A double walled tank has a space between its walls. Watching that space is the most direct release detection method there is, because product arriving in it has left the tank and has not yet reached the ground. 40 CFR 280.43(g) sets what the method has to achieve, and a set of state deadlines is currently moving the whole approach from periodic testing toward continuous monitoring.
What the rule asks
280.43(g) allows interstitial monitoring between the UST system and a secondary barrier immediately around or beneath it, but only if the system is designed, constructed and installed to detect a leak from any portion of the tank that routinely contains product, and meets one of two further requirements.
For double walled systems, the sampling or testing method has to be able to detect a leak through the inner wall in any portion of the tank that routinely contains product.
For systems with a secondary barrier within the excavation zone, the method has to detect a leak between the UST system and that barrier, and the barrier itself has to qualify. It must be artificially constructed material, sufficiently thick and impermeable, at least 10 to the minus 6 centimetres per second for the substance stored, to direct a leak to the monitoring point and permit its detection. It also has to be compatible with the product stored.
Read those conditions as a design specification rather than a monitoring one. A liner that does not direct a leak to the monitoring point is a liner that hides the leak somewhere else.
Testing the containment is a different job
Two words get used interchangeably and should not be. Monitoring the interstice is the ongoing release detection method. Testing the interstice establishes that the containment is still intact.
Connecticut separates them on its periodic testing table. Secondary containment testing on tank and piping interstices runs at three years under RCSA 22a-449(d)-103(d)(2). Its note 6 exempts double walled components that use continuous interstitial monitoring automatically covering both primary and secondary containment, such as brine filled or vacuum systems, on the reasoning that a system continuously watching both walls is already answering the question the periodic test asks.
The federal rule makes a similar trade in 280.35 for spill buckets and sumps: monitor both walls at least as often as the walkthroughs and the three year liquid tightness test does not apply.
The direction of travel
Connecticut's table carries two dates worth putting in a diary.
From August 5, 2025, all newly installed tanks must have continuous interstitial monitoring that covers both primary and secondary containment, so that inner and outer walls are continuously monitored using technology such as inert gas, liquid, or vacuum.
From May 7, 2027, all interstitial spaces must be continuously monitored for double walled tanks and piping. Tanks retrofitted with continuous interstitial monitoring are treated as category 3A on the table, which carries its own testing profile.
A site with double walled tanks and sensors in the sumps rather than continuous monitoring of the interstice has a project to plan, not just a date to note. Retrofitting brine or vacuum monitoring is capital work with lead times, and 2027 is the deadline rather than the start.
What a wet interstice means
A sensor calling out in an interstice is telling you the space is no longer dry. What put it there is a separate question with three common answers.
- Groundwater, through a failed outer wall. The tank is compromised from the outside, and the product is still contained.
- Rain or wash water, through a poorly sealed fitting or a cracked riser. Not a tank problem at all, though it is a problem.
- Product, through the inner wall. The reason the monitoring exists.
The three look identical on an alarm panel and have very different consequences. Sampling and identifying what is in the space is what separates a maintenance call from a suspected release, and the timing matters, because a confirmed release starts a reporting clock measured in hours rather than days.
How FastDragon handles it
The site profile records whether tanks and piping are double walled and whether the interstice is monitored, and the calendar builds from that. A site with continuous monitoring on both walls does not get raised a periodic containment test it is exempt from, and the exemption shows on the record with its reason.
Interstitial alarms and the daily tank figures feed the same view, so an interstice going wet and a reconciliation drifting are visible together rather than in two systems. Where a reading needs somebody to look, it becomes an issue with an owner and a date rather than an alarm somebody silenced.
Common questions
What is the interstice?
The space between the two walls of a double walled tank or pipe. Interstitial monitoring watches that space, on the logic that product reaching it has left the inner wall but has not reached the ground. Under 40 CFR 280.43(g)(1), for double walled systems the sampling or testing method has to be able to detect a leak through the inner wall in any portion of the tank that routinely contains product.
Does a liner in the excavation count?
It can. 280.43(g)(2) covers a secondary barrier within the excavation zone rather than a second wall on the tank, and sets conditions: the barrier must be artificially constructed material sufficiently thick and impermeable, at least 10 to the minus 6 cm per second for the substance stored, to direct a leak to the monitoring point and permit its detection, and it must be compatible with the product stored.
How often is the interstice tested?
Testing the containment and monitoring it are two different things. Connecticut's periodic testing table puts secondary containment testing on tank and piping interstices at three years under RCSA 22a-449(d)-103(d)(2), with an exemption at note 6 for double walled components using continuous interstitial monitoring that automatically monitors both primary and secondary containment, such as brine filled or under vacuum.
What changes in 2027?
Connecticut's table carries a note that as of May 7, 2027, all interstitial spaces must be continuously monitored for double walled tanks and piping, and that tanks retrofitted with continuous interstitial monitoring will be treated as category 3A. It also states that from August 5, 2025 all newly installed tanks must have continuous interstitial monitoring covering both the inner and outer walls.
Is a wet interstice always a leak?
No, and that is the reason it needs investigating rather than assuming. Water in an interstice can come from groundwater through a failed outer wall, from rain through a poorly sealed fitting, or from product through the inner wall. The three have different consequences and the same appearance in a sump. What the sensor tells you is that the space is no longer dry.