Correlating specific conductivity with total hardness in gypsum karst waters

Abstract Modern digital conductivity meters are readily portable, robust, cheap, and give precisely reproducible values of specific electrical conductivity (SpC, in µS cm −1 ). Here we investigate the accuracy of their estimates of the amounts of gypsum dissolved in waters collected in gypsum karst terrains, expressed as total hardness (TH) in mg L −1 of CaSO 4 ·2H 2 O (GYP). Total dissolved solid concentrations (TDS) are also considered. Curves obtained with the program PHREEQC, for the dissolution of pure gypsum in water at 25 C, are compared with 574 comprehensive water chemical analyses selected from gypsum karst studies in Europe and the Americas. Principal common and foreign ions encountered are the BNC group (bicarbonates, nitrates, chlorides). It is found that GYP = 1·12 · SpC + 62 where BNC < 33% (Cl − < 5%), with one standard error <5% for waters with SpC > 2400 µS cm −1 ; GYP = 0·74 · SpC + 777 where BNC < 33% (5% ≤ Cl − < 15%), with one standard error <10% for waters with SpC > 3100 µS cm −1 ; GYP = 0·97 · SpC − 209 where BNC < 33% and Cl − ≥ 15%, with one standard error <10% for samples with SpC > 4300 µS cm −1 . There are similar results for the more complex waters found in gypsum karsts where much carbonate rock or salt is also present, to the limit of BNC < 50% for what may reasonably be defined as ‘gypsum waters’. Values of R 2 for linear correlations of different subsets of the water samples range from 0·69 to 0·96, the majority being >0·8. Copyright © 2006 John Wiley & Sons, Ltd.