From my experience the best way to handle the old oil tank is to have in removed entirely by an expert in the field. Usually the removal expert will be able to determine if there has been leakage. if so they will take soil samples from the area. Depending on the age it's quite likely the tank can be emptied, removed and back filled without the involvement of outside agencies like the DEC. You may want to contact a local real estate expert for a referral of a few oil tank excavation companies to talk you through the process. In smaller towns you can frequestly find the firm that has serviced the account and they could pass along some of their own "local knowledge". This is a very common obstacle in rural areas, especially in older towns. Good luck. This process is much less daunting than it sounds. Many times you can negotiate with the seller (unless it's bank owned) to help cover the cost of such a process or ask for a credit at closing to handle the costs if funds are limited. At the very least this is a $1500 item to correct which is usually seen as a major structural defect. Depending on the RE practices in the area this is custonarliy grounds for re-negotiating.