Just when we environmental old-timers were just getting used to talk about PFAS concentrations in the low parts per trillion range, EPA has moved us squarely into uncharted territory.  EPA has released new, interim, lifetime drinking water health advisories of 4 parts per quadrillion for PFOA and 20 parts per quadrillion for PFOS.  I'll just note a couple of points: 

• I think it sort of charming that EPA still states the health advisories in the units of parts per trillion (0.004 ppt for PFOA and 0.020 ppt for PFOS, for arithmetically challenged readers). I can only imagine that EPA concluded it would be less terrifying to continue to use parts per trillion.
• These are interim standards (though EPA did issue a Final health advisory of 10 parts per trillion for GenX Chemicals today as well). Also, it's worth a reminder that, as EPA noted, health advisories “are not regulations and should not be construed as legally binding.”
• On the other hand, EPA's Federal Register notice states that EPA is working on development of Maximum Contaminant Level Goals and expects to propose them later this year.

Whatever the regulatory status of health advisories, it would be wise to assume, absent a successful legal challenge, that parts per quadrillion concentrations for at least these two PFAS are going to work their way into regulations under both the Safe Drinking Water Act and CERCLA.

That's when the real fun will begin.