Drainage issues running through my neighbor's property

The situation in brief:
About 20 years ago, my parents installed a concrete drainage pipe to handle storm runoff and waste from a three-tank septic system
The pipe runs across 150 meters of construction land, and we had a verbal agreement with the neighbor that he could tap into this infrastructure if he ever needed to
The property was sold, a house was built, and now the new owners have connected to the pipe without any prior notice or permission...
Is there any legal way for the neighbor to prevent me from using this pipe, or perhaps intentionally clog it?
Does he actually have the right to connect himself without asking me, even if the pipe was buried on his property two decades ago?

Well, I suppose there is nothing left to say....

Joshua Booth2 said:The situation in brief:
About 20 years ago, my parents installed a concrete drainage pipe to handle storm runoff and waste from a three-tank septic system
The pipe runs across 150 meters of construction land, and we had a verbal agreement with the neighbor that he could tap into this infrastructure if he ever needed to
The property was sold, a house was built, and now the new owners have connected to the pipe without any prior notice or permission...
Is there any legal way for the neighbor to prevent me from using this pipe, or perhaps intentionally clog it?
Does he actually have the right to connect himself without asking me, even if the pipe was buried on his property two decades ago?

Where does that pipe actually terminate—in a nearby ditch or the municipal sewer system?
If it connects to a public utility, then you don't need anyone's permission; connection is handled strictly according to the local water authority's regulations once the building permit for the house is issued.
If it isn't part of the public system, then connections follow local wastewater ordinances, though I suspect you wouldn't have much standing there either.
In any case, the neighbor purchased the land (the construction site) along with all its inherent rights and obligations. Frankly, I don't see an issue here beyond the fact that your feelings are slightly hurt because nobody consulted you. That pipe hasn't belonged to you for a long time, and the new neighbor is simply honoring the arrangement your parents made with the previous owner of the lot.

Thank you for such a clear and thorough response.
The pipe runs into a nearby creek, which sits right on my neighbor's property line.
I didn't take offense personally; I was mainly wondering about what happens if he decides to shut me off down the road... I assume I would have some legal recourse in that case...?

Hey Barack Obama, that pipe ends up right in the local creek 😵 What exactly was the neighbor building without a permit?
Look, if he decides to block the pipe, you might want to consider digging a small pond on your own property—think of it as a biological filtration system. Then, just report the neighbor to the EPA and see what happens. Honestly, it would be better for both of you if that pipe were used strictly for storm runoff, while you each handle your own sewage through individual septic systems with those small treatment units. It seems like you're out in a rural area with plenty of acreage, so setting that up shouldn't be much of an issue.