Diffusion Problem

[AkuI]

Hey

 

I have a diffusion cell where I have 50ml of water in both ends and bentonite tablet in the middle which is separated by a sinter from the water cells. Water goes through the sinter to the bentonite but bentonite maintains its compacted form. I would like to know how you can model this diffusion problem with GWB or can you ?

In X1t, there are only inlet and outlet boundaries and in X2t there is closed boundary. However if I try to do closed boundary in both ends, the calculation does not seem to start. Best way for me would be a radial column, which is in X1t with only diffusion, because the tablet is cylindrical, but I don't know if the boundary conditions will match the experiment. Could you give me some advices how I can do this kind of model?

 

Regards

 

Aku

[Brian Farrell]

Hi Aku,

 

You can model a pure diffusion type problem with GWB. I might start with a 1D model (linear coordinates) using X1t. You can set discharge to 0 and use the left and right commands (left = inlet, right = inlet) to allow for diffusion into the domain. If the water in your domain is initially dilute, set the inlet fluid to some higher concentration and it will diffuse in from both ends.

 

You can set the permeability to be heterogeneous if you'd like (bentonite and sinter might be different), to more accurately replicate your system.

 

You should check out section 2.9 (release 9.0) of the GWB Reactive Transport Modeling Guide for information about Boundary Conditions in GWB.

 

Hope this helps,

Brian

[AkuI]

Hey

 

Does the inlet option work so that the injected water can also change. Because in the experiment there is no injection but the water outside the bentonite will change too ? As far as I understood the injected water does not change during the modelling but retains its composition.

 

Regards

 

Aku

[Brian Farrell]

Hi Aku,

 

In X1t and X2t you can model two reaction intervals. You might have an initial chemistry for your domain, then one inlet fluid enters the domain (and optionally an injection fluid in X2t). After some time, a fluid of a different chemistry can enter the domain (through the inlet, or an injection well in X2t). People commonly use these two reaction intervals to simulate the introduction and subsequent remediation of a contaminant.

 

Hope this helps,

Brian