Hydrochemical fingerprints of karst underground river systems impacted by urbanization in Guiyang, Southwest China

Karst groundwater plays an irreplaceable role in the formation and development of urban areas, and land-use and land-cover change (LUCC) and the input of pollutants during the urbanization process would pose potential environmental risks to underground rivers. We analysed the relationship between urbanization processes and underground river hydrochemistry over nearly 35 years in Guiyang city, southwest of China, it was found that concentrations of various cations and anions, as well as total dissolved solids (TDS), gradually increased with the urbanization process, with significant fluctuations during the rapid urbanization periods. The Hydrochemical Facies Evolution Diagram (HFED) clearly showed the influence of urbanization on the hydrochemistry of the underground rivers. The ion ratios of γMg²⁺/γCa²⁺—γHCO₃⁻, γNa⁺/γCl⁻, Ca²⁺/Mg²⁺—Ca²⁺ or Mg²⁺/Σ cations, HCO₃⁻/SO₄²⁻—HCO₃⁻ or SO₄²⁻/Σ anions revealed two distinct phases in the hydrochemical evolution of the underground river system, highly consistent with the urbanization process. Before the rapid urbanization, acid deposition and agricultural activities affected the hydrochemistry, with HCO₃-Ca·Mg and HCO₃·SO₄-Ca·Mg as the dominant types controlled by limestone and dolomite dissolution in water-rock interactions. As acid deposition diminished, the input of SO₄²⁻ from urban sewage compensated for the reduced impact, but the increased impermeable surfaces reduced the infiltration of atmospheric precipitation, leading to a reduced dissolution of dolomite minerals in water-rock interactions, resulting in a decrease in Mg²⁺ and a change in the hydrochemical type. The hydrochemical type evolved from a single HCO₃·SO₄-Ca·Mg type and HCO₃-Ca·Mg type to multiple types, such as HCO₃·Cl-Ca, HCO₃·SO₄-Ca, HCO₃-Ca, and HCO₃·SO₄-Ca·Mg, and was highly unstable. With changes in land use, the proportions of various cations and anions in the hydrochemistry changed, especially NH₄⁺, NO₃⁻, SO₄²⁻, Na⁺, and Cl⁻, which were more sensitive to human activities. This study indicated the impact of urbanization on the hydrochemistry of the underground river system, with the input of SO₄²⁻ from human activities and the increase in paved surfaces due to urbanization collectively altering the hydrochemical types of the underground river system. The rapid response of karst underground river system hydrochemistry indicates a potential impact on groundwater system by urbanization that should not be ignored.