What is an underground storage tank (UST)?

An underground storage tank (UST) is defined as a tank and any underground piping connected to it that has at least 10 percent of its volume underground. This definition is significant because it sets the baseline for regulatory requirements and environmental protection measures that apply specifically to USTs. These tanks typically store liquids such as petroleum products and can pose environmental risks if they leak or are not properly managed.

The parameters set by having at least 10 percent of the tank's volume underground are essential for distinguishing USTs from above-ground storage tanks and ensuring that appropriate regulations are applied to minimize potential contamination of soil and groundwater. Understanding this definition is critical for compliance with state and federal regulations governing USTs, as well as for conducting effective site assessments and risk evaluations.

Other options present options that do not meet the criteria for USTs. The surface tank option does not pertain to underground storage, and containers used exclusively for hazardous waste may not necessarily fit the UST definition if they don't meet the volume criteria or if they are placed above ground. Thus, the clear definition provided in the correct answer encompasses the scope and regulations relevant to underground storage tanks.