Using chloride as a conservative tracer to define mixing proportions

[pknappet]

Hi, I have two waters mixing in a riverbank and I want to use chloride concentrations to define the mixing proportions between river water and the groundwater end-member to assess the mass transfers that occurred between phases and the aqueous solution while accounting for this dilution effect. I see that PHREEQC prefers to calculate the mixing proportions on its own, but is there a way to define these. Currently it doesn't like that I have chloride as a solute that is not included in any phases, but that is not why I have chloride in the model.

[dlparkhurst]

Are you using INVERSE_MODELING? If you are, you should include Cl in the -balances section so that PHREEQC will include a mole balance equation. By default, only elements included in the phases are given mole-balance equations, so Cl is not included. Adding -balances; Cl will allow the solutions to mix to account for the Cl, and Cl will be included in the charge-balance part of the calculation.