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[OLD] Benchmarking, Comparing GWB, PHREEQC, EQ3/6


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From: Yasushi Yoshida

We JNC (Japan Nuclear Cycle development institute) have developed the data format conversion program which enables to use the PHREEQE format database for GWB software. The coding of this conversion program is almost finished, and we would like to verify that database is precisely converted or not. For verification, we tried to do benchmark calculation between GWB and PHREEQE, comparing the calculation results of simple speciation and solubility calculation. In this process, we encountered problems. At first we made inputs of speciation and solubility. Inputs are like below:

 

temperature = 25

swap e- for O2(aq)

1 kg free H2O

Eh = -.2957

pH = 8.5

activity Ca++ = .01

activity Cl- = .01

activity Na+ = .01

activity Fe++ = .01

----------------

<input of solubility>

-----------------------

temperature = 25

swap e- for O2(aq)

swap UO2(am) for U++++

1 kg free H2O

Eh = -.2975

pH = 8.5

free mol UO2(am) = 20

activity Ca++ = .01

activity Cl- = .01

activity Na+ = .01

activity K+ = .01

activity Fe++ = .01

By these input, we obtain outputs of calculation, and I have one question about these inputs. Each input has 4 or 5 element and their activity, and if more element is added in the system, a calculation doesn't converge. Usually thermodynamic database has more than 30 kinds of elements, and each element is related with several other elements as they compose complexes (i.e. element C is distributed as HCO3-, NaCO3+, MgCO3(aq), KCO3-, .... ). If speciation or solubility calculation for GWB software can handle less than 4 or 5 elements, it is difficult for us to verify the converted database by using such calculation way. And I also think our process is not correct for our purpose (speciation and solubility calculation by using GWB software). If we did wrong, what kinds of calculation should we do for benchmarking?

And I have another minor question. I tried calculations shown above under the activity and mg/kg type input. As a result, mg/kg input type converge easier than activity input

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From: Robert C Lee

 

Your solutions look sort of electrically imbalanced - perhaps you should use a solution composition more realistic in its distribution of elements. For any benchmark calculations, I would suggest performing the classic aq. geochem. problems first B seawater speciation, K-spar dissolution, seawater evaporation, etc. A lot has been said about these problems so it should free you of having to figure out the both the conceptual aspects of the models and lower-level details of benchmarking at the same time. FYI: I ran a series of calculations that compared output of PHREEQC, GWB, and EQ3/6 (I did it for both single-point speciation/saturation calculations and reaction-path calculations (it was mostly an exercise for fun ).As you expect, if you use the same thermo database in across the programs, the results are almost identical, particularly for single-point speciation/saturation calculations. However, there are differences in certain scenarios where programs have differing capabilities and/or formalisms in charge balancing, mineral dumping, and predicting precipitations in a multiply-saturated solution, but these are issues the modeler can resolve through different conceptual approaches. As always, it ultimately boils down to the thermo data and its appropriateness.

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