You can sue the seller for fraud.  Fraud is the intentional misrepresentation of a material fact made with knowledge of its falsity and with the intent to induce your reliance upon which you justifiably relied to your detriment.In other words, you would not have bought the house had you known the true facts about the septic lines.Your damages (monetary compensation you are seeking in a lawsuit for fraud) would be either the benefit of the bargain or your out of pocket loss.Benefit of the bargain means that a defrauded purchaser may recover the difference between the real and represented value of the property purchased regardless of the fact that the actual loss suffered might have been less.Out of pocket determination for damages for fraudulent misrepresentations permits recovery of the difference between the price paid and the actual value of the property acquired.You would file one lawsuit naming both the seller and real estate agent as defendants.  Your cause of action (claim) against the seller would be fraud.It may be difficult to prove fraud against the real estate agent.  Therefore, you should sue the real estate agent for negligence.  Negligence is the failure to exercise due care (that degree of care that a reasonable real estate agent would have exercised under the same or similar circumstances to prevent foreseeable harm).  Your damages (monetary compensation) for negligence would be all expenses, costs, and other items caused by the real estate agent's negligence.