fluid flux with dilute species model

Hey all,

I'm sure this has been beaten to death, but I cannot seem to make a simple fluid flux across an arbitrary boundary work. My model consists of a container with a fixed amount of solute, and a C=0 boundary on one side. How do I get ahold of the normal variable names to generate a surface integration along a boundary of my choice? I can make a cut line or cut plane, but I don't see a way to get the plot I need.

Thanks,

John