[no subject]

["Andrew Mitchell" <andrew_mitche78@xxxxxxxxxxx>]

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