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

Karst water is a significant water resource 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. Such studies provide a scientific basis and theoretical support for promoting the sustainable development of regional water resources and maintaining 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 delineate the extent and dynamic changes within the karst water system.

To uncover the genesis mechanism of karst water in Pingliang City, we conducted a comprehensive analysis of its hydrochemical characteristics and sources using methods such as Piper trilinear diagrams, mathematical modeling, and ion ratio coefficients. The results indicate that the groundwater in the region can be mainly classified into two types: HCO₃-Ca and HCO₃·SO₄-Na, with Ca²⁺ and Na⁺ as the dominant cations and \rmHCO_3^- as the primary anion. The pH value ranges from 6.72 to 6.90, exhibiting overall weakly alkaline hydrochemical characteristics. Total Dissolved Solids (TDS) concentrations in the groundwater range from 158.01 mg·L⁻¹ to 1,519.40 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⁻, \rmSO_4^2- and \rmHCO_3^-. Additionally, the content of Mg²⁺ is significantly positively correlated with the contents of the main anions Cl⁻, \rmSO_4^2- and \rmHCO_3^- (r > 0.70, p < 0.01), and the content of Na⁺ also shows a significant positive correlation with the contents of Cl⁻ and \rmSO_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 karst water in Pingliang City. The Gibbs diagram combined with the analysis of mineral saturation indices further confirms that the dissolution of evaporites, carbonates, and silicate minerals is the natural source of hydrochemical components in Pingliang City. Ca²⁺, Mg²⁺ and \rmHCO_3^- in the karst groundwater of Pingliang City mainly originate from the combined dissolution of calcite and dolomite. Hydrogeological conditions and hydrogen-oxygen isotope analyses 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‰, averaging -10.07‰. These results indicate that all groundwater is recharged by infiltration of atmospheric precipitation.Based on the identification of the gensis model for karst water, it has been determined that there are two genesis modes of karst water in the Pingliang region. Mode 1: Atmospheric precipitation infiltrates through the overlying Quaternary loess layer, continuously dissolving sulfate minerals within 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 aquifers. In this mode, the chemical composition of karst water is mainly HCO₃-Ca. Although both modes receive atmospheric precipitation recharge from the windward side of the mountain and enter the aquifer through karst fissures in the overlying strata, differences in these strata result in distinct hydrochemical characteristics. This indicates that the mineral dissolution in the overlying strata is the main factor influencing the hydrochemical composition of karst water in Pingliang.