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Best Method in PHREEQC to Recombine Dissolved Gas Measures into the Solution


I have two types of data from one subsurface fluid- 1) A liquid phase ionic analysis and 2) an identical sample of liquid that I heated and measured the dissolved gas that was captured in the headspace. What is the best method to recombine the gas species into the solution at subsurface conditions in PHREEQC? I understand how to do this when a gas has separated out of the liquid phase naturally as it approaches a lower pressure, (using: Solution, Gas_phase, Reaction_temp, pressure, etc.) but I'm not sure how to do this when the produced fluid is fully in the liquid phase and I separately measured the gas to determine the noncondensable gases in the solution. Do I put the gas concentrations as a part of the Solution since they are dissolved measurements?

Well, it depends. For N2, CH4, Ar, H2, and O2 (probably not present) you think you can assume they are inert, and add them back to the SOLUTION (assuming you did not measure them in the "ionic" analysis"), or you could use REACTION or GAS_PHASE, depending on which is easiest. Then too, if they are inert at the time scale of sampling and measurement, they probably do not have much effect on any other PHREEQC calculations, except perhaps GAS_PHASE.

If you simply add back NH3 and CO2, you may be double accounting for N(3) and C(4). As you heated the solution, you may have produced the following reactions:

--- Code: ---HCO3- -> CO2(g) + OH-
NH4+ -> NH3(g) + H+

--- End code ---

and you probably have measured HCO3- (alkalinity) and NH4+ in your ionic analysis. Assuming the pH was not too high or too low in the ionic analysis, you probably do not want to add the gas values to the solution.

The other issue is, did any gas evolve from solution during the sampling process?


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