Legality of sealing a retention pond (US)

Hey all,
I have been trying to work with a client on a pond project and can't find any definitive answers to this question. The project is area is a roughly 1/2 acre wet/dry retention pond that the client wants to seal to the drain level and fill with a permanent level of water. The allotted land is surrounded on all sides by customer parking, and floods the whole lot during exceptionally large storms.
I've received word that he may be doing some legal gymnastics to get permission, and that I might not even HAVE legal permission, but once again I've really struggled to find answers. If it helps, the project is located in Gilmer county GA.
In all honesty, I have felt very bad about this project and just want some reassurance that firing this client is a good move. No money has been exchanged at this point.