Traditionally, fire-safety codes have required that aboveground storage tanks be surrounded by a dike. That is, the tanks must have earthen, steel, or concrete walls which create a basin around the tanks. If the tanks should rupture or be overfilled, the containment area created by the surrounding dike would be able to hold the spilled product.
Fire codes also provide for an alternative to diking, which is remote impounding. This is a technique that allows aboveground tank owners to construct what amounts to a drainage ditch, leading away from the tanks and sloping downgrade. The ditch continues to a location removed from the tanks. There, the ditch empties into a pond-like basin called a “retention area.” This area must be located a certain distance from the tank and from nearby buildings and property lines.
If one or more of the tanks protected by this arrangement should rupture, the spilled product, instead of collecting in the diked area around the tanks, would be diverted to the remote impounding pond away from the tanks.
Fire codes spell out specific requirements for designing and constructing remote impounding facilities.