Re: Phreeqci questions

["David L Parkhurst" <dlpark@xxxxxxxx>]

> First,  most of the data we are using gives alkalinity in the form of
bicarbonate.  The question that we have is should this data be entered
under the heading of "Alkalinity", "C", or "C(IV)" to produce the best
results.

You should enter the data as "Alkalinity" and add a field to show that it
is as HCO3, default is as CaCO3. I am assuming the units are mg/L. The
input file looks like this:

-units      mg/L
Alkalinity  73    as    HCO3

> We were also wondering if there was a straight forward way to essentially
simulate evaporation on our samples and see what mineral phases precipitate
out and at what concentrations.  We have tried to do this via the steps
outlined in Example 4 (p.209) of the PHREEQC user's guide but were met with
little success, and we're unsure if it was due to differences between the
programs or whether we were not fully grasping the concept of how to do
this type of reaction in PhreeqcI.

I can't tell what problem you were having. Evaporation is accomplished by
removing water. The initial solution normally has 1 kg water after the
initial solution calculation. That is about 55.5 moles of water. If you
want to evaporate 90 percent of the water, you must remove about 50 moles
of water. Example 2 removes 52.73 moles or 95 percent of the water.

For precipitation of solids, you should probably first simulate the
evaporation and then look at the saturation indices for minerals that you
think might precipitate. Minerals that have positive saturation indices
could precipitate.

One further complication is that you probably want to stay in equilibrium
with atmospheric CO2 during the simulations. You need to add an
EQUILIBRIUM_PHASES data block including CO2(g).

Once you decide which minerals you want to let precipitate, add them to the
EQUILIBRIUM_PHASES data block with 0 moles present initially.  PHREEQC will
precipitate them when they reach saturation by evaporation.

David

Any help or suggestions you may be able to offer us would be greatly
appreciated.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