My question precisely. If you do not have legal title or an easement to the property in question, you have no case.
To get an easement if you don’t have one, you’d have to claim easement by pre-existing use, which may not be allowed by your state.
You need a land use lawyer to sort this out.
I have an easment,it’s in my Title
Wouldn’t ya think if his drainfield was on someone ELSE’S property, he would have gotten the legal right to do it? Did not some bureaucratic inspector have to sign off on a septic system? Is a permit required to install a system in your jurisdiction? If all the answers were yes, than your beef is with your neighbor who allowed you to use his property and put in a drainfield. He had the obligation to prevent any damage to the land he effectively rents to you.
Just as a landlord has the obligation to protect the building as a whole.