What is the definition of “interstitial space” in USTs?

The definition of "interstitial space" refers specifically to the space between the inner and outer walls of a double-walled underground storage tank (UST). This space serves a critical function in monitoring for leaks and providing secondary containment. In a properly designed double-walled tank system, the interstitial space can serve as a barrier against environmental contamination, as any leakage from the inner tank can be detected by monitoring systems placed within this space. This is an essential aspect of USTs to ensure compliance with environmental regulations and to enhance the safety of storing hazardous materials.

The other options do not accurately reflect the specific characteristics of interstitial space. The area surrounding the UST is broader and encompasses more than just the structural integrity of the tank itself. The volume of liquid contained in the UST pertains to the tank's capacity rather than its structural components. The air gap at the top of the tank refers to a separate part of the tank system that relates more to vapor space and does not capture the essence of the interstitial space.