Buyer guide
AST vs. UST: start with the regulatory definition, then the site
Aboveground and underground storage systems solve different site problems and enter different regulatory programs. The first step is understanding what EPA actually considers a UST.
EPA's 10 percent rule is the starting point
EPA defines an underground storage tank system as a tank or combination of tanks and connected underground piping with at least 10 percent of the combined volume underground. That means an installation can create a UST question even when one visible tank sits aboveground if enough connected system volume is underground.
ASTs and USTs enter different federal programs
Federal UST requirements are administered under 40 CFR Part 280 for covered UST systems. Aboveground oil storage can instead become subject to SPCC under 40 CFR Part 112 when the facility meets the non-transportation, capacity, and reasonable-discharge-potential tests. State and local rules can be more stringent or use different definitions.
| Decision factor | AST | UST |
|---|---|---|
| Visual access | Tank shell and many components remain accessible for routine visual inspection. | More of the system is concealed, which changes monitoring and leak-detection strategy. |
| Site footprint | Uses visible surface area and can create setback, traffic, and barrier questions. | Preserves surface space but adds excavation, subsurface work, and closure considerations. |
| Regulatory starting point | Fire code, state AST rules where they exist, and possibly federal SPCC. | Federal and state UST rules if the system meets the applicable definition and is not excluded. |
| Future relocation | Some shop-fabricated AST systems may be easier to replace or relocate, subject to code and inspection requirements. | Removal and replacement typically require excavation and formal closure work. |
Do not choose on purchase price alone
Compare installation cost, civil work, usable site area, vehicle protection, environmental risk, inspection access, expected service life, closure cost, and the regulator that will own the project. A less expensive tank can create a more expensive project if the site forces major civil or permitting work.
Before choosing AST or UST: six decisions
Ask whether the site has enough aboveground space, what the local fire authority permits, whether excavation or groundwater creates added construction risk, how deliveries and dispensing will work, who will maintain inspections and leak monitoring, and what eventual removal will cost. The cheaper tank is not always the less expensive facility.
Practical example: A fleet yard replacing an aging underground system should compare the cost and closure process for the existing UST against an AST design that may require impact protection, fire-code spacing and a different site layout. Confirm the regulatory classification with the agencies before comparing quotes.
Primary sources
- U.S. EPA: Learn About Underground Storage Tanks
- U.S. EPA: UST Technical Compendium
- U.S. EPA: Does SPCC Apply to Your Facility?
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