Largely firefighting foams, industrial and manufacturing, and landfill sources, but it's still an interesting problem. They don't really break down (that's why they're so useful both in a materials science sense and as a medication) which implies they'll stick around for an extremely long time.

One thing you can be sure of is that the vats of PFAS being produced year after year for this drug aren't going away anytime soon. They're called "forever chemicals" for a reason.

Being dispersed in the environment is not the same as being concentrated into our drinking water supply with each measure resulting in 1ppt contamination of a trillion measures of water.