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Help needed with NETPATH modelling



Dear Dr Parkhurst,

Kirk Nordstrom suggested I contact you regarding some problems I am 
having doing mass-transfer calculations with NETPATH. Just to give 
you some background, I am a PhD student at the University of Cape 
Town, in the closing stages of writing up my thesis (hoping to be 
done by Christmas). 

My research is on the geochemistry and quality of coal mine waters in 
South Africa. I have sampled over 200 water bodies and done extensive 
analyses on these (not including isotopes). I then sampled some 
borehole cores on the mines to get an idea of the surrounding geology 
and am hoping to be able to use this information to determine the 
processes involved in the water-rock interactions. In other words, I 
would like to be able to say why the water in regions is different to 
others, based on the geology there.

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 ?

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. 

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.

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).

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. 

Kind regards
Bernadette



Bernadette Azzie
c/o Geology Dept.
U.C.T.
Private Bag
Rondebosch
7700
South Africa
E-mail: bazzi@xxxxxxxxxxxxxxxxx
082-576-3602 (Cell)
(021) 650-2912 (O)



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