An underground storage tank (UST) is any tank, including its connected underground piping, that has at least 10 percent of its total volume beneath the ground surface. Most USTs hold petroleum products like gasoline and diesel at gas stations, but they also store hazardous chemicals, waste oil, and other regulated substances at commercial and industrial sites across the country. There are hundreds of thousands of them in the United States, and because they sit in direct contact with soil and groundwater, they’re subject to strict federal and state regulations designed to prevent leaks.
How Federal Law Defines a UST
The 10 percent threshold is the key legal dividing line. If 10 percent or more of a tank’s combined volume (tank plus connected underground piping) sits below the ground surface, it’s classified as a UST under federal regulations in 40 CFR Part 280. That classification triggers a wide range of requirements for installation, monitoring, leak detection, and eventual closure.
Not every buried tank falls under these rules, though. Federal UST regulations specifically exempt several categories:
- Farm and residential tanks of 1,100 gallons or less holding motor fuel for noncommercial use
- Heating oil tanks storing oil that’s consumed on the same property where it’s stored
- Tanks in underground areas like basements or tunnels, as long as they sit on or above the floor
- Septic tanks and systems collecting stormwater or wastewater
- Small tanks with 110 gallons or less of capacity
- Emergency spill and overfill tanks
These exemptions mean that many residential heating oil tanks and small farm fuel tanks don’t face federal oversight, though state regulations sometimes fill that gap.
What USTs Are Made Of
Older USTs were almost exclusively bare steel, and corrosion was their biggest vulnerability. Steel buried in moist soil acts like a slow battery, gradually corroding from the outside in until holes form and the contents seep into the ground. That’s exactly what happened at tens of thousands of sites from the 1950s through the 1980s.
Since the 1960s, fiberglass-reinforced plastic (FRP) has been an alternative. FRP tanks don’t corrode the way steel does. They’re lighter, resist chemical degradation, and have a high strength-to-weight ratio. Today, most new UST installations use either full FRP construction or a hybrid design: a steel tank with a fiberglass interior coating and an external fiberglass overwrap. This composite approach gives the structural strength of steel with the corrosion resistance of fiberglass.
Steel tanks that remain in service typically rely on cathodic protection to slow corrosion. This works by running a small electrical current through the soil to the tank, which shifts the chemistry at the tank’s surface so it resists rusting. Two systems are common. Sacrificial anode systems attach blocks of a more reactive metal (usually zinc or magnesium) to the tank; those blocks corrode instead of the steel. Impressed current systems use an external power source to push protective current to the tank continuously. Either way, these systems need regular testing to confirm they’re still working.
How Leaks Are Prevented
Leak prevention starts with the tank itself but extends to every point where fuel enters or leaves the system. During fuel delivery, the two biggest risks are spills at the fill pipe and overfilling the tank.
Spill buckets are basins sealed around the fill pipe opening. When a delivery driver disconnects the hose, any fuel left in the hose (a typical delivery hose holds about 14 gallons) drips into the bucket instead of onto the ground. The collected fuel is then pumped out.
Overfill prevention devices work from inside the tank and come in three types. Automatic shutoff devices use a float mechanism to slow and then stop fuel flow when the tank reaches 95 percent capacity. Overfill alarms trigger an audible alert when the tank hits 90 percent full or is within one minute of overfilling, giving the driver time to close the truck’s valve. Ball float valves sit inside the tank’s vent line and physically restrict vapor flow as the tank fills, creating back pressure that slows incoming fuel. These must activate by the time the tank reaches 90 percent full.
Secondary Containment and Monitoring
Modern UST systems often use double-walled construction, essentially a tank within a tank. The space between the inner and outer walls (the interstitial space) is monitored for the presence of liquid or vapor. If fuel leaks from the inner tank, it’s caught in the outer wall before reaching the soil, and sensors detect it.
Federal regulations require secondary containment for any new or replaced tanks and piping installed within 1,000 feet of a community water system or a potable drinking water well. This rule, established by the 2005 Energy Policy Act, reflects the direct connection between UST leaks and drinking water contamination.
Why Leaking Tanks Are a Major Problem
When a UST leaks, the risk to soil and groundwater is high. Petroleum products release volatile organic compounds that move readily through soil and can reach aquifers. The contaminants of greatest concern include benzene, toluene, ethylbenzene, and xylenes (collectively called BTEX), all of which pose health risks at low concentrations. Another problematic compound is MTBE, a fuel additive that dissolves easily in water and can make drinking water undrinkable even at very low levels.
The scale of the problem is enormous. As of March 2024, the EPA had confirmed 575,275 releases from UST sites nationwide. Of those, 518,843 cleanups have been completed, leaving 56,432 sites still awaiting full remediation. Congress funds this work through the Leaking Underground Storage Tank (LUST) Trust Fund, which held a balance of approximately $1.5 billion as of early 2024. In fiscal year 2024, over $62 million from the fund was directed toward cleanup activities, with nearly $53 million going to states for assessing and remediating contaminated sites.
Closing or Removing a UST
When a UST is no longer needed, federal rules require a formal closure process. Owners must notify their state regulatory agency at least 30 days before beginning permanent closure. The tank must then be emptied and cleaned, with all liquids and accumulated sludge removed. After cleaning, the tank is either physically pulled from the ground, filled with an inert solid material like sand or concrete slurry, or closed in place using a method approved by the state agency.
Before closure is complete, the site must be assessed for contamination. This means sampling soil and groundwater in the area most likely to show signs of a release, typically the excavation zone around and beneath the tank. The sampling strategy takes into account what the tank stored, the depth to groundwater, the type of backfill surrounding the tank, and how the closure is being performed.
If that assessment turns up contaminated soil, contaminated groundwater, or free product (liquid fuel sitting on top of the water table), the owner must begin corrective action. This can mean anything from excavating and hauling away contaminated soil to installing groundwater treatment systems that operate for years. The cost of cleanup varies widely, but contamination discovered during closure is one of the most common triggers for expensive, long-term remediation projects, which is one reason the LUST Trust Fund exists.