Problems defining the parameters for kinetic calcite dissolution

[ACSchubert]

Hi All,

I am pretty new to PhreeqC and I hope that I did not miss the answer to my question in another thread in this forum.
What I would like to do, is to model the kinetic dissolution of calcite together with transport and I have troubles understanding the parameters that have to be definded, especially Parm(1).
The version of PhreeqC I'm using is 2.18.00 and I am using the Wateq4f database.
The database states that parm(1) = Area/Volume, cm^2/L (or cm^2 per cell). So what does this mean exactly? Do I have to determine the area of the calcite that one liter of water comes in contact with? Or does it refer to the cell length definded in the transport-block, so the area of calcite per cell (of in this case one meter length)?

Any help is very much appreciated!

Best greetings,
Agnes

[dlparkhurst]

You should use version 3.

For 2.18, you want the area in cm^2 per liter of water, and normally, you use SOLUTION definitions with 1 kgw (~1 L).

Note, for version 3, parm(1) is different. It is cm^2 per mol calcite.

[ACSchubert]

Dear Mr. Parkhurst,

thank you a lot for your quick answer! I will try to stick with version 2.18 for now, because I'm a bit afraid that, when changing versions mid-work, I might have to redo the models of which I am now using the results for the calcite dissolution model. So cm^2 per liter of water it is!

Thanks a lot!
Agnes

[ACSchubert]

Dear Mr. Parkhurst,

I have one more follow-up question:
Parm(1) is now in the range of 500 cm2 per liter water. This results in Ca-concentrations of 91 mg/L after only 36 seconds of reaction time. It seems very high to me. Maybe I misunderstood you here, but: Should I normalize the area (in cm2) by 1 liter oder by 1000 cm3, so that the units match up?

Best greetings,
Agnes

[dlparkhurst]

I believe the units are cm^2/L = cm^2/dm^3, but you will have to check. If you are using the default definition, comments are in the RATES block of the database. Calcite rates are reasonably fast, on the order of hours to days for equilibrium.

There are several discussions of calcite rates in the forum, for example an extensive one at http://phreeqcusers.org/index.php/topic,1131.msg3392.html#msg3392