My advice from experience? Never EVER buy a home with an underground tank, in use or not. Have the seller remove it. If your attorney at closing doesn't tell you to have the soil tested or have the tank removed if it is not in use, get another attorney.
You obviously have quite an interesting mess on your hands. Contact me, I need more details before I can recommend a course of action. We recently dealt with an REO property that had a leaking oil tank and learned a great deal from that process.
Jacobus "Jack" Vollenberg
RE Appraiser - Vollenberg Appraisers
Asset Manager - ERA Statewide Realty
Cell (973) 590-0142