Can you tell me if this difference is significant and if so where I might be going wrong? I would not consider the difference significant. Both numbers indicate calcite is reacting fast enough to maintain equilibrium. I'm not sure how you are doing the hand calculation, but PHREEQC prints the molalities, activities, and log activity coefficient for each aqueous species. You should be able to compare directly to the hand calculation. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 "Garrett Kilroy" <kilroyg@xxxxxxx To: <dlpark@xxxxxxxx> tcd.ie> cc: In-Reply-To: <LPBBLDFBJADNFPNHNHGJCEGGCBAA.kilroyg@xxxxxxxxxxx> Subject: Manual calculations versus phreeqc 07/21/01 10:50 AM Please respond to kilroyg Dear Sir/Madam, I would be very grateful if you could assist me in this query. I am using Phreeqc to calculate calcite saturation indices for my PhD. However, my manual calculations are coming out different to the Phreeqc results. As a test I used data from a hydrogeology textbook (Fetter(2001) applied hydrogeology, p360-1) which calculates calcite saturation manually. The textbook gives a saturation index of 0.05 while Phreeqc gives a saturation index of -0.02. Can you tell me if this difference is significant and if so where I might be going wrong? I would welcome any suggestions. Best regards, Garrett Kilroy PhD Candidate Department of Geology/ Centre for the Environment Trinity College Dublin 2 Ireland Tel: +353 1 608 2403 Fax: +353 1 671 8047
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