Dear all,

I am running a 3D transient fluid-flow and mass-transport model to simulate the propagation of a conservative contaminant dissolved in the fluid phase for more than 40 years.
The main transport medium is a shallow phreatic highly conductive aquifer which I am simulating using the Unconfined option - Phreatic mode for all the slices.
The contamination source was set as a mass-source term (+0.005 g/m3/d) located in the saturated zone (just one/two slice below the water table) as shown in the attached Fig_1.

I am struggling to understand why after few time steps (10-20 d) the mass concentration moves also in the unsaturated zone? (the advective transport is predominant respect to the diffusive!!!)

I am attaching two figures to explain the geometry and settings of the model. In Fig_1 you can see a local cross-section along the flow direction that crosses the contamination source and the Darcy flux vectors.
In Fig_2 I exported the mass concentration isosurfaces after 60 days (a), the forward streamlines (b) and the forward randomwalks (c) from the nodes of the mass-source (can someone help me understanding why randomwalks are elongated along the Z dimension? seems that immediately particles are moving up- and downwards from the contamination source along the Z dimension?)

Thanks in advance for your time!