Secondarily-contained tanks that use interstitial monitoring must be tested every _____.

• A. Every year
• B. Every two years
• C. Three years
• D. Five years

Answer: (C)

Interstitial monitoring detects leaks by watching the space between the primary tank and its secondary containment. Since components like sensors, seals, and connections can wear or drift over time, the monitoring system has to be checked on a regular schedule to ensure it will actually detect a release if one occurs. The standard interval for this verification is every three years. This cadence provides a practical balance: it keeps leak-detection capability reliable without imposing excessive testing costs or downtime, while still catching degradation before a release goes unnoticed. Testing more frequently (such as annually) is usually more than necessary for most systems, and waiting longer (like every five years) increases the risk of missing a developing problem.