PHREEQCI is a widely-used geochemical computer program that can be used to calculate chemical speciation and specific conductance of a natural water sample from its chemical composition (Charlton and Parkhurst, 2002; Parkhurst and Appelo, 1999). The specific conductance of a natural water calculated with PHREEQCI (Appelo, 2010) is reliable for pH greater than 4 and temperatures less than 35 °C (McCleskey and others, 2012b). An alternative method for calculating the specific conductance of natural waters is accurate over a large range of ionic strength (0.0004–0.7 mol/kg), pH (1–10), temperature (0–95 °C), and specific conductance (30–70,000 μS/cm) (McCleskey and others, 2012a). PHREEQCI input files for calculating the specific conductance of natural waters using the method described by McCleskey and others (2012a) have been created and are presented in this ScienceBase software release. The input files also incorporate three commonly used temperature compensation factors which can be used to determine the specific conductance at 25 °C: the constant (0.019), the non-linear (ISO-7888), and the temperature compensation factor described by McCleskey (2013) which is the most accurate for acidic waters (pH < 4). The specific conductance imbalance (SCI), which can be used along with charge balance as a quality-control check (McCleskey and others, 2012a), is also calculated: SCI (%) = 100 x (SC25 calculated – SC25 measured) / (SC25 measured) where SC25 calculated is the calculated specific conductance at 25 °C and SC25 measured is the measured specific conductance at 25 °C. Finally, the transport number (t), which is the relative contribution of a given ion to the overall electrical conductivity, for 30 ions is also calculated. Transport numbers are useful for interpreting specific conductance data and identify the ions that substantially contribute to the specific conductance. References Cited Appelo, C. A. J. 2017. Specific conductance: how to calculate, to use, and the pitfalls, [http://www.hydrochemistry.eu/exmpls/sc.html] Ball, J.W., and Nordstrom, D.K., 1991, User's manual for WATEQ4F, with revised thermodynamic data base and test cases for calculating speciation of major, trace, and redox elements in natural waters: U.S. Geological Survey Open-File Report 91-0183, p. 193. Charlton, S.R., and Parkhurst, D.L., 2002, PhreeqcI--A graphical user interface to the geochemical model PHREEQC: U.S. Geological Survey Fact Sheet FS-031-02, 2 p. McCleskey, R.B., Nordstrom, D.K., Ryan, J.N., and Ball, J.W., 2012a, A New Method of Calculating Electrical Conductivity With Applications to Natural Waters: Geochimica et Cosmochimica Acta, v. 77, p. 369-382. [http://www.sciencedirect.com/science/article/pii/S0016703711006181] McCleskey, R.B., Nordstrom, D.K., and Ryan, J.N. 2012b, Comparison of electrical conductivity calculation methods for natural waters. Limnology and Oceanography: Methods, v.10, p 952-967. [http://aslo.org/lomethods/free/2012/0952.html] McCleskey, R.B., 2013, New Method for Electrical Conductivity Temperature Compensation: Environmental Science & Technology, v. 47, p. 9874-9881. [http://dx.doi.org/10.1021/es402188r] Parkhurst, D.L., and Appelo, C.A.J., 1999, User's guide to PHREEQC (Version 2)--a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations: U.S. Geological Survey Water- Resources Investigations Report 99-4259, 312 p.