Surprisingly enough, when you look at a map Westmalle is about 100 miles due west of Dusseldorf, Germany. Not that it has anything to do with the water profile, but I would think they get it from a well. They are very secrative with their brewing process.
BLAM may offer some tips. I don't recall what was stated off the top of my head, but I don't think it was very specific about the water.
I'm not sure if any of Michael Jackson's books offered any clues but I'm sure some others may chime in with some info.