I don't know this is off topic or not but look the map that John posted over RDM:
http://hotcopper.com.au/post_single.asp?fid=1&tid=1440729&msgid=8179504
We need 2 main things for Potash mining:
Gas and Water.
We already know there are Gas fields around.
By looking the map, TRH location much close to the river (water's source) than RDM.