, Available online , doi: 10.11932/karst2026y006
Abstract:
The southwestern Hunan region is a significant coal resource accumulation zone in China. Within its coal mining areas, carbonate rocks and coal-bearing clastic rock strata are distributed in an interbedded manner, resulting in complex geological structures. Hydrogeochemical processes are jointly influenced by mining activities and karstification. This study focuses on a typical small watershed in southwestern Hunan, with a total area of approximately 20.51 km2. The landform is predominantly characterized by dissolution-tectonic features of low mountains and wide valleys, and denudation-tectonic features of clastic rock hills and valleys. The regional strata are primarily composed of the Quaternary System of the Cenozoic Erathem and the Carboniferous System of the Upper Paleozoic Erathem. Groundwater types mainly include pore water in loose rocks, fissure-karst water in carbonate rocks, and pore-fissure water in clastic rocks. To systematically reveal the hydrochemical characteristics and dominant controlling factors of surface water and groundwater in this typical coal mining area of southwestern Hunan, 16 surface water and 13 groundwater samples (including 4 mine water samples) were collected. A comprehensive multi-indicator analysis was conducted using mathematical statistical analysis, Piper trilinear diagrams, Gibbs diagrams, and ion ratio methods to investigate the hydrochemical composition, spatial distribution patterns, and formation mechanisms of various water bodies in the region.The results indicate that both surface water and groundwater in the study area are generally weakly alkaline, with pH values ranging from 7.06 to 8.33 and TDS values between 236 and 884 mg·L−1. Groundwater is minimally affected by mine water, with its hydrochemical types predominantly being HCO3·SO4-Ca, followed by HCO3-Ca. For surface water, the major hydrochemical indicators (${\rm{SO}}_4^{2-}$, Ca2+, Mn, TDS, and toxic metals) exhibited the spatial distribution pattern: Shiqiao Creek (North Branch) > Shiyan Creek (Downstream) > Shijing Creek (South Branch). In contrast, the trends for pH and ${\rm{HCO}}_3^{-}$ concentration were the opposite. The North Branch (Shiqiao Creek), influenced by mine water input, has a SO4-Ca hydrochemical type. The South Branch (Shijing Creek), less affected by mining activities, is primarily of the HCO3·SO4-Ca type. After their confluence, the main stream of Shiyan Creek generally exhibits a SO4-Ca hydrochemical type.Gibbs diagrams show that most samples from the study area plot within the rock weathering dominance field, indicating that the chemical composition of the water bodies is primarily controlled by mineral dissolution within the aquifers, with relatively weaker influences from evaporation concentration and atmospheric precipitation. However, most mine water samples deviate from the model's distribution range, and their spatial heterogeneity may originate from geochemical disturbances caused by historical coal mining activities. The end-member diagram illustrating the relative contributions of rock weathering and dissolution shows that regional water samples are concentrated towards the carbonate rock end-member, with some samples trending towards the silicate rock end-member. Ion ratio analysis indicates that Ca2+, Mg2+, and ${\rm{HCO}}_3^{-}$ are mainly derived from the dissolution of carbonate minerals. ${\rm{SO}}_4^{2-}$ primarily originates from the dissolution of gypsum interbeds within the Zimenqiao Formation limestone and the oxidation of pyrite in the coal-bearing strata. Cl− and ${\rm{NO}}_3^{-}$ are mainly attributed to inputs from human activities such as domestic sewage and agricultural fertilization. Mine water samples exhibit significantly higher concentrations of Fe and Mn and are locally acidic, indicating that historical mining disturbances have enhanced sulfide oxidation, creating an acidic environment that promotes the dissolution and migration of metallic elements. In surface water, the concentrations of ions like ${\rm{SO}}_4^{2-}$, Ca2+, and TDS show a positive correlation with the intensity of mine water input, reflecting the significant impact of mining activities on surface water chemistry.Principal Component Analysis results reveal that groundwater chemical composition is primarily controlled by three factors: PC1, with a variance contribution of 39.65%, reflects the dominant role of carbonate rock dissolution on regional groundwater chemistry; PC2, with a variance contribution of 36.08%, represents the influence of Cl− and Na+ inputs from human activities; PC3, with a variance contribution of 11.35%, signifies the effect of gypsum dissolution. Surface water chemistry is mainly governed by three factors: PC1, with a variance contribution of 54.28%, characterizes the dual influence of gypsum dissolution and coal seam sulfide oxidation on surface water chemistry; PC2, with a variance contribution of 24.69%, embodies the coupled effects of silicate weathering from the Carboniferous Ceshui Formation and anthropogenic NaCl input; PC3, with a variance contribution of 11.80%, reflects ${\rm{NO}}_3^{-}$ input from agricultural fertilization.Integrating the multi-indicator analysis results, rock weathering/dissolution and human activities are identified as the two main factors controlling the hydrochemical evolution in the study area. Specifically, carbonate rock dissolution governs the variations of Ca2+, Mg2+, and ${\rm{HCO}}_3^{-}$; sulfide oxidation and gypsum dissolution control ${\rm{SO}}_4^{2-}$ enrichment; and human activities primarily influence the distribution of Cl− and ${\rm{NO}}_3^{-}$. This study systematically reveals the hydrochemical characteristics and formation mechanisms of surface water and groundwater in a typical coal mining area of southwestern Hunan, clarifying the hydrochemical relationships and spatial distribution patterns within the "surface water-groundwater-mine water" ternary system. The results demonstrate that the coupled effects of carbonate rock weathering/dissolution and sulfide oxidation from coal-bearing strata jointly determine the chemical evolution of regional water bodies, while human activities have intensified ion migration in localized areas. The comprehensive multi-indicator analytical methodology proposed in this study can provide a scientific basis and methodological reference for identifying hydrochemical characteristics, preventing and controlling pollution, and protecting regional water resources in karst coal mining areas.
