> 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 precipitate readily. 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 model. > 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 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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