Hydrochemical Evolution of Underground River System in Karst Trough Valley of Northern Guizhou: A Composite Mechanism of Natural Dissolution and Anthropogenic Forcing

ABSTRACT In karst landscapes, trough valleys are key negative topographic units that act as both natural groundwater catchments and preferential pathways for contaminant transport. Their well‐developed vertical karst features, such as sinkholes and fissures, facilitate the rapid entry of surface contaminants into groundwater systems, exerting significant control over flow paths, hydrochemical evolution, and pollutant fate. However, a systematic understanding of how this unique “catchment‐conduit” functionality governs the spatial differentiation of groundwater chemistry, and how anthropogenic pollutants quantitatively interact with and evolve within this natural karst background, remains elusive. This study investigates an underground river system in a typical karst trough valley of northern Guizhou. Using hydrogeological surveys, tracer tests, and hydrochemical sampling, we employed integrated approaches including Piper diagrams, Gibbs diagrams, ion ratios, correlation analysis, and principal component analysis (PCA) to systematically characterize groundwater chemistry, spatial evolution patterns, and controlling factors. A mineral dissolution equilibrium model was further applied to quantify the contribution of anthropogenic nitric and sulfuric acids to carbonate dissolution. The results indicate that: (1) Groundwater is predominantly of the HCO 3 –Ca·Mg type, primarily controlled by carbonate dissolution, which constitutes the regional hydrochemical background. However, a distinct spatial pattern of “significant pollution input—self‐purification recovery—localized recontamination” is observed along the subsurface flow path, a variability driven largely by anthropogenic activities. (2) PCA quantitatively identified three principal controlling factors. The anthropogenic pollution factor (PC1), represented by Na + , NH 4 + , K + , and Cl − , exhibits a contribution rate of 44.718%, surpassing that of the natural dissolution factor (PC2, 17.578%). This confirms that anthropogenic activities have become the primary driver of spatial hydrochemical variations. (3) The mineral dissolution equilibrium model estimated that anthropogenic nitric and sulfuric acids contribute an average of 21.0% to carbonate dissolution, demonstrating that human activities significantly accelerate karst dissolution through the input of acidic substances. This study quantifies anthropogenic acid contribution (~21%, upper limit) to carbonate dissolution and reveals a spatial pattern of pollution input, self‐purification, and recontamination driven by the valley's “catchment conduit” functionality, providing a scientific basis for groundwater protection in similar karst settings. These findings provide a mechanism‐based scientific basis for groundwater protection and pollution control in similar geomorphic settings.