Not much is both water soluble and smaller than sodium. For the most part your basically left things like individual molecules of lead or other heavy metals which you would need to drink a lot of water from vary contaminated sources before you had a problem.
Ed: Individual sodium atoms are smaller than individual lead atoms. However, under the assumption that there is some variation in filtering capability's that will let a low % of things in that are sodium sized and slightly larger through you may have to deal with some larger atoms like lead.
It would have to pass through a fairly hydrophobic (the pore) layer. The water can barely get through since it's polar but uncharged; most metals in solution are charged ions and wouldn't be able to pass through the barrier.
