The neighbors drain field and part of the septic field is on my son's new property

My son purchased a property on 3/31/2022, he had it surveyed the end of May 2022. The survey identified part of the neighbor's septic tank and drain field on his property (and has been marked to Wisconsin regulations, the survey will be registered to the county soon). the neighbor's property has not been surveyed and along with the septic they have also located other personal property on approximately .2 acres of my son's land. The neighbor's are also recent purchasers of their property (I believe not over 3 years ago). I have informed the other property owners that their septic / drain field is residing partially on my son's property. What are some of the outcomes that can occur? such as: put in a correct boundary markers/fence and ignore them (I think this would be an issue with the next owner), swap an = amount of property with them, sell the portion of the land that they are currently using?
can the septic be forced to relocate
and if selling how would a value be determined

Horsejmr said:

I have informed the other property owners that their septic / drain field is residing partially on my son's property.

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So?

What did they say when you told them that?

Presumably you didn't just say "your septic/drain field is residing partially on my son's property" and then just turn around and walk away.

Did your son check his deed to see if the neighbors had an easement for the septic field? It's rare but it does happen.

Also why are you dealing with this and not your son?

In addition I would want to know if the septic and drain field were installed while both parcels in the topic were a single parcel at one point in the past. If so, it might be seen as an implied easement for the septic and drain field. There are other possible situations, but we don't know at this point.

As for a change of property boundaries, I would put that idea well behind a much simpler and lower cost grant of an easement. Probably much less likelihood of a due-on-transfer clause being triggered if there are mortgages involved, and much less scrutiny by county or local authorities.