A lot of houses built before natural gas reached the neighborhood were heated with oil. The tank sat in the basement or, worse for you, in the yard. When the house converted to gas, the honest job was to pump the tank, pull it, and fill the hole. The common job was to disconnect the line, leave the tank, and forget it. A buried tank that still holds sludge or oil leaks when it rusts. The leak is a soil cleanup. In many states the current owner is in the chain of responsibility whether they buried the tank or not.

"Converted to gas" is a heating fact, the same way a new furnace is not proof the old equipment left the property. It is not a tank fact.

What it is

A steel tank, often 275 to 1,000 gallons, buried in the side yard, sometimes under a driveway that came later. Clues are a fill pipe, a vent pipe, a copper supply line that goes through the foundation to nowhere, and a basement patch where the old tank legs were. Some tanks were abandoned in place under a permit with sand or foam fill. That is a different, usually acceptable, status if the paperwork exists and the tank was empty. An abandoned tank with no paperwork is a buried question.

How to spot it in the listing

Age of the house plus the history of the gas utility. Remarks may say "gas heat" and stop. Disclosures in some states ask about underground storage tanks directly. A "no" is only as good as the seller's knowledge, and sellers who bought in 2005 may honestly not know. Look at old listings. Look at aerials for a rectangle of dead grass or a newer patch of concrete. The research guide includes the fire marshal or the local tank registry where one exists. Some towns kept abandonment permits. That file is the best five-minute call in this entire process.

Photos of the basement sometimes show the copper line capped at the wall. That cap is a flag, not a conviction.

How to spot it at the showing

Walk the foundation and the yard. A fill pipe is often a black or rusted metal cap a few inches across, sometimes cut off flush with the soil, sometimes hidden in a shrub bed. A vent pipe is smaller and sticks up. In the basement, look for a capped fuel line and a stain on the floor where a tank used to stand. If the tank is still indoors, you can see it. Indoor tanks leak too, but they are not a soil remediation. Price them as a removal and a floor, not as an environmental project.

If the clues are there and the paperwork is not, a tank sweep (metal detector and probe by someone who does this) is the test. Do not rent a metal detector and declare the yard empty. Water lines and rebar will lie to you.

The real risk

A release to soil and groundwater, reportable to the state environmental agency, with cleanup that can be a few loads of soil or a long monitoring case. The transaction risk is time. A cleanup can outlast a mortgage contingency. Buyers who discover the tank on the day before closing either postpone or inherit the file.

Insurance and lending

Standard homeowners policies exclude pollution. There are sometimes state cleanup funds for residential heating-oil tanks, with eligibility rules, caps, and deadlines. Do not assume the fund will adopt you. Ask the state's program what a newly discovered residential tank qualifies for before you count on it. Lenders do not want an open environmental condition. A documented removal with a clean soil sample closes. An open leak report can stall underwriting. Title insurance often excludes underground tanks. Read the exception.

Typical cost ranges

Typical ranges, not quotes.

Tank sweep: often $200 to $500.
Pump, remove, and backfill a simple residential tank with no contamination: commonly $1,500 to $4,000, more under a deck or a tight side yard.
Soil sampling at removal: often included or a few hundred dollars. Do not skip it to save the afternoon. The sample is the document you will need later.
Limited soil removal when the lab comes back hot: often $5,000 to $20,000.
A real remediation with groundwater: five figures and up, and it is too site-specific for a national range to be kind. This is why the sweep happens during the inspection, not during the move.

Pass, negotiate, or investigate

Investigate when the house is old enough to have had oil and you cannot find an abandonment or removal permit. The sweep is cheap relative to the question. Negotiate removal and sampling, or a credit that covers the typical clean-removal range, when a tank is confirmed and samples are not back yet. If samples are already bad, negotiate from the environmental contractor's number, not from the clean-tank number. Pass when there is an open leak case, the fund will not accept it, and the seller wants you to take title before the soil is characterized. The red-flag guide assumes you know which of those situations you are in. The permit search is how you know.

Underground Oil Tanks: The Fill Pipe in the Yard and the Leak You Inherit