F’ing Septic Tanks

You may be able to go after the po for non-disclosure, I expect the laws vary by state, but the costs of litigation may outweigh anything you may recover. Best suggestion I can make is look for a lawyer that will give you an informed opinion. Unless the rainwater is all feeding into the tank, it's located in a low spot on the property, and short of building up the area surrounding it, you may need to relocate it, no inexpensive solution IMO. Have you made any attempts to correct the situation in the 18 months you've been there? I can't imagine living in those conditions for any length of time.