[Temporal Response of Subterranean Karst Stream Hydrochemistry to Urbanization]

The hydrochemical responses of underground rivers to urbanization were studied using a 25-year groundwater observation dataset and remote sensing. We found that as urbanization progresses, the mineralization degree of underground rivers gradually increases; time-series data for dominant hydrochemical indicators changed from HCO₃·SO₄-Ca·Mg during the dry season and HCO₃-Ca·Mg during flood season to HCO₃·Cl-Ca, HCO₃·SO₄-Ca, HCO₃-Ca, and HCO₃·SO₄-Ca·Mg. Influenced by surface precipitation input, the groundwater chemistry of underground rivers varies greatly during the dry season and the flood season. Prior to urbanization,[Mg²⁺]/[Ca²⁺] and[HCO₃^-]/[SO₄^2-] molar ratios are affected by water-rock interactions, agricultural activities, and acid rain infiltration, the average values of which were 0.86 and 29.34, respectively. After urbanization, agricultural activities and the contribution from acid rain decreased gradually. During the periods 1990-1995, 1996-2010, and 2011-2015, the main sensitive geochemical cations were Ca²⁺, Mg²⁺, Na⁺, and NH₄⁺, and the main anions were HCO₃^-, HCO₃^-, and SO₄^2-, and Cl^-. The hydrochemical response of underground rivers to urbanization was characterized by clear temporal phases.