Using conservative mass transport to determine water origin

Hello Forum,

i am facing the following problem. My model domain represents a vertical slice of aquifer which is orientated transverse to a river. The river is represented with a transient hydraulic head boundary. The remaining boundary condition is a recharge value implemented as a constant flux BC on top of the domain. I am investigating the influence of a clogging layer (implemented as a low hydraulic conductive layer next to the river boundary) to the hyporheic zone. The size of the hyporheic zone is defined as 10 % river water content.
Currently i try to use a non reactive species to show this 10% isoline and thus represent the size of the hyporheic zone. Therefore i add a mass concentration BC to the aquifer - river interface with 1 mg/l and 0 mg/l to the recharge boundary condition. Unfortunately with my setup i am not able to produce a conservative transport. As soon as i assign the dispersivity to 0 the model is no longer transporting mass. I already switched to the divergence form of the transport equation to enable also conductive transport.

I would be very glad if you could help me. Maybe i am just missing a setting which is important or my attempt is completely wrong and there is a much better solution.

Thanks in advance
Ludwig

Dear Ludwig,

A non-reactive tracer does not mean that dispersivity values should be zero. The conservative tracers do not consider chemical reaction (degradation rate) and solute interchange between solid and liquid phases (i.e. adsorption). In Groundwater literature, one typically speaks about the [b]retardation coefficient[/b] in order to distinguish between reactive and conservative tracers, you may be interested to read about this.
The selection of convective and dispersive mass transport formulations in FEFLOW depends on model boundary conditions. You may take a look on White Papers Vol. I.

In addition, I recommend you the following things:
1) Try using a 3rd kind BC to simulate the interaction river-aquifer. Here you can indicate low values of conductance (transfer rates) to account for clogging effect.
2) If you want to delineate hyporheic zone, you can use the new class [b]AGE [/b] introduced recently in FEFLOW 6.2. For example, 1) a so-called [b]"age"[/b] specie can be used to identify the mixture zones of two water bodies with different origins; and 2) apply the so-called [b]"exit"[/b] specie to delineate the capture zone of the river boundary.

Cheers,

Carlos