> 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). Alkalinity is, for a first approximation, HCO3- - H+ - HSO4-. The HCO3- is negligible at pH 3, and HSO4- is probably small relative to H+ unless SO4 concentrations are really large, so the negative alkalinity is primarily due to H+ ion. > If I assign a concentration of zero alkalinity, the alkalinity result is the same. The program ignores concentrations of zero. > 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). You need to think about what you are requiring. You are fixing the pH at about 3, but you are requiring HCO3- - H+ to be about 2e-6 (.0001/50) or essentially HCO3- = H+. At pH 3, the ratio of HCO3- to CO2 is less than 1e-3 (HCO3- + H+ -> CO2 + H2O, which has log K of -6.3, and do a little math for pH 3), so you need moles of carbon, most of which is CO2, to have a millimole of HCO3-. > Is a negative value for alkalinity valid? Yes. However, PHREEQC does not allow defining negative alkalinity, it simply does not have this feature. > Conceptually is this equivalent to the amount of acidity associated with the sample? Not quite. Acidity is the amount of base consumed in titrating a sample to pH 8.3 (usually), so CO2(aq) would be titrated in addition to H+ and HSO4-; Al, Fe, and anything else that has pK less than 8.3 will also get into the act in an acidity titration. > 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? No. It should work correctly. You should consider how to handle carbon. For the initial solution: SOLUTION C 1 CO2(g) -3.5 ... will add enough CO2 to be in equilibrium with the atmosphere; if this makes geochemical sense. For the mixture you need to add an EQUILIBRIUM_PHASES data block MIX ... EQUILIBRIUM_PHASES CO2(g) -3.5 ... to add CO2 to atmospheric equilibrium. 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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