D.C. Mun. Regs. tit. 20, r. 20-6010
A release detection method that tests or monitors for regulated substances in the groundwater or in the tank excavation zone shall meet the requirements of this section.
The regulated substance stored shall be immiscible in water and have a specific gravity of less than one (1).
The groundwater shall never be more than twenty feet (20 ft) from the ground surface, and the hydraulic conductivity of the soil(s) between the UST system and the monitoring wells or devices shall not be less than one hundredth of a centimeter per second (0.01 cm/s). The soil should consist of gravel, coarse to medium sand, coarse silt, or other permeable materials.
The slotted portion of the monitoring well casing shall be designed to prevent the migration of natural soils or filter pack into the well, while allowing entry of any regulated substance on the water table into the well, under both high and low ground water conditions.
Monitoring wells shall be sealed from the ground surface to the top of the filter pack in accordance with the requirements of 21 DCMR Chapter 18.
Monitoring wells or devices shall intercept the excavation zone or be as close to the excavation zone as is technically feasible.
Before using groundwater monitoring methods, the owner or operator shall assess the excavation zone and area immediately below the excavation zone to ensure compliance with §§ 6010.2 through 6010.6, and determine the number and position of monitoring wells or devices that will detect releases within the excavation zone from any portion of the tank that routinely contains a regulated substance. The owner or operator shall install monitoring wells or devices in accordance with the assessment before operating the UST system. A minimum of two (2) monitoring wells shall be required in each excavation zone.
The continuous monitoring devices or manual methods used shall be capable of detecting the presence of at least one eighth (1/8) of an inch of free product on top of the groundwater in a monitoring well.
Each monitoring well shall be clearly marked and secured to avoid unauthorized access and tampering.
An owner or operator of an UST system installed after February 8, 2007, may use groundwater monitoring a release detection method only if secondary containment and interstitial monitoring methods are also used.
D.C. Mun. Regs. tit. 20, r. 20-6010