X1T heterogeneity problem

[AkuI]

Hi

 

I have two problems with my model file. I am trying to model 1d column model where there is 50ml water in both ends of the column and clay in the middle. The diffusion problem works fine. The clay in the middle has it's own water inside which calculates the ionexchange sites correctly. The water cells have only 0.1 molar NaCl in equilibrium with CO2 partial pressure of 1 bar. Both ends have inlet condition the same as the water cells.

 

First problem is that the model assumes that every cell has ion exchanger phase. Can this be somehow changed because ion exchange is not coupled to any mineral the sameway as surface complexation file. Can it be coupled or somehow done differently only to those cells with clay?

 

The second problem is that GWB somehow calculates equilibrium to those water cells at zero timestep with minerals in clay phase when there is supposed to be only 0.1M NaCl present. Why is that? the nodes for water are 0 and 11 and 1-10 are clay cells.

 

Can you help me?

 

attached are database, ionexchange and surface files and the model file.

 

 

-A

MX80IonEx.dat

MX80Surfthermoddem.dat

Testi1atmCO2.x1t

Thermo_thermoddem.zip

[Brian Farrell]

Hi Akul,

 

Exchange capacity is a field variable, so you can in fact set it to be heterogeneous. You need to go to the Sorbing surfaces dialog to do so. In X1t, go to File - Open - Sorbing surfaces, then click the "+" button next to exchange capacity for your ion exchange surface, then choose from the various options. You can also just add an in-line table from the command pane as you've done for your other heterogeneous values. Depending on the version of GWB you're using, there might be a problem reading the node-by-node editor (as described here), but the tables should work fine.

 

As for the clay minerals, when I run your script I don't see any mineral mass in either of the two outer nodes. Can you clarify your question? Also, do you have any particular reason for setting up the outer nodes as you have? If they are the same composition as the inlet fluid, I'm not sure they even need to exist.

 

Regards,

 

Brian Farrell

Aqueous Solutions LLC

[AkuI]

Hi Akul,

 

Exchange capacity is a field variable, so you can in fact set it to be heterogeneous. You need to go to the Sorbing surfaces dialog to do so. In X1t, go to File - Open - Sorbing surfaces, then click the "+" button next to exchange capacity for your ion exchange surface, then choose from the various options. You can also just add an in-line table from the command pane as you've done for your other heterogeneous values. Depending on the version of GWB you're using, there might be a problem reading the node-by-node editor (as described here), but the tables should work fine.

 

As for the clay minerals, when I run your script I don't see any mineral mass in either of the two outer nodes. Can you clarify your question? Also, do you have any particular reason for setting up the outer nodes as you have? If they are the same composition as the inlet fluid, I'm not sure they even need to exist.

 

Regards,

 

Brian Farrell

Aqueous Solutions LLC

 

Hi

 

Yes I am using 8.012 version of GWB so there is only two options in the sorbing surfaces section for ion exchange and those are to choose between eq and eq/g and to put the value in. There is no node to node editor there. I will try the other way if it is possible in v. 8.0?

 

The other equilibrium problem I meant was that yes there is no mineral phases in node 0 but there are for example lots of Ca2+ present 0.02mol/L and other ions which can only come from the clay minerals. My configure says there should be 1E-6 of Ca2+ which means only traces and other ions too excluding Na+, Cl- and CO2 complexes with these. The original pH in the node 0 should 3.9 at zero time step like in the inlet cell so why it is different? Only place where aluminum can come from is montmorillonite in this case and there is also Al complexes present in the node 0 which does not have any minerals so only explanation I can think of is that it somehow calculates some kind of equilibrium with the clay minerals.

 

- Aku

[Brian Farrell]

Hi Aku,

 

Sorry to be slightly misleading - I didn't realize that you were using an older version of GWB. GWB 9 has expanded the options for medium heterogeneity, including the exchange capacity, which as a field variable can be set to heterogeneous values. This is not possible in version 8, however.

 

I think when you swap a mineral into the Basis it has to be in equilibrium everywhere in the domain (at least to begin the run). So, although you've set the mass of Montmorillonite, Calcite, and Gypsum to 0 in the two outer nodes, those nodes are still in equilibrium with those minerals everywhere in the domain. The chemistry is different, however, because you've specified different fugacities/ concentrations for the other Basis species.

 

Regards,

Brian

[AkuI]

Hi

 

Thanks for this

 

If I understood correctly, in this case it is impossible in GWB to calculate so that the initial concentrations would be as told in the input at water cell nodes. Should this be something you should consider in future updates of GWB?

In the experiment there is no equilibrium with the mineral cells at time step 0 but of course this is something which will happen during long time periods.

However in this case GWB gives erroneous results.

 

Regards

 

Aku

[Brian Farrell]

Hi Aku,

 

I think there are a couple solutions to this problem. The first, and most straightforward approach, is to remove the two outer nodes which are supposed to be identical in composition to the inlet fluid (and not in equilibrium with the minerals in the rest of the domain).

 

For the second option, you wouldn't swap your equilibrium minerals into the system. Rather, you would specify concentrations (or activities) for your components such that the desired minerals would be in equilibrium. For example, if you wanted a domain to be partially in equilibrium with quartz, and partially undersaturated, you wouldn't swap quartz into the domain, then set its mass to 0 where you don't want it to exist - this won't work. Instead, you could make the initial concentration of SiO2(aq) a heterogeneous value.

 

As an example, try figuring out the concentration of SiO2(aq) in equilibrium with quartz. Using SpecE8, add SiO2(aq) to the Basis, then swap in Quartz and set its mass. Run the model and you'll find the equilibrium concentration at 25 C is ~ 6 mg/kg. Now instead of swapping Quartz in for SiO2(aq) in X1t, you can specify a concentration of 6 mg/kg where Quartz should be in equilibrium, and a smaller value where it should not be in equilibrium. You can always incorporate kinetics from the Reactants pane if you'd like.

 

Hope this helps,

Brian