Why is my solute-concentration in unsaturated zone?

Hi all (Bjorn!)

Apologies again for the (probably) silly question,

But I need to know why I've got a mass concentration measurement in my unsaturated zone (see picture).

I guess it's related to my unsaturated flow equations, so I've listed these below;

Saturation Limits;
s_s = 1
s_r = 0.15

Van Genutchen;
a = 0.8
n = 3

I'm assuming it should be totally dry, as there's no recharge modelling from the top.

Having spelt it all out here, I'm starting to think it's the residual saturation parameter.

Is it ever sensible to set this to 0?

I would not set the parameter to zero. However, you can use a small value closed to zero. Please note that this parameter has not only an impact on the physics, but also on the numerical stability. For more details you may also contact the support: https://www.mikepoweredbydhi.com/support

If im doing sea water intrusion modelling, should i need to give recharge from top of aquifer any mass boundary condtion or feflow will take rainfall recharge as 0mg/l?

If you use the In-/Outflow on Top/bottom parameter in a 3D model, FEFLOW will assume a concentration of 0 mg/l for recharge. When doing a cross-sectional model, however, you'll have to use a Fluid-flux BC for recharge, and this will (by default) NOT assume freshwater, but a gradient of 0, meaning that recharge will enter with the current concentration of the node the BC is set to.