Re: Help needed with NETPATH modelling

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

> My first problem is that when I enter my data into DB and try to
speciate it, I get an error message on all those waters with a low pH
(i.e pH < 4.2). I have not measured any alkalinity in waters with a
pH below 4.3 and therefore have enterred my alkalinity value as
<0.01. Could this be the problem ?  Is there some way of enterring
acidity to compensate for this ? Perhaps a negative alkalinity (to
indicate that it is consumed in the reaction) ?  Or should I just add
a miniscule amount as HCO3, so that it has some value to work with ?

NETPATH does not like negative alkalinity. To make it work, try changing
the input to total inorganic carbon instead of alkalinity.

> Secondly, the .dat file which I have does not offer some of the more
important iron and aluminium minerals as a possible phase. The
minerals which are not available, and that I need include
ferrihydrite, jarosite, jurbanite, melanterite and schwertmannite. I
have tried to enter these into the database but am not certain I have
done it correctly ....... largely because I have no idea what "I1",
"I2" and "I3" are, not to mention my uncertainty regarding the
calculation of a redox state for these.

Goethite works just as well as ferrihydrite for mole balance modeling. The
stoichiometry of the mineral is all that is important. Additional water in
the formual also makes no difference. I1, I2, and I3 are isotope values for
C13 (permil), C14 (pmc), S34 (permil).

> Do you have an updated .dat file ? The one I downloaded from the web
is dated August 1997. Alternatively some advice on how to enter these
minerals would be greatly appeciated.

You probably have the latest version, NETPATH has not been modified in a
long time. You can enter more phases in NETPATH with Add on the main screen
followed by Phases, followed by a number as listed on the next screen to
create a new phase.

> Thirdly, I find that if I use an initial (rain water) and a final
(mature mine water) water with no iron or aluminium (eg. pH ~7
waters), but include phases such as pyrite, goethite, gibbsite, etc,
a mass balance transfer still occurs using these minerals. For
example, all the pyrite converts to goethite but I can't figure out
where the S goes. ALL into solution ?  Similarly with the aluminium
.... I can account for most of it: K-spar + kaolinite ~= albite. Is
the rest in solution ? I do not want to leave the iron and aluminium
phases out, as I know they do exist (XRD traces are evidence).

You need to look at the mole transfers and the concentration in solution.
The amount of S in the final solution should equal the amount of S in the
initial solution plus and minus the transfers from and to the minerals.
NETPATH does not automatically perform mole balance on elements, you must
specify S as one of the constraints. You also need to specify Redox as a
constraint to account for the redox reactions correctly.

> Perhaps I should be using PHREEQC (which I used to speciate my
waters and calculate SI, but know very little else about) ?  I would
greatly appreciate any help you can offer.

If you are not using isotopic data, you could use the interactive program
PhreeqcI. The latest version is a complete interface to PHREEQC that runs
under Windows https://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc). You
need to define the beginning and ending solutions with SOLUTION data blocks
(flask icon in the interface) and the details of inverse modeling in the
INVERSE_MODELING (f-1 icon in interface). Mole balance on all elements
included in the phases selected will automatically be enforced. Any
conservative elements (not contained in any minerals) will need to be
included through the Balances tab. PhreeqcI also enforces charge balance in
its calculations by adjusting element concentrations within specified
limits. The uncertainties also affect the inverse models that can be found.

Good luck,

David




David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225