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A basic underground storage tank is a tank that holds petroleum or other products, and has 10% or more of its mass under the ground. They are most commonly made out of steel or aluminum, but developments in composite materials have advanced the construction of underground storage tanks, and they are far less likely to leak or rust. The US Environmental Protection Agency (EPA) has regulated the use of an underground storage tank for commercial purposes, and legislation now states that groundwater has to be monitored around the perimeter of the wells to check for any leakage which might pollute the water table.
When a person buys a tank of gas at a fuel pump, he or she does not notice the large underground storage tank that is holding the fuel. These large tanks can generally hold anywhere from a few hundred gallons to tens of thousands of gallons of product. Mostly used for petroleum, an underground storage tank can hold any type of liquid.
Rural homes also may have a smaller underground storage tank for fuel for farm equipment. These mostly do not fall under US EPA regulation. The US EPA defines a regulated underground storage tank as a holding tank for commercial purposes which holds 1,100 gallons (4,164 liters) of liquid or more, so most residential or farm underground storage tanks are not regulated.
Because of the capacity for the tanks to rust and degrade over time, many tanks have leaked throughout the US and beyond, polluting soil and ground water. In the 1980s, the US EPA began regulating the use and leak detection efforts at underground storage tank sites. By 1989, the US EPA mandated that property owners must remove, repair or replace underground storage tanks on their property.
Property owners who do not want the cost of underground storage tanks may convert to an above-ground storage tank to allow for closer monitoring of leaks and easier maintenance. Many states, however, do not allow fuel to be resold from an above-ground storage tank. Therefore, options are limited and the costs associated with locating, installing, repairing or removing an underground storage tank can be significant.
Some leak detection methods can be built into the construction of the underground storage tank, but sometimes the only way to determine if there was or is an underground storage tank leak is to install monitoring wells around the piece of land. Devices like bailers or pumps will bring up samples of the ground water table. The samples are then tested by labs for priority pollutants defined by the US EPA.