Ground water monitoring
Secondary containment with interstitial monitoring
Automatic tank gauging systems
Tank tightness testing and inventory control
Manual tank gauging
Leak detection for underground suction piping
Leak detection for pressurized underground piping
Statistical inventory reconciliation
Secondary Containment With Interstitial Monitoring
NOTE: Secondary containment with interstitial monitoring is required for hazardous substance USTs, and the requirements are different from those for petroleum USTs. Consult your state or local agency for regulations on hazardous substance USTs.
Will I be in compliance?
When installed and operated according to manufacturer's specifications, secondary containment with interstitial monitoring meets the Federal leak detection requirements for new and existing underground storage tanks (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 (see the later sections on leak detection for piping).
You should find out if State or local requirements allow all of the types of secondary containment and interstitial monitoring or have other restrictions that are different from those described below. In some jurisdictions, secondary containment is required for all USTs.
How does it work?
Secondary containment provides a barrier between the tank and the environment. The barrier holds the leak between the tank and the barrier long enough for the leak to be detected. The barrier is shaped so that a leak will be directed towards the monitor. Barriers include:
- Double-walled tanks, in which an outer tank partially or completely surrounds the primary tank;
- Leakproof excavation liners that partially or completely surround the tank;
- Leakproof liners that closely surround the tank (also known as "jackets"); and
- Concrete vaults, with or without lining.
Clay and other earth materials cannot be used as barriers.
Interstitial 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 pressure or a change in the level of water 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 10-6 cm/sec;
- Be compatible with the product stored in the tank;
- Not interfere with the USTs cathodic protection;
- Not be disabled by moisture;
- Always be above the groundwater and the 25-year flood plain; and
- Have clearly marked and locked monitoring wells, if they are used.
Will it work at my 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.
What other information do I need?
Correct installation is fairly difficult yet is crucial both for the barrier and the interstitial monitor. Therefore, trained and experienced installers are necessary.
The purchase of secondary containment with interstitial monitoring is similar to any other major purchase. You should "shop around," ask questions, get recommendations, and select a method and company that can meet the needs of your UST site.
How much does it cost?
The costs of the secondary containment depend on the size and number of the tanks, how much of the tank is surrounded by the barrier, the product, the type of containment, and the site conditions. The cost of the interstitial monitor depends on the number of tanks, degree of automation, and type of monitor.
Possible costs for containment and monitoring at a typical station with three 10,000-gallon tanks:
- Monitored double-walled tanks cost $15,000 - $30,000 above the cost of single-walled, unmonitored tanks; or
- An excavation liner (not covering the top of the tank) and monitoring could cost $10,000 - $16,000.