SIc–Abacus: An in–situ tool for estimating SIc and Pco2 in the context of carbonate karst

This article aims to provide an on the spot, handy field tool that can be used in estimating the Saturation Index with respect to calcite in karst aquifers in a limestone context. It relies on an abacus that gives the SIc by using measured pH and bicarbonate concentration as coordinates. The analytical expressions of the carbonate equilibrium equations are derived in order to calculate SIc from values that can be measured on field. Using these established relationships, we propose an Abacus built from the SIc=f(Pco2eq) reference frame, hence called SIc–Abacus. Accuracy of SIc calculation is also discussed. SIc is calculated with values measured considering uncertainty coming from pH (±0.05 pH units), temperature (±0.1 °C), bicarbonate and calcium concentrations (±2.5 mg/L and ±2 mg/L). This gives, for each sample, a range of possible SIc values of ±0.060 pH units (for a bicarbonate concentration close to 300 mg/L). The effect on SIc range of possible values given by bicarbonate and calcium uncertainties is about 10 times lower than the effect brought by pH uncertainty. Also, bicarbonate and calcium concentration can be estimated from electrical conductivity leading to a possible range of SIc value of ±0.087 pH units. In both cases, the range of possible values of SIc varies according to the bicarbonate concentration. Using Cussac and Lascaux sites, an evaluation was done. Results show the goodness of fit (R2=0.96) between the calculated SIc from measured pH, bicarbonate and calcium concentration and the estimated SIc values generated from the SIc–Abacus. Apart from calculating SIc, another method which is an alternative use of the generated SIc equation focused on obtaining pH using parameters such as electrical conductivity and dissolved CO2 measurements is presented. The study shows that SIc–Abacus can be a new in–situ tool for karst aquifer surveys. It can facilitate in making a swift decision that tackles carbonate karst issues (e.g. when selecting sites or in deciding if extensive monitoring should be performed or not based from a specific objective).