁺ also has a significant positive correlation with the contents of Cl⁻ and ${\rm{SO}}_4^{2-}$ (r > 0.98, p < 0.01). In the analysis of ion proportional coefficients, the hydrochemical characteristics of karst water in Pingliang City are mainly influenced by rock weathering. Na⁺ not only originates from the dissolution of rock salt but may also come from the dissolution of sulfate or silicate minerals; Mg²⁺ is derived from the combined action of calcite and dolomite; Ca²⁺ is produced not only from the dissolution of dolomite but also from the dissolution of calcite and dolomite. During the rock weathering process, dolomite, calcite, and evaporites dominate the hydrochemical formation of Pingliang karst water. Gibbs diagram combined with mineral saturation index analysis further confirms that the dissolution of evaporites, carbonates, and silicate minerals is the natural source of hydrochemical components in Pingliang City. Ca²⁺, Mg²⁺, and ${\rm{HCO}}_3^{-}$ in the karst groundwater of Pingliang City mainly originate from the combined dissolution of calcite and dolomite. Hydrogeological conditions and hydrogen-oxygen isotope results show that δD values range from -75.45‰ to -64.80‰, with an average of -69.20‰; while δ¹⁸O values range from -11.26‰ to -9.16‰, with an average of -10.07‰, indicating that all groundwater is recharged by atmospheric precipitation infiltration. Based on the identification of the karst water genetic model, it is determined that there are two genetic modes of karst water in the Pingliang area. Mode 1: Atmospheric precipitation infiltrates through the overlying Quaternary loess layer, continuously dissolving sulfate minerals in the loess, and enters the limestone confined aquifer through karst fissures formed by faults, eventually emerging as springs at the interface between the loess and limestone aquifer. In this mode, the chemical type of karst water is mainly HCO₃·SO₄-Na. Mode 2: Atmospheric precipitation infiltrates through the overlying Quaternary sandstone layer, continuously dissolving carbonate minerals in the sandstone layer, and enters the limestone confined aquifer through karst fissures formed by faults, eventually emerging as springs at the interface between the sandstone and limestone aquifer. In this mode, the chemical type of karst water is mainly HCO₃-Ca. Although both modes receive atmospheric precipitation recharge from the front of the mountain and enter the aquifer through karst fissures through the overlying strata, the differences in the overlying strata lead to different hydrochemical characteristics. This indicates that the mineral dissolution in the overlying strata is the main factor in the formation of the hydrochemical composition of Pingliang karst water.