Hydrogeochemical Characteristics and Evolution Processes of Multilayer Karst Aquifer in the Huayingshan Coalfield, Southwest China

Abstract

The Huayingshan coalfield is one of the most important coal districts in Southwest China. Coal mining may have an impact on the hydrochemical characteristics and regional evolution of karst groundwater. This study aims to analyze the hydrogeochemical characteristics, identify the evolution processes, and influencing factors that govern the hydrochemistry in multilayer karst aquifers in the coalfield. Statistical methods and conventional techniques were utilized to gain a thorough understanding of the origin and hydrogeochemical evolution of karst groundwater. The results revealed that the groundwater was fresh water and natural to mildly alkaline. It suggested that the relative abundance of main ions was proposed to be Ca²⁺ \( \gg \) Mg²⁺ > K⁺ + Na⁺ for cations and \({\text{HCO}}_{3}^{ - }\) \( \gg \) \({\text{SO}}_{4}^{{2 - }}\) > Cl^– > \({\text{NO}}_{3}^{ - }\) for anions. A Piper diagram of the investigated water samples demonstrated that most groundwater was of the HCO₃-Ca type. The results showed that dissolution of carbonate, gypsum, halite, and silicate minerals highly influenced the formation of \({\text{HCO}}_{3}^{ - }\), \({\text{SO}}_{4}^{{2 - }}\), Ca²⁺, Mg²⁺, and Na⁺. Cation exchange and/or absorption was another important regulatory process. \({\text{NO}}_{3}^{ - }\) concentrations were excessively high, proving that karst water was affected by agricultural activities in certain aquifers. Moreover, S^2– concentrations were high in the borehole and mine tunnel samples, suggesting great acidification potential. Coal mining carries a risk of deteriorating the local water environment. This exposes sulfide minerals to oxygen and water, increases \({\text{SO}}_{4}^{{2 - }}\) concentration, and reduces groundwater pH. Scientific research must focus on specific recharge area locations, runoff and drainage pathways, and hydrochemical evolution processes of karst water, and the contact of sulfide with water and oxygen must be controlled to protect groundwater quality and reduce pollution. The results suggest it may be helpful for investigation and treatment of water environment pollution, aid the protection of karst groundwater in the Huayingshan coalfield, and serve as a model for other comparable studies.