Cost-effective reconstruction of high-frequency ion dynamics in karst groundwater using electrical conductivity and PHREEQC inverse modeling

ABSTRACT High-frequency hydrochemical monitoring of karst aquifers is essential but often constrained by the high cost of direct ion sampling. This study aims to establish an efficient inversion approach that combines continuous electrical conductivity (EC) measurements with PHREEQC-based geochemical modeling to reconstruct high-resolution ion concentration time series. The method was applied to three karst spring sites in the Li River Basin, China, representing contrasting hydrogeological conditions. Results demonstrate that the approach reliably reproduces 30-min concentration dynamics of major ions (Ca2+, Mg2+, HCO3−, etc.) even in low-mineralization groundwater (EC ≈ 280 μS/cm). Compared with linear regression, the EC–PHREEQC method achieved higher accuracy and interpretability. Systematic deviations, such as underestimation of HCO3− and overestimation of SO42−, were mainly associated with site-specific factors, including evaporation and groundwater abstraction. Overall, the method enables cost-effective and continuous ion monitoring, providing a robust framework for improving the characterization and management of karst groundwater systems.