CORRELATING SPECIFIC CONDUCTIVITY WITH TOTAL HARDNESS IN KARST WATERS

Under field conditions modern digital conductivity meters give standardized, rapid and reproducible measurements. Here we investigate the accuracy of their estimates of  the composition of karst waters, as total hardness (TH, as mg/l CaCO₃) for carbonate rocks, and as Ca²⁺ (meq/l) for sulfates. PHREEQC curves for the dissolution of pure calcite/aragonite, dolomite and gypsum in water at 25°C are compared with >2400 comprehensive analyses from karst studies worldwide. Other principal ions encountered are the “SNC” group (S=sulfate in bicarbonate waters; N=nitrates, C=chlorides, in all waters). In carbonate karsts, 2309 spring, well and stream samples were divided into uncontaminated (SNC<10%), moderately contaminated (10<SNC<20%), and contaminated (SNC>20%) classes. Where SpC<600 mS/cm, a clear statistical distinction can be drawn between waters having little contamination and substantially contaminated waters with SNC>20%: as sometimes claimed in manufacturers’ literature, in “clean” limestone waters TH is close to ½ SpC,  with a standard error of 2-3 mg/l. In the range 250 <SpC<600 mS/cm, dolomite waters can be readily distinguished from limestone waters. Where SpC>600 mS/cm in bicarbonate waters, errors become larger due to common and foreign ions. 140 samples from karsts with gypsum range ~100 to 2700 mS/cm, with Ca²⁺ up to 40 meq/l, and R²=0.90 for their correlation.