Groundwater hydrogeochemical formation and evolution in a karst aquifer system affected by anthropogenic impacts

Karst groundwater, an important water source, is often highly influenced by human impacts, causing environmental damage and threats to human health. However, studies on the anthropogenic influences on the hydrogeochemical evolution of karst groundwater are relatively rare. To assess hydrogeochemical formation and evolution, we focused on a typical karst groundwater system (Jinan, China) which is composed of cold groundwater (av. temperature 13-17 °C), springs and geothermal water (av. temperature > 30 °C) and is significantly affected by human activities. The study was performed by means of water samples collecting and analyzing and isotope analysis (²H, ¹⁸O and ¹⁴C). The statistical analysis and inverse models were also applied to further understand geochemical processes and anthropogenic influences. The ²H, ¹⁸O and ¹⁴C results indicate that the cold karst groundwater is easily influenced and contaminated by the local environment, while geothermal water is relatively old with a slow rate of recharge. The hydrochemical types of cold karst groundwater are mainly HCO₃-Ca and HCO₃·SO₄-Ca, while geothermal water hydrochemical types are SO₄-Ca·Na and SO₄-Ca. Groundwater Ca²⁺, Mg²⁺, HCO₃^- and SO₄^2- are mainly controlled by carbonate equilibrium, gypsum dissolution and dedolomitization. Groundwater Na⁺, K⁺ and Cl^- are mainly derived from halite dissolution, and in geothermal water, they are also affected by incongruent dissolution of albite and K-feldspar. Anthropogenic nitrogen produces ammonium resulting in nitrification and reduction in CO₂(g) consumption and HCO₃^- release from carbonate dissolution. Principal component analysis and inverse models also indicate that nitrification and denitrification have significantly affected water-rock interactions. Our study suggests that karst groundwater quality is dominated by water-rock interactions and elucidates the influence of anthropogenic nitrogen. We believe that this paper will be a good reference point to study anthropogenic influences on the groundwater environment and to protect karst groundwater globally.