Hydrochemical-thermal coupling based on temperature field measurements in a karst spring

LBC Spring is a representative karst groundwater discharge system in southwestern China, characterized by elevated spring temperatures. In this paper, hydrochemical analyses and high-density temperature–depth measurements were combined to investigate the coupling between groundwater chemistry and thermal behavior in a karst geothermal system. Hydrochemical results indicate that groundwater is mainly of Ca·Mg–HCO3 and Ca·Mg–SO4 types, controlled by carbonate rock weathering. As the main regional discharge point, LBC Spring gathers water from different depths, and the chemical characteristics of its outlets show significant differences. Compared with the main thermal outlets, the cold-water outlet exhibits higher Ca2+ and lower Mg2+ concentrations, suggesting shallower circulation and shorter water–rock interaction. Gibbs diagram analysis shows that cold groundwater is dominated by rock weathering, whereas thermal groundwater is influenced by both rock weathering and evaporation–crystallization processes. Temperature–depth measurements identify multiple spring vents with stable temperature profiles under rainfall conditions, with thermal outlets exhibiting higher and stable temperatures, and cold-water outlets showing lower temperatures. The SiO2 geothermometer estimates a reservoir temperature of 45.34 °C. Mixing calculations indicate that LBC Spring represents a mixture of deep thermal water and shallow cold groundwater, with a mixing ratio of 28.1:71.9 and a circulation depth of 981–1174 m. The coupled hydrochemical–thermal analysis provides robust constraints on groundwater circulation and geothermal potential in karst systems.