Deriving Major Ion Concentrations at High Resolution from Continuous Electrical Conductivity Measurements in Karst Systems

Time series of hydrochemical parameters support the investigation of dominant karst hydrological processes and conceptual model structures. Nevertheless, high costs for sample collection and analyses cause hydrochemical data to be rarely available at a sufficiently high temporal resolution, e.g. hourly. The electrical conductivity (EC), however, can be cheaply and continuously measured by an EC sensor. To overcome this issue, a method is proposed to derive continuous major ion concentrations from continuous EC using both low-frequency (i.e. weekly) and high-frequency (i.e. every 3 h) ionic measurements. Due to the large ion concentrations and complex speciation characterizing karst springs, the concentrations of each element as free ion and as aqueous complexes are computed separately. The first is computed based on their contributions to the total EC, whereas the concentrations as aqueous complexes are obtained from speciation calculations. The method is tested in two karstic watersheds with different types of bedrock and temporal resolution of the available hydrochemical datasets, i.e. the Kerschbaum dolostone system in Austria and the Baget limestone system in France. The results show that complexation needs to be considered for SO42-, Ca2+, Mg2+ and HCO3-, whose neglection would lead to an underestimation of the total concentrations. The proposed methodology allows us to obtain high-resolution major ion concentrations without performing costly laboratory analyses. The investigation of the proper temporal resolution, required to apply this method to different geologies, can support future fieldwork and sampling campaigns.