I recommend expressing ...

I recommend expressing concentration as a mobile fraction  i.e.  expressing mass of metal mobilised, normalised to sample mass, with a mobile fraction expressed in mg/kg. This tends to eliminate differences in liquid to solid ratios, flow rates and so on.

The pore volume in a column is very dependent on how the column has been packed and may or may not represent the pore fraction in the undisturbed soil. It's often easiest to use simple methods.

One other thing, I try and avoid using the term "heavy metal" outside its musical context. Have a look at:

Duffus, J H. ""Heavy Metals" - a Meaningless Term? (Iupac Technical Report)." Pure and Applied Chemistry 74, no. 5 (2002): 793-807.