Is the Langelier Index (LSI = pH - pHs) too simplistic for determining calcium carbonate precipitation, when the water system also contains iron precipitation which will tend to drive down pH, thereby using available alkalinity for buffering (and not calcium precipitation)?
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From: David L Parkhurst [mailto:dlpark@xxxxxxxx]
Sent: Tuesday, July 02, 2002 12:27 PM
To: Richard Kane
Subject: RE: Phreeqc simulations
> I've run a number of simulations using exact monitoring point data for
the major cations, along with alkalinity and sulfate (the only significant
anion data I have - nitrate, phosphate are miniscule). Regardless of the
data set, I'm always short in the anions department for the charge balance.
You need to get a better analysis.
> So, I decided to use total carbon as C(4) with a charge balance instead
of alkalinity for an input, because I can think of no other way of
balancing the solution. I converted alkalinity as CaCO3 (430 ppm) to HCO3-
(525 ppm) (assuming the 2eq/mole ratio thereby getting a higher
concentration of HCO3-), and added it to the field measured CO2 (560 ppm)
to get an approximate total dissolved carbonate species value of 1085 ppm.
I used this value for C(4).
This assumption is driving the results of the calculation. If there is not
as much alkalinity as you are defining, no calcite will precipitate at all.
The iron will oxidize and generate low pH water that is undersaturated with
calcite. My guess is that the alkalinity is not high enough to buffer the
pH if there are large iron conentrations. With these high iron waters,
alkalinity should be done in the field, by precipitating iron on th way to
the lab, the lab-measured alkalinity values would be erroneous. For
landfill leachate, some of the missing anions may be organic acids, so a
titration to a very low pH would be a good idea to see if there are any
organic acids that are titrated in the pH 2-4 range.
> The solution is then allowed to react with the specified mineral
assemblage, which includes keeping CO2(g) and O2(g) near atmospheric. The
model is predicting 3.032e-003 moles of calcite, 1.880e-003 moles of
goethite, and 3.458e-004 moles of manganite will precipitate at equilibrium
conditions (all have SI=0.0). The pH shifted from 6 to 8.8 and the pe from
0 to 11.8. Accordingly, with this shift, the dominant carbon species have
changed from CO2 and HCO3- to HCO3- and CO3-2. Other minerals are
oversat'd w.r.t their components in solution, but due to the fact that they
are unreactive (slow formation kinetics), they are not expected to
> So, with that said, my major uncertainty remains. If my main concern is
to see whether or not enough carbonate is present in the pH>7 / aerobic
trench waters (modeled by designating the CO2(g) and O2(g) is the phase
assemblage) to precipitate all the Ca2+, then is it inaccurate to have the
charge balance on the C(4)?
I think so.
> In this way, it doesn't matter what initial C(4) you enter, 1 or
1,000,000, PHREEQ will always adjust this initial concentration upward, to
come into balance with the cations. So, this elevated C(4) concentration
is carried into EQUILIBRIUM PHASES, where it is allowed to react with the
phase assemblage. If the model is always elevating the C(4), won't I
always have all the calcium precipitating out?
Yes, your assumption is producing the alkalinity that you need. Run the
calculation with the original alkalinity (236?) and look at the results.
The key is whether there really is enough buffering capacity to buffer the
acid generated by iron oxidation and precipitation; you need better
analytics or a backup (lime?) to make sure you don't generate more acid
than can be buffered.
> And if so, is this correct? I feel as though I'm not hitting the mark on
what I'm trying to accomplish.
I don't think you can answer your question without knowing the alkalinity
and the missing anions with more certainty.
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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