How to set flux boundary at slice between aquifer and aquitard

Hi! I'm a new user of FEFLOW. Now I'm very confused when I set flux boundary condition. I only have the data of hydraulic gradient and approximate hydraulic conductivity of different layers near the boundary. The boundary should be set as a flux boundary. And the flux value on boundary only can be estimated according to Darcy's law.
The first layer is an aquifer and the secend layer is an aquitard. Then when I set the flux boundary condition on the second slice between the aquifer and aquitard, how could I get the flux value according to Darcy's law, using the hydraulic conductivity of the upper aquifer or the lower aquitard?
Looking forward to your reply! Thanks!

While the fluid-flux boundary condition is technically set on nodes, it has to be set to nodes enclosing an entire element face in 3D models to actually work. So to specify an inflow into the aquifer, you'd set it to the nodes in slice 1 and 2 to specify and inflow over the entire vertical element face at the boundary.

Thanks for your reply!

But when I set flux value on the boundary of slice2 which is between an aquifer and aquitard, I have to use Darcy's law (q=KI ) to estimate the flux value. Then which "K" should I use? The "K" of upper aquifer or the "K" of lower aquitard, or an another integrated parameter?

Thanks!

Perhaps you need additional layers so that you can assign flux boundaries to discrete layers.

Otherwise, why not use a head dependent transfer boundary and let FEFLOW decide the flux?

The flux boundary is assigned on slice. In my model I have two layers- the upper is aquifer and the lower is aquitard, and I need to assign flux value on the boundary of slice2 which is between those two layers. So what did you mean by "additional layers"?

If I use transfer boundary, what dose "Reference hydraulic head" mean in my model and how to determine it. So I think the flux boundary is better.

Are you trying to impose a certain flow rate between the aquifer and aquitard? This should be an output from your model you should attempt to replicate through model setup and parameterisation.  All of the boundary conditions are used to add/remove flow to/from the model domain.  Fluxes between nodes (horizontally and vertically) is a model output (not an input).

If you insert any boundary condition on slice 2, it will inject or abstract some volume of water along this interface, which I assume is not your goal.

Sorry I probably didn't express my intention clearly. In my description before, I just simplified the condition. In fact, boundaries of slice1 and slice2 both  are indeed flux boundaries according to hydrogeological analysis. So I would assign flux value on those two slice boundaries according to Darcy's law.

When calculating flux value at slice1 boundary, I use "K" of layer1 which is an aquifer. But when calculating flux value on slice2 boundary, I'm confused how to determine the "K", The "K" of upper aquifer or the "K" of lower aquitard, or an another integrated parameter?

Thanks!