Brine won't move in polluted water

I had some pockets of brine in a large vat of polluted water under a pwater vent.  They were kind of scattered around, and wouldn't move around to coalesce.  I thought I could build some tiles to force them to move together into one spot where a pump could remove them all, but whenever I tried building a tile in the brine to force it to move, it instead got deleted.  It seems that under no circumstance will the brine move to either side when it is surrounded by pwater.