D.C. Mun. Regs. tit. 20, r. 20-6009
A release detection method that monitors or tests for vapors within the soil gas of the excavation zone shall meet the requirements of this section.
The materials used as backfill (such as gravel, sand, crushed rock, or similar materials) shall be sufficiently porous to readily allow diffusion of vapors from releases into the excavation zone.
The stored regulated substance, or a tracer compound placed in the tank system, shall be sufficiently volatile to result in a vapor level that is detectable by the monitoring devices located in the excavation zone in the event of a release from the tank.
The monitoring device measuring vapors 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 fifteen (15) days.
The level of background contamination in the excavation zone shall not interfere with the vapor monitoring method used to detect releases from the tank.
The vapor monitor used shall be designed and operated to detect any significant increase above the background concentration in the excavation zone of:
Before using vapor monitoring, the owner or operator shall assess the excavation zone to ensure compliance with §§ 6009.2 through 6009.6 and determine the number and positioning of monitoring wells required to detect releases within the excavation zone from any portion of the tank that routinely contains regulated substances. The owner or operator shall install monitoring wells in accordance with the assessment before operating the UST system.
Monitoring wells shall be clearly marked and secured to avoid unauthorized access and tampering. Monitoring wells shall not be marked in any way that could be associated with a regulated substance stored at the facility.
An owner or operator of an UST system installed after February 8, 2007, may use vapor monitoring as a release detection method only if secondary containment and interstitial monitoring methods are also used.
D.C. Mun. Regs. tit. 20, r. 20-6009