ELECTRICAL CONDUCTIVITY: THEORY AND APPLICATIONS FOR NATURAL WATERS

A new method has been developed for calculating the specific conductance of a wide range of natural waters including acid mine waters, geothermal waters, seawater, dilute mountain waters, and river water impacted by municipal waste water. The method presented in this study has significant advantages over existing specific conductance methods by improving the calculations of ionic molal conductivities, accurately accounting for ion pairs, and including more species relevant to natural waters. High-quality electrical conductivity data for numerous electrolytes exist in the literature, but the data do not span the concentration or temperature ranges of many electrolytes in natural waters. Thus, the electrical conductivities of 34 electrolyte solutions pertinent to natural waters ranging from 10^-4 to 1 m in concentration and from 5 to 90 °C have been determined. These data provide a better basis for calculating the ionic molal conductivity for the ions important to natural waters. In addition, the concentrations of ion pairs and complexes were determined using geochemical speciation models. By using the speciated ion concentrations, the method can be used to calculate the specific conductances of natural water samples having a large range of ionic strength (0.0004 – 0.7 m), temperature (0 – 96 °C), pH (1.1 – 9.80), and specific conductance (33 – 70,000 µS/cm). For 1,553 natural water samples, the mean difference between the calculated and measured specific conductances was -0.7% ± 5%. For the wide range of natural waters tested in this study, transport numbers were calculated and the ions that contribute significantly to the specific conductance were identified as H⁺, Na⁺, Ca²⁺, Mg²⁺, NH₄⁺, K⁺, Cl^-, SO₄^2-, HCO₃^-, F^-, Al^3+, Fe²⁺, NO³⁺, and HSO₄^-. Transport numbers can also be used to better predict the concentrations of ions in natural waters. Another important application of the specific conductance method is checking the accuracy of water analyses by coupling charge imbalance and specific conductance imbalance. We artificially adjusted either the major cation or anion concentrations for 50 different water samples and the constituent or measurement most likely in error is easily identified. Considering the importance of accurate chemical analyses, the ability to identify inaccurate determinations is critical.