Based on a quick search, they are probably referring to this study[0] which states “One fish serving can be equivalent to drinking water for a month at 48 ppt PFOS”.

The EWG map of PFAS in fish[1] provides some specific measurements on fish in the Mississippi and Minnesota rivers.

[0] https://www.sciencedirect.com/science/article/pii/S001393512...

[1] https://www.ewg.org/interactive-maps/pfas_in_US_fish/map/