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An aboveground storage tank (AST) is a tank whose entire surface area is completely aboveground. Also, the entire surface area of the tank (including the tank bottom) can be visually inspected (40 CFR 260.10).
An underground storage tank (UST), on the other hand, is a tank whose entire surface area is totally below the surface of and covered by the ground (40 CFR 260.10). Note that EPA underground storage tank regulations at 40 CFR part 280 define UST as “a tank (including piping) which has at least 10 percent of its volume underground.” USTs containing petroleum or hazardous substances are regulated by (40 CFR 280).
Unlike USTs that are covered under an entire section of regulations, in addition to select areas under the hazardous waste regulations, ASTs, at the federal level, are regulated in a somewhat piecemeal way. Consequently, it is necessary to look in several different places for AST requirements.
Concerning the storage of hazardous waste in ASTs, you must look in the hazardous waste regulations (Parts 260 through 268) to determine what is required. Parts 262, 264, and 265 of the Resource Conservation and Recovery Act (RCRA), Subtitle C, also impact ASTs as these requirements apply to hazardous waste generators and treatment, storage, and disposal facilities.
If you store oil in an AST, you may be regulated under the Spill Prevention, Control and Countermeasure (SPCC) regulations at 40 CFR Part 112. The SPCC regulations establish procedures to prevent the discharge of oil into navigable waters.
Some ASTs fall under OSHA requirements (29 CFR 1910.106) if they contain flammable or combustible liquids. The rules apply to liquids with flashpoints below 200° F. In this case, the objective of the regulations is to protect workers from fires, rather than protecting the environment from spills.
Aboveground tank means a tank that is situated in such a way that the entire surface area of the tank is completely above the plane of the adjacent surrounding surface and the entire surface area of the tank (including the tank bottom) is able to be visually inspected.
Flammable liquid means any liquid having a flashpoint at or below 199.4°F (93°C). Flammable liquids are divided into four categories as follows:
When liquid with a flashpoint greater than 199.4°F (93°C) is heated for use to within 30°F (16.7°C) of its flashpoint, it shall be handled in accordance with the requirements for a Category 4 flammable liquid.
Freeboard means the vertical distance between the top of a tank or surface impoundment dike, and the surface of the waste contained therein.
Operator means the person responsible for the overall operation of a facility.
Owner means the person who owns a facility or part of a facility.
Tank means a stationary device, designed to contain an accumulation of hazardous waste which is constructed primarily of non-earthen materials (e.g., wood, concrete, steel, plastic) which provide structural support.
Tank system means a hazardous waste storage or treatment tank and its associated ancillary equipment and containment system.
Underground tank means a device meeting the definition of “tank” in §260.10 whose entire surface area is totally below the surface of and covered by the ground.
The majority of regulations affecting ASTs are found at the state and local level. Check with your state and local agencies to see if there are more stringent AST requirements that you need to know about.
WY Department of Environmental Quality: https://deq.wyoming.gov/shwd/storage-tank/
Wyoming State Fire Marshal: https://wsfm.wyo.gov/