Release Detection for Underground Storage Tanks (USTs) - Interstitial Method

EPA allows owners and operators to use this interstitial method – secondary containment with interstitial monitoring – for detecting releases from their UST systems. 

This method detects leaks in the space between the UST and a second barrier. The federal UST regulations describe general performance requirements for interstitial monitoring with double walled USTs, USTs fitted with internal liners, and USTs using interception barriers.

• Secondary containment
• Interstitial monitors
• What are the regulatory requirements?
• Will it work at your site?
• Anything else you should consider?
• Will you be in compliance?

Secondary Containment

• Secondary containment provides a barrier between the tank and the environment. The barrier holds the leak between the tank and the barrier so that the leak is detected. The barrier is shaped so that a leak will be directed towards the interstitial monitor.
• Barriers include:
  • Double walled or jacketed tanks, in which an outer wall partially or completely surrounds the primary tank;
  • Internally fitted liners (bladders); and
  • Leakproof excavation liners that partially or completely surround the tank.
• Clay and other earthen materials cannot be used as barriers.

Interstitial Monitors

• Monitors are used to check the area between the tank and the barrier for leaks and alert the operator if a leak is suspected.
• Some monitors indicate the physical presence of the leaked product, either liquid or gaseous. Other monitors check for a change in condition that indicates a hole in the tank, such as a loss of vacuum or a change in the level of a monitoring liquid between the walls of a double walled tank.
• Monitors can be as simple as a dipstick used at the lowest point of the containment to see if liquid product has leaked and pooled there. Monitors can also be sophisticated automated systems that continuously check for leaks.

What are the regulatory requirements?

• The barrier must be immediately around or beneath the tank.
• The interstitial monitor must be checked at least once every 30 days.
• A double walled system must be able to detect a release through the inner wall.
• An excavation liner must:
  • Direct a leak towards the monitor;
  • Not allow the specific product being stored to pass through it any faster than 0.000001 cm/sec;
  • Be compatible with the product stored in the tank;
  • Not interfere with the UST's cathodic protection;
  • Not be disabled by moisture;
  • Always be above the groundwater and the 25-year flood plain; and
  • Have clearly marked and secured monitoring wells, if they are used.
• Beginning on October 13, 2018 you must either test your containment sumps used for interstitial monitoring at least once every three years to ensure the equipment is liquid tight by using vacuum, pressure, or liquid testing or use a double-walled containment sump where the space between the sump is periodically monitored.
  
  Testing must be performed in accordance with manufacturer’s requirements; a nationally recognized code of practice; or requirements determined by your implementing agency to be no less protective of human health and the environment.
• Beginning on October 13, 2018 you must perform the following, as applicable, on your release detection equipment annually to make sure it is working properly:

For hand held non-electronic equipment:

• Keep records of these checks for one year
• Check for operability and serviceability (includes dipsticks)

For other equipment:

• Verify the system configuration of the controller
• Test alarm operability and battery backup
• Inspect sensors for residual build-up
• Ensure sensor communication with controller
• Keep records of these tests for three years

These activities must be performed in accordance with manufacturer’s requirements; a nationally recognized code of practice; or requirements determined by your implementing agency to be no less protective of human health and the environment.

Will it work at your site?

In areas with high groundwater or a lot of rainfall, it may be necessary to select a secondary containment system that completely surrounds the tank to prevent moisture from interfering with the monitor.

Anything else you should consider?

This method works effectively only if the barrier and the interstitial monitor are installed correctly. Therefore, trained and experienced installers are necessary.

Will you be in compliance?

USTs installed or replaced after April 11, 2016 must be secondarily contained and use interstitial monitoring.

When installed and operated according to the manufacturer's specifications, secondary containment with interstitial monitoring meets the federal leak detection requirements for new and existing USTs. Operation of the monitoring device at least once each month fulfills the requirements for the life of the tank. Secondary containment with interstitial monitoring can also be used to detect leaks from piping.