Which of the following requires secondary containment?

The requirement for secondary containment is particularly relevant for new underground storage tank (UST) systems. Secondary containment refers to a structure or system that is designed to prevent the release of hazardous substances into the environment in case of a leak or spill from the primary containment, which in this case is the UST itself.

In the context of new UST systems, regulatory agencies require secondary containment to provide an additional layer of safety. This means that if there is a failure of the tank (the primary containment), the secondary containment system is designed to capture the leaked material, preventing it from contaminating the surrounding soil and groundwater. This is crucial for protecting public health and the environment, as spills from UST systems can lead to serious environmental hazards.

While existing UST systems may have different regulations depending on their condition and age, and old underground pipes may not be covered by such requirements, new systems are mandated to include these safety features as a standard part of their design. Similarly, UST systems that are removed may not require secondary containment as they are no longer in operation or capable of leaking.

Thus, the correct answer emphasizes the importance of secondary containment in new UST systems to enhance environmental protection against potential leaks and spills.