42 U.S.C. § 300g

Subject to sections 300g–4 and 300g–5 of this title, national primary drinking water regulations under this part shall apply to each public water system in each State; except that such regulations shall not apply to a public water system— (1) which consists only of distribution and storage facilities (and does not have any collection and treatment facilities); (2) which obtains all of its water from, but is not owned or operated by, a public water system to which such regulations apply; (3) which does not sell water to any person; and (4) which is not a carrier which conveys passengers in interstate commerce.