I'm sorry to hear about your dispute with your neighbor.
From the information you provided, if the drain field was located there for 50 years, you've probably got a valid claim for adverse possession if you meet the statutory requirements, and probably had a valid claim after only 10 years.
There are other theories of possession that may apply in your favor, including "mutual recognition and acquiescence," and perhaps "parol agreement." You can read the statutes yourselves, or consult an excellent attorney to advise you on those claims since the statutes are requirements can be confusing.
Your neighbors request for money and relocation of the boundary line would likely be dismissed by a Court if you meet the required elements for any of the theories of possession. I have several clients currently in litigation for boundary line disputes, and one thing is for sure, litigation over the issue will be very, very expensive, and you'll want to avoid litigation if you can.
If you can keep friendly communications going with your neighbor, there is nothing keeping you and your neighbor from working something out that will satisfy everyone involved. You can always file documents (such as a boundary line adjustment) with the County (some fees and other requirements will apply) if you both agree on a solution.
Good luck. Mark Arend
I agree with Mr. Arend and if the neighbor wants something let him pay for it and the septic system most likley will not meet the elements of advers possession because it was underground and not visable.
the elements are;
Open and notorious
For the statutory period
and underground it is impossible to see, in this sort of case a temporary license until the system needs to be replaced and then place it on the correct property.