Its really quite simple, for anyone that has ever read mining reports the first thing you find out is that grades change and pollutants differ from one area to the next.
Don't care if you are mining Epsom Salt or Borax that same thing is in effect. NEVER use non USP Epsom Salt unless you feel lucky and never internally use non USP Borax.
After reading the homeo for Borax I would be more worried about Borax than Epsom Salt it has a higher infinity to bond with a positive ion charge to many more minerals.