I’ve been looking all over the EPA website trying to find that particular drinking water standard. The closest I can find is a standard for gross beta, at 4 mrem/year.
I’m starting to believe the standard is not an official one, perhaps even made up.
http://water.epa.gov/drink/contaminants/index.cfm#Radionuclides
This does not mention I-131 but has standards for activity in general. It needs to be converted from pCi/l to Bq/l, to match the article, I’m quite busy so I’ll leave the conversion up to FReeperdom.