Dr. Parkhurst, I have a question regarding how PhreeqC calculates alkalinity. I am running a simple speciation calculation on a low pH mine water sample. When I enter the solution composition without indicating a value for alkalinity, Phreeqc calculates a negative alkalinity (-2.551e-3 eq/kg). If I assign a concentration of zero alkalinity, the alkalinity result is the same. If however I specify a nominal amount of alkalinity (0.0001 mg/L as CaCO3), Phreeqc adds a significant amount of total carbon to the solution (5.85 mol/kg). The carbon is essentially all present as H2CO3 with a minor amount of HCO3- (2.431e-3 mol). Is a negative value for alkalinity valid? Conceptually is this equivalent to the amount of acidity associated with the sample? This water quality is to be mixed with other waters to simulate the water quality of a pit lake. Will inclusion of a negative alkalinity make the results invalid? Thank-you for your time, Cheryl Ross PhreeqC input file SOLUTION 6 # SOLUTION surficial run-in to the pit from the escombrera interior Temp 10 pH 2.81 pe 5.1 # assume 300 mV redox pe units mg/kgw density 1 Alkalinity 0.0 as CaCO3 Cl 5.26 S 2000 as SO4 Co 1.60 U 0.0790 as U Ni 2.70 Na 48.0 K 9.8 Ca 283 Mg 194 Al 22 Fe 71.7 Mn 29.4 Cu 0.800 Zn 7.1 END _____________________________ Cheryl Ross, M.Sc. Project Hydrogeochemist Golder Associates 18300 NE Union Hill Road., Suite 200 Redmond, WA 98052 Office: 425-883-0777 (ext. 2208) Fax: 425-882-5498
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