In addition to the inventory control requirements in Regulation .04 of this chapter, an owner and an operator of a UST system shall perform one or a combination of the methods of release detection described in §§B—H of this regulation at least monthly. Statistical Inventory Reconciliation. An owner and an operator of a UST system utilizing statistical inventory reconciliation as a monthly method of release detection: May only use statistical inventory reconciliation if the UST system is a metered UST system; Shall use statistical inventory reconciliation to detect a spill, release, or discharge through application of statistical principles to inventory data, or another test of equivalent performance, with a probability of detection of not less than 0.95 and a probability of false alarm of not more than 0.05; Shall use a method of statistical inventory reconciliation that meets the following requirements: Uses inventory volume measurements collected in accordance with COMAR 26.10.04.01; Produces a quantitative result with a calculated leak rate; Is capable of detecting: A leak rate of 0.2 gallon per hour; or A spill, release, or discharge of 150 gallons within a month; and Uses a threshold that does not exceed one-half the minimum detectible leak rate; and Shall have a Department-approved third party conduct the statistical inventory reconciliation. Manual Tank Gauging. At least weekly, an owner and an operator of a UST system shall perform manual tank gauging using a storage tank gauging stick maintained in accordance with COMAR 26.10.04.01 as follows: Take UST liquid level measurements at the beginning and ending of an at least 36-hour period, during which liquid may not be added to or removed from the UST; Base the level measurements on an average of two consecutive stick readings at both the beginning and ending of the period; and Using a UST gauge chart specific to the UST gauged, convert the liquid levels to the gallons present in the UST and review for any variations between the starting and ending measurements. An owner and an operator of a UST system shall report a suspected spill, release, or discharge and follow the investigation and confirmation procedures in COMAR 26.10.08 if the variation between beginning and ending manual tank gauging measurements performed in accordance with §C(1) of this regulation exceeds the weekly or monthly standards in the following table: An owner and an operator of a UST system may use manual tank gauging: As the sole method of release detection if the nominal capacity of a UST is 550 gallons or less; and For a UST with a nominal capacity of 551 to 2,000 gallons only in conjunction with conducting a precision tightness test, as defined in COMAR 26.10.02.02, at least every 5 years in accordance with the requirements of COMAR 26.10.03.05. An owner and an operator of a UST system may not use manual tank gauging to meet the requirements of this chapter for a UST that has a nominal capacity of more than 2,000 gallons. Precision Tightness Testing. At least monthly, an owner and an operator of a UST system shall conduct a precision tightness test, as defined in COMAR 26.10.02.02, as a method of release detection in accordance with the requirements of COMAR 26.10.03.05. Automatic Tank Gauging. At least monthly, an owner and an operator of a UST system shall perform automatic tank gauging using equipment that tests for the loss of regulated substance and conducts inventory control and that meets the following requirements: The automatic regulated substance level monitor test is capable of detecting a 0.2 gallon per hour leak rate from any portion of the UST; The automatic tank gauging equipment meets the inventory control, or another test of equivalent performance, requirements of COMAR 26.10.04.01, and Regulation .04 of this chapter; and An automatic tank gauging system tests for loss of a regulated substance by operating in one of the following modes: In-tank static testing conducted at least once monthly; or Continuous in-tank leak detection operating on an uninterrupted basis or operating within a process that allows the automatic tank gauging system to gather incremental measurements to determine the leak status of the UST at least once monthly. Groundwater Monitoring. Beginning June 13, 2022, an owner and an operator of a UST system may only use groundwater monitoring as a monthly method of release detection with prior written approval from the Department. An owner and an operator of a UST system shall conduct groundwater monitoring in accordance with the following requirements: Perform a site assessment 60 days before the implementation of the groundwater monitoring method; Provide a written report of the site assessment to the Department upon request and, beginning June 13, 2022, ensure the written report is signed by a professional engineer or professional geologist, or equivalent licensed professional with experience in environmental engineering, hydrogeology, or other relevant technical discipline acceptable to the Department; and Demonstrate that the testing or monitoring to detect a regulated substance on the groundwater meets all of the following requirements: The regulated substance stored is immiscible in water and has a specific gravity of less than one; Groundwater is never more than 15 feet from the ground surface and the hydraulic conductivity of the soil or soils between the UST system and the monitoring wells or devices is not less than 0.01 centimeter/second, with the soil consisting of gravels, coarse to medium sands, coarse silts, or other permeable materials; The slotted portion of the monitoring well casing is designed to prevent migration of natural soils or filter pack into the well and to allow entry of regulated substance on the water table into the well under both high and low groundwater conditions; Monitoring wells are sealed from the ground surface to the top of the filter pack; Monitoring wells or devices intercept the UST excavation zone or positioned as close to the excavation zone as is technically feasible; The method of measuring the contents of the well, whether automatic or manual, is capable of detecting the presence of at least 1/8 of an inch of free product on top of the groundwater in the monitoring wells; At least monthly, the contents of the groundwater monitoring well are measured; Within and immediately below the UST system excavation zone, the site is assessed to ensure compliance with the requirements in §F(2)(c)(i)—(v) of this regulation and to establish the number and positioning of monitoring wells or devices that will detect discharges or releases from any portion of the UST system; Monitoring wells are clearly marked and secured to avoid unauthorized access and tampering; and A key, a lock combination number, or access tool for all monitoring wells is provided to the Department upon request and made available on the site where the UST system is located. Interstitial Monitoring. Beginning June 13, 2022, an owner and an operator of a UST system may only use interstitial monitoring as a monthly method of release detection between a UST system and a secondary barrier as described in §G(2)(b) of this regulation with prior written approval from the Department. An owner and an operator of a UST system may use interstitial monitoring between the UST system and a secondary barrier immediately around or beneath the UST system as a method of release detection if the UST system is designed, constructed, and installed to detect a spill, release, or discharge from any portion of the UST system and one of the following conditions are met: For a double-walled UST system, the owner and operator of the UST system uses a sampling or testing method that can detect a spill or release through the inner wall in any portion of the UST system; For a UST system with a secondary barrier within the excavation zone, the owner or operator of the UST system uses a sampling or testing method that can detect a spill, release, or discharge between the UST system and the secondary barrier, provided that: The secondary barrier around and beneath the UST system consists of artificially constructed material that is sufficiently thick and impermeable, at least 1×10-6 centimeter/second for the regulated substance stored, to direct a spill, release, or discharge to the monitoring point and allow the detection of the regulated substance; The barrier is compatible with the regulated substance stored so that a spill, release, or discharge from the UST system will not cause a deterioration of the barrier and allow a spill, release, or discharge to pass through the barrier undetected; For cathodically protected UST systems, the owner and the operator installs a secondary barrier so that it does not interfere with the proper operation of the cathodic protection system; The groundwater, soil moisture, or rainfall will not make the testing or sampling method used inoperative so that a spill, release, or discharge could go undetected for more than 30 consecutive days; The site is assessed to ensure that the secondary barrier is always above the groundwater and not in a 25-year flood plain, unless the barrier and monitoring designs are for use under these conditions; and Monitoring wells are clearly marked and secured with bolts or a lock to avoid unauthorized access and tampering; and For USTs with an internally fitted liner, the owner and operator of the UST uses an automated device that can detect a spill or release between the inner wall of the UST and the liner, and the liner is compatible with the regulated substance stored. Other Methods. Upon the approval of the Department, an owner and an operator of a UST system may use one or combination of alternative methods of monthly release detection if: The method can detect a 0.2 gallon per hour leak rate or a release of 150 gallons within a month with a probability of detection of not less than 0.95 and a probability of false alarm of not more than 0.05; or The owner and the operator of the UST system can demonstrate to the Department that the method can detect a spill, release, or discharge as effectively as any of the methods allowed in §§B and D—G of this regulation. If an alternative method of monthly release detection proposed by an owner and an operator of a UST system is approved by the Department, the owner and the operator of the UST system shall comply with any conditions imposed by the Department on the use of the method to ensure the protection of human health and the environment. Vapor Monitoring. Except as provided in §H of this regulation, an owner and an operator of a UST system may not use vapor monitoring as a monthly method of release detection after April 1, 2009.