Study on Hydrochemical Evolution Mechanism of Groundwater Driven by Small-scale Mining in Southwest Karst Areas: A Case Study of a Lead-Zinc Sulfide Mine in Huanjiang County, Guangxi

Southwest China hosts extensive karst carbonate sequences, whose groundwater constitutes an indispensable resource sustaining regional ecological security and socioeconomic water supply. However, the highly heterogeneous hydrogeological framework and intimate hydraulic connectivity between surface water and groundwater render karst aquifers extremely vulnerable to anthropogenic mining disturbances. Small-scale lead-zinc sulfide mines are widespread across Guangxi's karst terrains, typically featuring low access thresholds, rudimentary mining techniques and inadequate environmental protection infrastructure. Sustained sulfide oxidation generates acid mine drainage (AMD), which triggers dramatic shifts in groundwater hydrochemistry. Yet systematic long-term comparative investigations into small karst polymetallic mines remain scarce, largely constrained by the lack of pre-mining historical monitoring datasets. This study takes a lead-zinc sulfide mine in Huanjiang County, northern Guangxi, as a representative case, integrating archived hydrochemical records from 1982 (pre-mining period) and field sampling data collected in 2025 (long-term mining stage). A total of 32 water samples encompassing mine drainage, surface streams, borehole groundwater and karst springs were collected across wet and dry seasons. Hydrochemical trilinear plotting, characteristic ion ratio discrimination and principal component analysis (PCA) were comprehensively deployed to quantitatively elucidate the spatiotemporal evolution patterns, coupled geochemical mechanisms and dual ion sources of karst groundwater under prolonged small-scale mining perturbation. The results reveal that long-term underground exploitation and massive groundwater dewatering have fundamentally transformed the original hydrochemical facies of groundwater. Natural undisturbed karst groundwater is predominantly HCO₃-Ca type, whereas post-mining groundwater evolves into SO₄-Ca·Mg type, with total dissolved solids (TDS) surging by nearly an order of magnitude and sulfate concentrations rising drastically. This directional hydrochemical transition is governed by the coupled geochemical feedback between sulfide oxidation and carbonate dissolution. Oxidation of pyrite and other sulfide minerals releases substantial H⁺ and ${\rm{SO}}_4^{2-}$; the resultant hydrogen ions further accelerate incongruent dissolution of calcite and dolomite, concurrently elevating aqueous Ca²⁺ and Mg²⁺ concentrations within aquifers. Pronounced spatial differentiation of hydrochemical facies exists along gradients of mining influence: SO₄-Ca·Mg water occurs within the footprint of groundwater depression cones, transitional HCO₃·SO₄-Ca water extends along major karst runoff channels, and undisturbed HCO₃-Ca water persists only in peripheral zones isolated from mining impacts. Distinct seasonal disparities in hydrochemical signatures emerge between wet and dry periods. Dilution dominates water-rock interactions during the wet season, attenuating pollution signals induced by mining; by contrast, retarded groundwater circulation during the dry season prolongs water-rock contact time, facilitating continuous accumulation of sulfate and base cations and amplifying typical AMD geochemical fingerprints. This study innovatively establishes a vertical hydrogeochemical zoning conceptual model of “shallow acidification − deep buffering” driven by mining-induced hydrodynamic field reorganization. Large-scale groundwater dewatering lowers the water table by over 136 m, exposing sulfide orebodies to the aerated zone and generating shallow acidified groundwater characterized by low pH and high ${\rm{SO}}_4^{2-}$. Acidic infiltrates migrate downward into deep saturated carbonate aquifers, where carbonate minerals neutralize hydrogen ions and yield high-alkalinity, high-hardness buffered groundwater with concurrently elevated ${\rm{SO}}_4^{2-}$ and ${\rm{HCO}}_3^{-}$. Horizontal hydrochemical gradients are interpreted as planar projections of vertical acidification-buffering processes modulated by depression cone geometry and karst conduit distribution. Notably, divergent pH evolutionary trajectories between surface water and groundwater before and after mining are identified and proposed as a novel diagnostic indicator for tracing mining disturbances. Prior to exploitation, surface water exhibited weak acidity while groundwater maintained neutral to weakly alkaline conditions buffered by carbonate dissolution. Post-mining, surface water pH shifts toward neutrality and weak alkalinity owing to direct discharge of high-alkalinity deep buffered groundwater via flood drainage tunnels, whereas shallow groundwater pH declines to weak acidity within the core mining zone. PCA further quantifies the dual regulatory mechanisms of natural geological background and superimposed anthropogenic mining inputs. In the wet season, the primary principal component encapsulates combined water-rock interactions of carbonate dissolution and sulfide oxidation, while the secondary component reflects miscellaneous anthropogenic inputs including mine explosive residues and domestic wastewater. In the dry season, mining activities emerge as the dominant factor shaping hydrochemical composition, with secondary factors corresponding to evaporite dissolution and evaporative concentration effects. The conservative ion Cl⁻ exhibits negligible spatial variability, corroborating that water-rock interactions rather than simple hydraulic mixing govern overall hydrochemical evolution. This research fills the gap in long-sequence comparative research on small karst lead-zinc mines, and delivers an original vertical zoning model and pH diagnostic indicator for AMD disturbance identification. The differentiated control strategies proposed herein, targeting shallow acidified source zones and deep high-mineralization runoff zones, provide robust theoretical support for groundwater environmental impact assessment, pollution source apportionment and targeted pollution abatement in analogous small sulfide mines across karst regions of Southwest China.