Principles, caveats and improvements in databases for calculating hydrogeochemical reactions in saline waters from 0 to 200 degrees C and 1 to 1000 atm

Databases distributed with PHREEQC may give widely different results for concentrated solutions. Only the database that uses Pitzer's interaction coefficients provides both correct solubilities and mean activity coefficients. The applicability of this database for predicting scaling by mineral precipitation is extended by fitting interaction coefficients for the Na-K-Mg-Ca-Ba-Cl-CO2-HCO3-SO4-H4SiO4 system from isopiestic and solubility data at high temperatures. The pressure dependence of equilibrium constants is calculated from the reaction volume, in which the apparent molar volume of the solutes is derived from density measurements. The apparent volumes are a function of temperature, pressure, and ionic strength, and incorporate complicated changes of the partial molar volumes of the water molecules. Fugacity coefficients for CO2 can be obtained reliably with the Peng-Robinson equation of state for gases. The CO2 ion interaction parameters given by Harvie et al. (1984) for 25 degrees C are also valid for calculating the CO2 solubility at high temperatures, pressures and salinities. PHREEQC input files are available for download, comparing experimental and calculated solubilities of (Na, K, Mg, Ca, Ba) minerals of chlorides, sulfates and carbonates, and of amorphous silica and CO2 in concentrated solutions. (C) 2014 Elsevier Ltd. All rights reserved.