The PFOS plume travels 2.5 mi SSE from the 3M Oakfield disposal site, then the concentration drops by a factor of two (from 0.27 ppb to 0.136 ppb) over a distance of 0.19 mi (the width of the orange band just below the red).

3M Oakfield well 5
      and PFOS plume

This seemed a bit odd considering the much longer distances covered by the other color bands.

I remembered that the most contaminated Oakfield municipal well was #5 so I decided to look up the Oakfield well protection plan.

Oakfield Well

The location of well 5 capture zone appears to roughly correspond to the steep reduction in PFOS concentration.

I suspect that something similar is happening at Madison well 15 and that the groundwater plume map will look similar there.

3M paid for installation of a granular activated carbon filter at Oakfield well 5 so the PFOS is being removed from the environment.
It remains to be seen what Madison will do with well 15.