Here are some suggestions.
(1) Discuss the matter with your buyers agent
(2) Discuss the matter with your attorney and make sure you order your own survey (which I am assuming you may have already done.
(3) Interview the neighbors, they can hold a plethora of information; and in my view most importantly
(4) Go to the township (or ask your buyer's agent to do so) to determine their position and/or there are any past permit or report issues regarding issues with this basin.
Francesca Patrizio, Broker Sales Associate, ePro
It goes along the right side of my property.
I have a temp shed over it.
I didn't think twice about it when I bought the home and it has not presented any issues.
However, you are concerned about it as a Buyer.
When you sell, others may also wonder.
That may effectthe price.
Things to consider:
Is the drainage basin unsightly?
Does it fill often.
Does it look like it might attract bugs?
A 10' drain easement will prevent you from doing anything that would disturb the flow of water to the basin. If you have plenty of remaining property for expanding, should that be the future plans, then it's would be of no real concern. Should new water drains piping be planned on your street, in the coming future, then you may be able to remove the easement through the town's planning and zoning boards. You would have to argue the point of the easement became obsolete once a better system was put in place.
Since, an easement could be a negative, to full enjoyment of the property, I would suggest presenting a reduced offer on the premise of restricted use.
Without knowing the exact property in question, I can only offer a limited number of suggestions. But, you get the idea.
Good Luck !