How to Spot the Signs of a Buried Oil Tank

Why this matters

Millions of older houses heated with fuel oil at some point, and when they converted to gas or a heat pump the buried tank in the yard was often left where it was: sometimes properly closed, often just disconnected with oil still in it. A landscaper setting a patio, a plumber trenching a sewer lateral, an electrician running conduit to a garage and a fence crew augering posts all dig in the band of yard where those tanks sit.

A leaking tank is an environmental problem governed by state rules and a real estate problem that surfaces at sale, and a tank punctured or crushed by your work turns the owner's problem into yours. The signs are visible above ground on most houses that have one. This HowTo is the read you do before you dig, and the steps run the way a visit does: the question, the outside, the inside, the ground, then the hold, the paper and the route.

The steps

1. Before any digging near an older house, ask the question out loud. "Did this house ever heat with oil?" Reason: the owner often knows, and a yes changes where you look. What breaks if you skip it: the owner bought the house five years ago, never thought about it, and you find out with an auger. Note that 811, the one-call utility locate, does not mark a private oil tank or its lines; they are not utility-owned. A clean locate tells you nothing about a tank.

2. Walk the foundation for the fill pipe and the vent. A buried tank needs two pipes to the surface: a fill pipe, usually about 1-1/2 to 2 inches, ending at or just above grade with a screw cap, often within a few feet of the foundation on the side the delivery truck could reach; and a smaller vent pipe, often run up the outside of the foundation or the wall, ending in a return bend or a mushroom-style cap a couple of feet up. Look in lawns, in planting beds, along the driveway, and in concrete or asphalt where a cap may sit flush. A vent pipe on the wall with no indoor tank behind it is a strong clue. Reason: these are the only parts of the tank you can see, and a cut-off stub still marks where the tank was. What breaks if you skip it: the stub under the mulch is the one your bucket finds.

3. Look at the foundation wall from inside. On the wall nearest the fill pipe, look for two small lines (usually 3/8-inch copper, the supply and return between tank and burner) entering through the wall, cut off or crimped, or patched holes where they were. Look for an old oil filter bracket on a joist, a gauge or a tank-whistle fitting, or an abandoned indoor tank. Reason: the lines show the tank was buried outside rather than sitting in the basement, and which wall it is behind. What breaks if you skip it: you find an abandoned indoor tank, assume that was the only tank, and miss that the copper lines also run out through the wall.

4. Read the heating history the house still carries. A chimney serving nothing, or a gas furnace venting into a chimney sized for an oil burner; a gas meter that looks newer than the house; a capped flue thimble in the basement. None proves a tank, but together they say the house converted, and conversions are where tanks get left behind. See related: Dating a House by What You Find in It.

5. Look at the ground itself, and treat contamination cues as a stop. A rectangular settlement in the lawn, a patch where grass dies back, dark stained soil, an oily sheen in a sump pit, an oil odor in the basement or in soil you have already turned. Reason: a tank that leaks changes the ground above and below it. If you see stained soil, a sheen or a fuel odor in a hole, stop digging there, keep the spoil where it is rather than spreading or hauling it, keep sources of ignition back from any strong vapor, and tell the owner the same day. Many states require a discharge of petroleum to be reported to the state environmental agency within a short window; that duty usually falls on the owner, and your record of what you saw is what they report from.

6. Do not probe for it with a rod. Pushing a steel probe into the ground to find the tank is the tempting next move and the wrong one. A tank that has been in the ground for fifty years may have a shell thinned by corrosion, and a probe can puncture it; the same probe can strike a buried electric line, the oil lines themselves or a gas service that 811 did mark but was off by a foot. And a probe hitting something hard tells you there is a rock as often as a tank. Locating a tank is done by an environmental professional with a magnetometer or metal detector sweep and ground-penetrating radar, and sampling the soil is done by the same people.

7. Hold the work off the suspected area, and say how far. A common shop default is no excavation, post setting or heavy equipment within 10 feet of a fill or vent pipe until the tank's position is known. Tune it to your work, but write a number on the ticket. Reason: you do not know which end of the tank the fill pipe is on or how long the tank is. What breaks if you skip it: the patio goes on top of the tank, and the first buyer's tank sweep finds it under the new pavers.

8. Ask for the paper. A tank that was properly closed usually leaves a record: a removal receipt and disposal ticket, an abandonment-in-place certificate (the tank emptied, cleaned and filled with sand or foam), a fire department or building permit, a tank sweep report from a past sale, a seller's disclosure. Reason: "the tank was taken care of" in conversation is not evidence; a dated document is. What breaks if you skip it: you rely on the owner's memory of what the previous owner said.

9. Record it and route it. Write it as "suspect buried fuel oil tank: fill pipe stub 3 ft from foundation at northeast corner, vent pipe on north wall, cut copper lines through north basement wall; no closure paperwork; not probed or excavated." Photograph each pipe with a tape for scale. Route the owner to an environmental professional for a tank sweep and, if a tank is found, to a contractor licensed for tank removal or closure where the house is. See related: How to Photograph and Record a Suspect Material.

10. Make the fill pipe undeliverable, through the owner. Recommend the owner have the fill and vent pipes removed or permanently sealed as part of closure. Oil companies have pumped deliveries into fill pipes of tanks that were removed or disconnected, filling a hole in the ground or a basement. Do not cut or remove the pipe yourself; it may still lead to a tank with oil in it.

Who regulates it

Federal underground storage tank rules (40 CFR Part 280) exclude tanks storing heating oil for use on the premises where it is stored, so a home heating oil tank is generally outside them. The rules that apply are state and local. They range widely: New Jersey, for one, runs a state program for residential heating oil tank discharges with certified contractors doing the work, while other states leave residential tanks largely to the local fire code and to buyers, lenders and insurers who ask for a sweep at sale. Removal or abandonment of a tank is commonly regulated through the fire code, often NFPA 31 in the edition your jurisdiction has adopted, which reaches the owner and their contractor through a local permit. Check your state environmental agency and your local fire official. This is not legal advice, and whether a past leak is the owner's obligation to investigate is a question for the owner's attorney.

Worked example: a patio bid on a 1948 house

A landscaper is bidding a 16 by 20 foot paver patio against the back wall of a 1948 house, starting at the foundation and running 16 feet out into the yard. The house heats with gas.

Steps 2 and 3 turn up a capped 2-inch pipe stub in the lawn 3 feet off the back wall, about 6 feet in from the corner, and a 1-1/4 inch vent pipe on the back wall above it. In the basement, two cut copper lines come through that same wall.

TECH: Before I price this, did the house ever heat
      with oil?
CUSTOMER: Before us, maybe. We've always had gas.
TECH: There's a fill pipe and a vent out back, and
      the old oil lines come through your basement
      wall. That usually means a tank is buried there.
CUSTOMER: Is it leaking?
TECH: Nothing I saw says so. I can't tell from up
      here, and I don't want to find out with a
      shovel. Do you have papers from when you bought
      it, a tank sweep or a removal receipt?
CUSTOMER: I'd have to look.
TECH: If it was removed or filled, the patio's fine.
      If it's still there, it's better you know before
      it's under pavers.

The arithmetic of the hold: the fill pipe sits 3 feet out and 6 feet in from the corner, and the patio covers everything from the wall to 16 feet out along the first 20 feet of wall. A 10-foot hold around the fill pipe reaches from the wall out to 13 feet, and along the wall from the corner (6 minus 10 falls past it) to 16 feet. Clipped to the patio's footprint, that circle takes in about 180 of the patio's 320 square feet (16 times 20), a little over half, including the whole strip along the wall from the corner to about 15-1/2 feet. There is no version of this patio that avoids the area.

A typical buried residential tank in that era might be a 550-gallon cylinder about 4 feet in diameter and 6 feet long, with its top a foot or two below grade; those are illustrative proportions, not this tank's. A patio base dug 8 inches deep may not reach the tank top, which is the trap: the dig seems safe, and then a plate compactor runs over a corroded shell, and the finished patio sits on the only access to a tank a future buyer's lender may require removed.

The call. The bid goes in with the patio held pending the owner's paperwork or a tank sweep. Three outcomes, and what each does to the job:

• Paperwork shows the tank was removed and the hole backfilled: the patio proceeds, and the stubs are removed as part of it with the owner's agreement.
• Paperwork shows abandonment in place (filled with inert material): many owners proceed, but the patio now sits over a closed tank, and some buyers and lenders ask for removal anyway. That is the owner's decision with full information, written on the ticket.
• No paperwork: the sweep comes first. If a tank with product is found, it is closed or removed by a licensed contractor, with soil sampling, before the patio is built.

What would change the answer. A house that has always been on gas, with no fill pipe, no vent and no cut lines, gets no hold; the absence of every sign is evidence. A fill pipe with fresh-looking delivery tickets on the fridge means a tank in service, which is a different question: work around an active tank with its owner and supplier informed. And a fuel odor in the first shovel of soil moves the whole job to step 5 immediately.

References

• See related: Recognize, Record, Route - the Rule for Legacy Materials; How to Plan Work Around a Suspect Material You Cannot Test Today; Telling a Customer About an Old Material Without Scaring or Understating
• EPA, Underground Storage Tank regulations, 40 CFR Part 280 (definition excluding heating oil for consumptive use on the premises)
• NFPA 31, Standard for the Installation of Oil-Burning Equipment, in the edition adopted by your jurisdiction
• Your state environmental agency's heating oil tank and petroleum discharge guidance