TOUGHREACT (v1.2) not considering fluid density due to salinity?

Hello,

I am using EOS9 to model a plume of leachate entering the top of an aquifer to evaluate the resulting changes to water & mineral chemistry. I have noticed that when injected, the leachate does not experience any buoyancy driven transport, and as a result, tends to stay at the very top of the aquifer and is carried by advection (and a small amount of diffusion). 

To test this, I created another model and fixed all of the boundaries such that there is no more advection occurring. I then set the diffusion coeff to be 0. Then I bumped up the concentration of the leachate by a few order of magnitudes. This should isolate and magnify buoyancy-driven fluid flow. The result is that there was still absolutely zero flow, the highly saline leachate remained in the same spot for 1000 years. 

Why is this happening? I would expect the injected leachate to sink downward to some degree.

FWIW I am using Petrasim 

Thanks in advance for any help. I am fairly new to this.