Modelling of contaiment Pond with HDPE and Clay liners

We are trying to model a containment Pond with 1.5 mm thick HDPE liner with K 1x e-12 m/s on top of 300 mm clay liner (K=1x1e-8 m/s). The normal flux through the liners is about 10 times higher than what is expect. The head of water in the pond and on the liner is 24.2 m. The embankment of the pond is 25.2 m high. Our questions are:

1. how to model a containment pond with liners of such thicknesses.

2. is it appropriate to use equivalent k values for the liners by increasing their thickness

3. is FEFLOW capable of simulating such thin layers without causing excessive fluid/mass balance errors

We will appreciate your suggestions. Thank you.

kobonsu

Hard to say what causes this problem.
I tried to reproduce this using a 3 layer pseudo-1D-model (layer 1-2 HDPE and clay, following description above, all layers horizontally), plus a 2 m layer below (with standard FEFLOW parameters). top BC is head = 24.2 m, bottom BC = 20.2 (no-Flow BC else)
The resulting flux brings a nearly perfect match with the analytic solution (deviation below 0.001 %). Do you think my model is representative?

regards,
Alex