> 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
Please note that some U.S. Geological Survey (USGS) information accessed through this page may be preliminary in nature and presented prior to final review and approval by the Director of the USGS. This information is provided with the understanding that it is not guaranteed to be correct or complete and conclusions drawn from such information are the sole responsibility of the user.
Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.
The URL of this page is:
Last modified: $Date: 2005-09-13 21:04:21 -0600 (Tue, 13 Sep 2005) $
Visitor number 5424 since Jan 22, 1998.