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Dear Dr. Parkhurst,

I am writing regarding the use of PHREEQCi (v 2.4.2) for inverse modelling. I have troubled you before on this matter, and I apologise for bothering you, but I am having a bit of trouble working out what the model has done. I have been inverse modelling water chemistry from a subglacial environment. After running the model, and achieving a number of feasible models, I have chosen a few models to represent different weathering scenarios. 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 find I have a large deficit in H+ production; i.e. far more acidity is supposedly consumed than is produced. In my models, H+ is produced by CO2 dissolution, pyrite oxidation, and the precipitation of ferrihydrite and gibbsite. Much of the H+ consumed appears to form H4SiO4, and if this was to dissolve to release H+, then this could account for much of the required acidity. However, I don?t see how this is a feasible reaction process. I would be really grateful if you could tell me if my understanding of the models is wrong, and if not, why my H+ consumption and production does not balance. Am I missing an obvious H+ source? I look forward to hearing from you.

Many thanks,

Andrew

Andrew Mitchell,
Centre for Glaciology,
University of Wales,
Aberystwyth,
Ceredgion.
SY23 3QW

+44( 0) 1970 625716







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Attachment: Mitchell-problem.xls
Description: MS-Excel spreadsheet



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