Charge balances/ speciation

[Laurie Tremblay]

Hi,

 

There is 3 of us newly working with your software on different databases and we can't figure this out!!!! I have been spending 3 days on this so please anwser my questions. I am familiar with other sofware such as PHREEQC, AQUACHEM. I am trying to use Geochemist's workbench. The error on the chemical balance is twice as big in your sofware and I can't figure out why. How does it calculate the major elements or does it calculate the charge balance anyway? I usually enter the bicarbonates as alkalinity for CaCO3 wish is measured in the field in mg/L. All of my units are in mg/L. My elements are analysed under the form total Fe, Ca, Na.... Even if I use the same database wateq4f, it is not giving me the same results and the error is large with geochemist workbench??? Does-it even take in consideration the pe I entered in the input.

 

So my questions are;

 

1) Can you look at my data table and let me know if there is something wrong with my input

2) I received the results of S2- for H2S- under the fiorm of S2- (mg/L so in GWB do I enter it under HS- or S)???

3) Is there units incompatibitlty, should I change all my units in mg/Kg

4) Is there another way to enter data or a better way in a better form.

 

My goal is to make species diagrams and pe-pH diagrams with your software so first of all my analytes need to balance. The redox couples Fe2+, Fe3+, S2-/SO4, NH4, NO3, NO2 and other redox couple are important in speciation calculations.

 

I incuded my GWB sheet can someone tell me what is wrong?

 

 

Please help me find a solution.

 

Laurie Tremblay

Ph.D srudent in Hydrogeology (Institut national de la recherche Scientifique)

Temporarely in UQUAT in Amos Qc. Canada

819-732-8809 poste 8302

Essai_1.gss

[Tom Meuzelaar]

Hi Laurie:

 

Finally getting around to your question. The issue of charge balance calculation differences among the various codes is a significant one that has not been discussed yet on this forum- so I will post it here for reference.

 

There are two very different methods for calculating charge balance, both with an apparently significant following:

 

1. Imbalance calculated relative to the sum of anionic and cationic charge, popular among water engineers:
    
   

   Charge balance = 100*[((Sum cations-Sum anions)/(Sum cations + Sum anions))/2)]

   
   Example reference: Weight, W.D. (2008), Hydrology Field Manual, McGraw Hill, 751 p. (see Chapter 7)
    
   

2. Imbalance calculated as average of anion-cation charge sum:
    
   

   Charge balance = 100*[(Sum cations–Sum anions)/(Sum cations + Sum anions)]

   
   Example reference: Zhu, C. and Anderson, G. (2002), Environmental Applications of Geochemical Modeling, Cambridge University Press, 284 p. (see p. 95)
   

 

I will leave it to other to discuss why this apparent significant discrepancy exists, as I know little about it myself...

 

GWB uses the latter method. I'm not familiar with how other codes calculate this, but it may well be that the other method is used- as you can see, the difference is a factor of two, and you can easily make a correction.

 

The other significant aspect to note is that GWB calculates charge imbalance based on species concentrations after the speciation calculation is performed. If you want to double-check the calculation, simply open the output file, sum the anions and cations and apply formula 2 (above).

 

As far as your input file is concerned, it looks fine. You have chosen to enter your reduced sulfur concentration as total Sulfur. If your laboratory reports S2, you can either enter it as such, or adjust your total Sulfur by a factor of 2.

 

As far as redox is concerned, if you enter a pe value into the GSS spreadsheet, GWB will speciate all coupled redox species according to system O2 values reflected by your pe value, while decoupled redox species do not react with each other, and thus keep their independently constrained values.

 

I hope this helps...

 

Regards,

 

Tom Meuzelaar

RockWare, Inc.