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Re: -ve vs. +ve mole transfers & Closed vs. Open system

>            1). For -ve CO2 mole transfers, should I interpret this
             as CO2 release/respiration ? And, +ve CO2 mole transfers
             as CO2 uptake/dissolution ?

For mole-balance modeling, you are correct. Negative indicates a loss from
the aqueous phase and positive indicates an increase in concentration in
the aqueous phase.

(For forward modeling with equilibrium_phases, negative indicates a
decrease in the amount of the phase and consequent dissolution into the
aqueous phase; positive indicates an increase in the amount of CO2(g) and
degassing from the aqueous phase.)

>            2). In several of the inverse models, I noticed consistent
negative mole
              transfers for gypsum, but I believe gypsum doesn't

You should check the saturation index for gypsum in your solutions. It is
fairly soluble (~15 mmol/kgw) and is commonly undersaturated. If that is
the case, then your model has a problem and you need to consider other
reactions that could account for the change in chemistry (or determine
whether the decrease in sulfate is significant). If saturated or
supersaturated, gypsum will precipitate readily.

>            3). Is an open system assumed for inverse modeling ?

If you include CO2(g) as a reactant (and dissolution is allowed), the
program will react as much CO2 as necessary to achieve a mole-balance

 > i.e., the partial pressure of CO2 is not fixed ?

Mole-balance modeling does not directly take into account saturation
indices and partial pressures of gases; it simply solves for mass balance.
It could as easily be dissolving into a supersaturated as undersaturated
solution. You must make the comparisons with SI and partial pressure to
determine plausibility. The confusing part of your question is that a
system open to CO2 may very well have a fixed partial pressure, as a
solution in contact with the atmosphere. The solution is open to CO2 (that
is CO2 may enter or leave the solution) but it is reasonable to expect the
PCO2 to be fixed at atmospheric pressure.


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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