aquifer recharge with contaminated water

Hallo Feflow users !
I have a very important question for you. I have modelled in 3D the time -  varying surface and ground - water flow in an italian basin. I have set the recharge as daily net rainfall directly applied as flow on top slice of my model.
Now I want to develop a mass transport model derived from the flow one. But even if I know the daily contaminant concentration associated to the net rainfall (or the mass daily infiltration per square meter), I can't find the way to easily apply it to my transport model. The In - Out flow on top button give us the possility to apply a known water infiltration rate directly on the ground surface. On the contrary a Source of mass on the surface must be expressed as g/day related to the unit volume of the first layer (if I have understood the feflow manual) and this is not so easy to calculate.
Do you have some suggestions?
Thank you very much
Gianmarco

Gienmarco,

In my opinion there is no "easy way" to discribe those boundary condition in FEFLOW.
But there do exist several possibilities to do it.

1.) You could define a first type mass boundary condition on the top slice of your model. This would represent the concentration in your soil. All water passing through that slice (so also your recharge) will get that concentration.

2.) You could define a second type mass boundary condition on the top slice of your model. This would a mass flux (concentration in soil x water flux = mass flux).

3.) You could define the source in the material property menu. This is probably the method you were talking about. In this case you have to take the volume of the layer into account. For your problem I would not use it (this is ment to some problems where mass concentration are "produced" by the soil and not brought in from outside).

Zebra

Thank you zebra for your useful advices.
Gianmarco

For case 2 of zebra's recommendations, please note that you have to use the divergence form of the transport equation. Please see the corresponding White Paper for details about the difference between the convective and divergence forms.

Peter,

any plans to incorporate a coupled flow-transport-boundary condition? In the case of recharge the ways described above are more or less straight forward. But in some cases, e.g. river infiltration from a 3. kind boundary the user defined input of concentration may become quiet tricky.

zebra

At the moment, there are no plans to incorporate such a boundary condition. However, I fully agree that there are cases where it could be quite useful. In the meanwhile, you could easily implement such a boundary condition on your own using the programming interface IFM, calculating the values for a 4th kind boundary condition for mass transport. In this case, you would wish to use the divergence form of the transport equation, too.