· Can you please confirm whether PHREEQC always assumes a 1 liter solution, or if the volume of solution is determined from the density and mass of solution entered. Phreeqc takes the input concentrations and converts them to molality, mol/kg water. Normally, the initial solution, after conversion, contains 1 kg of water (volume is not known). If an amount of water other than 1 kg is specified, it then multiplies by the mass of water specified to geth the total moles of elements, including H and O that are in the specified solution. · Does the mass of water refer to jus the mass of H2O (meaning that 1 liter of any solution would have a mass of 1 kg) or the mass of solution? The mass of water is the mass of water in the solution, normally 1 kg, not counting any solutes. The H and O associated with SO4-2, HCO3-, are also not counted as part of the mass of water. · What information does PHREEQC use to calculate molality? Does PHREEQC calculate density independent of the estimated input value? Phreeqc takes the input concentrations in mg/L. It divides by density (kg solution/L solution) to get mg/kg solution, which is ppm. PHREEQC then sums the mg of solutes in solution and subtracts that from 1 kg of solution to arrive at a mass of water in the solution. The ppm values are converted to moles and divided by the mass of water to get molality and normally, the moles of solute and mass of water are adjusted so that the solution contains 1 kg water (or it is adjusted to give the mass of water specified with -water). David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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