D.C. Mun. Regs. tit. 20, r. 20-6011
Interstitial monitoring between an UST system and a secondary barrier immediately around or beneath the UST system shall meet the requirements of this section.
The owner or operator of an UST system installed or replaced after February 8, 2007 shall check for evidence of a release at least once every thirty (30) days using interstitial monitoring.
An interstitial monitoring system shall be designed, constructed, and installed to detect a leak from any portion of the tank or piping that routinely contains a regulated substance.
Where vacuum monitoring is utilized, the vacuum shall be maintained at not less than five (5) inches of mercury, and shall not exceed manufacturer's instructions.
If the vacuum falls below five (5) inches of mercury, the owner or operator shall follow the requirements of Chapter 62 for a suspected release.
A vacuum shall not be re- instituted more frequently than once every three (3) months without prior approval of the Department.
For double-walled USTs, the sampling or testing method shall be capable of detecting a leak through the inner wall in any portion of the tank that routinely contains a regulated substance.
For tanks with an internally fitted liner, an automated device shall be used that is capable of detecting a leak between the inner wall of the tank and the liner. The liner shall be compatible with the substance stored.
For UST systems with a secondary barrier within the excavation zone, the secondary barrier shall meet the following requirements:
An UST with a secondary barrier within the excavation zone shall use a sampling or testing method that is capable of detecting a release between the UST and the secondary barrier.
The testing or sampling method used shall not be rendered inoperative or less effective by groundwater, rainfall, soil moisture, or any other known interference to the point that a release could go undetected for more than thirty (30) days.
The owner or operator of an UST system with a secondary barrier within the excavation zone shall assess the facility to ensure that the secondary barrier is always above the groundwater and not located in a twenty- five (25) year floodplain, unless the barrier and monitoring designs are designed for use under those conditions.
The monitoring wells for each UST with a secondary barrier within the excavation zone shall be clearly marked and secured to avoid unauthorized access and tampering.
Interstitial monitoring alarms are an unusual operating condition that shall be reported as specified under § 6202.5.
If a system test confirms a leak in either the inner or outer tank wall or liner, effectively rendering the tank a single wall tank, the owner or operator shall repair, replace, upgrade, or close the UST as specified in § 6203.
D.C. Mun. Regs. tit. 20, r. 20-6011