Re:

["David L Parkhurst" <dlpark@xxxxxxxx>]

> I have tried to calculate the protons produced and consumed by the moles
of phases
dissolved and precipitated (I have attached an example calculation as a
spreadsheet). By my understanding of the inverse model, (i) mass should
always be conserved (i.e. the number of protons produced and consumed
should
be ~ equal), and (ii) pH in solution definition is only used to speciate
the
initial solutions, and has no bearing on the mass balance calculation
itself. However when I perform these calculations (see attached
spreadsheet).

(i) Unless the pH is the same in the initial and final solution, protons
are not conserved. The program does not have a mass balance on protons, but
it does have mass balance on H and O and a charge balance constraint. Each
of these states: initial + reactions = final. There is also a constraint
that electrons are conserved, which is a similar equation to the other
balances with the stipulation that initial and final are 0. Another
constraint is conservation of alkalinity, which may be what you are
thinking about. There is a section on the alkalinity balance in the manual.
It also states that initial alkalinity + alkalinity from reactants = final
alkalinity. Alkalinity of the reactants is calculated by summing the
alkalinities of the aqueous species in the chemical equation defining the
reactant.

(ii) The pH is used to distribute the species in the initial solution,
however this does affect the mass balance calculations. For a specified
alkalinity, a pH of 12 versus 7 would make a factor of two difference in
the total carbon for example, which would figure into the mole balance on
carbon. Even a difference from pH 2 to pH 3 affecting no other element
concentration would affect the mass balance for alkalinity and for charge.

Hope this helps.

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