Estimation of mineral saturation and CO2 partial pressure in natural waters using electrical conductivity and pH: A fast and versatile on-site tool

• Estimate mineral saturation using electrical conductivity, temperature, and pH. • Simplify field applications by removing the need for detailed water chemistry. • Support CO 2 equilibrium assessment in karstic environments. • Validated method for karst waters with ion proportions below 20 %. • Statistical equations provide the best results, comparable to PHREEQC calculations. This study presents a methodology for estimating the saturation index (SI) of halite, gypsum, anhydrite, and calcite, as well as the partial pressure of CO 2 (logP CO2 ), using easily measurable water parameters: electrical conductivity, temperature, and pH. Our approach eliminates the need for extensive chemical analyses, allowing an efficient on-site application. PHREEQC was used to generate water composition datasets for single-salt systems (NaCl, CaCO 3 and CaSO 4 ) and mixed-salt systems, and to calculate SI and logP CO2 . Multiple linear regression (MLR) equations were developed to enhance estimations and provide practical expressions. The estimations for the NaCl and CaCO 3 systems have a higher accuracy compared to those for the CaSO 4 system. MLR equations significantly improve the estimation accuracy over theoretical equations, both in single and mixed systems. Validation against experimental and natural karst water datasets from the Rull Cave area (SE Spain) confirmed that the theoretical equations reliably estimate SI and logP CO2 when the ionic contribution from other salts is below 20 %, with optimal accuracy under 10 %. A Microsoft Excel spreadsheet is provided to facilitate the use of both theoretical and MLR equations. Additional applications of the methodology to the KCl, Na 2 SO 4 , MgCl 2 , and Na 2 CO 3 systems are also included in the Supplementary Material. This methodology offers an efficient tool for evaluating water–mineral equilibria (particularly in karst environments) by identifying oversaturation states, assessing CO 2 exchange conditions, and guiding sampling strategies. Finally, our study highlights the key role of electrical conductivity in hydrogeochemical modeling and geochemical assessments in both natural and engineered systems.