Trouble with Saltwater Head Boundary Condition

Hello all,
I am currently trying to model seawater intrusion in a coastal aquifer using FEFLOW v6.01 and, as a starting point, I am trying to build a simplified 2D cross sectional model. I have the following boundary conditions:
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[li]Along the ground surface (top of the model domain) I have a Fluid Flux BC with a 0mg/L Mass Concentration BC[/li]
[li]Along the inland boundary (left of model domain) I have a Fluid Flux BC and a 0 mg/L Mass Concentration BC[/li]
[li]Along the slanted seaward boundary (right of model domain) I have a 0m Saltwater Head (SWH) BC and a 35000 mg/L Mass Concentration BC with a Minimum Mass Flow Constraint of 0 g/d[/li]
[/list]
The density ratio is constant throughout the domain at 0.0245
My run of 1000days shows a saltwater wedge encroaching landward as predicted.

My problem is that when I change the SWH BC to a higher value (simulating sea level rise), the mass contours are the same for 1m, 10m, even 100m of SWH after 1000 days.
Shouldnt the contour lines look significantly different (more inland) with increasing SWH?
Is there something that I am missing?

Thanks so much for the help!

Indeed I'd also expect that. May it be that you have applied a large dispersivity, hereby influencing the isoline locations by dispersivity rather than gradient?

Or perhaps too coarse a grid.  Mass error is maybe overwhelming the result.

Hello

Try a head BC instead of flux on the landward margin.

Good luck

Gianmarco