PURPOSE: This rule contains the requirements that specific underground storage tank leak detection methods must meet.

1. (1) Methods of release detection for underground storage tanks (USTs) used to meet the requirements in 10 CSR 26-2.041 must be conducted as follows:

   1. (A) Inventory Control. Regulated substance inventory control (or another test of equivalent performance) must be conducted monthly to detect a release of at least one percent (1%) of flow through plus one hundred thirty (130) gallons on a monthly basis in the following manner:

      1. 1. Inventory volume measurements for

regulated substance inputs, withdrawals, and the amount still remaining in the tank are recorded each operating day on forms provided by the department or on forms previously approved by the department;

1. 2. The equipment used is capable of

measuring the level of regulated substance over the full range of the tank’s height to the nearest one-eighth inch (1/8");

1. 3. The regulated substance inputs are

reconciled with delivery receipts by measurement of the tank inventory volume before and after delivery;

1. 4. Deliveries are made through a drop

tube that extends to within one foot (1') of the tank bottom;

1. 5. Product dispensing is metered and

recorded within the local standards for meter calibration or an accuracy of six (6) cubic inches for every five (5) gallons of product withdrawn;

1. 6. The measurement of any water level

in the bottom of the tank is made to the nearest one-eighth inch (1/8") at least once a month; and

1. 7. The practices described in the

American Petroleum Institute Publication 1621, Recommended Practice for Bulk Liquid Stock Control at Retail Outlets, revised 2001, may be used, where applicable, as guidance in meeting the requirements of this subsection;

1. (B) Statistical Inventory Reconciliation (SIR), which is a statistical inventory analysis method that tests for the loss of a regulated substance. SIR must meet the following requirements:

   1. 1. Report a quantitative result with a cal-

culated leak rate;

1. 2. Be able to detect a two-tenths (0.2)

gallon-per-hour leak rate from any portion of the tank system that routinely contains a regulated substance;

1. 3. Must be conducted for each indepen-

dent tank system;

1. 4. Be done in conjunction with invento-

ry control that meets the requirements in 10 CSR 26-2.043(1)(A);

1. 5. Use a threshold that does not exceed

one-half (1/2) the minimum detectible leak rate;

1. 6. Be conducted in accordance with the

National Work Group on Leak Detection Evaluations listing and the manufacturer’s requirements. To obtain copies of equipment listings, contact the National Work Group on Leak Detection Evaluations, www.nwglde.org; and

1. 7. The SIR analysis report must include

the supporting data, inventory measurements of the regulated substance and water, delivery data, and analysis or reporting date;

1. (C) Manual Tank Gauging. Manual tank gauging must meet the following requirements:

   1. 1. Tank liquid level measurements are

taken at the beginning and ending of a period of at least thirty-six (36) hours during which no liquid is added to or removed from the tank;

1. 2. Level measurements are based on an

average of two (2) consecutive stick readings at both the beginning and ending of the period;

1. 3. The equipment used is capable of

measuring the level of regulated substance over the full range of the tank’s height to the nearest one-eighth inch (1/8");

1. 4. A leak is suspected and subject to the

requirements of 10 CSR 26-2.050–10 CSR 26-2.053 if the variation between beginning and ending measurements exceeds the following weekly or monthly standards:

1. A. Tanks of five hundred fifty- (550-)

gallon capacity or less are allowed a weekly standard of ten (10) gallons per reading and a monthly average of five (5) gallons per reading, with a minimum test duration of thirtysix (36) hours;

1. B. Five hundred fifty-one to one thou- Substance Storage Tanks

sand- (551–1,000-) gallon capacity tanks are allowed a difference of thirteen (13) gallons per week and a monthly average of seven (7) gallons, with a minimum test duration of thirty-six (36) hours, and when combined with a tank tightness test in accordance with subsection (D) of this section;

1. C. One thousand one to two thou-

sand- (1,001–2,000-) gallon capacity tanks are allowed a difference of twenty-six (26) gallons per week and a monthly average of thirteen (13) gallons, with a minimum test duration of thirty-six (36) hours, and when combined with a tank tightness test in accordance with subsection (D) of this section;

1. D. Five hundred fifty-one to one

thousand- (551–1,000-) gallon capacity tanks with a diameter no greater than sixty-four inches (64") are allowed a difference of nine (9) gallons per week and monthly average of four (4) gallons, provided that a period of at least forty-four (44) hours during which no liquid is added to or removed from the tank is allowed to pass between tank liquid level measurements, without requiring an additional tank tightness test; and

1. E. Five hundred fifty-one to one thou-

sand- (551–1,000-) gallon capacity tanks with a diameter no greater than forty-eight inches (48") are allowed a difference of twelve (12) gallons per week and a monthly average of six (6) gallons, provided that a period of at least fifty-eight (58) hours during which no liquid is added to or removed from the tank is allowed to pass between tank liquid level measurements, without requiring an additional tank tightness test; and

1. F. Tanks of greater than two thousand

(2,000) gallons nominal capacity may not use this method for release detection;

1. (D) Tank Tightness Testing. Tank tightness testing (or similar test) must be capable of detecting a one-tenth- (0.1-) gallon-per-hour leak rate from any portion of the tank that routinely contains regulated substance while accounting for the effects of thermal expansion or contraction of the regulated substance, vapor pockets, tank deformation, evaporation or condensation, and the location of the water table;

2. (E) Automatic Tank Gauging. Equipment for automatic tank gauging that tests for the loss of regulated substance and conducts inventory control must meet the following requirements:

   1. 1. The automatic regulated substance

level monitor test can detect a two-tenths- (0.2-) gallon-per-hour leak rate from any portion of the tank that routinely contains a regulated substance;

1. 2. The test must be performed with the

automatic tank gauging system operating in one of the following modes:

1. A. In-tank static testing conducted at

least once every thirty (30) days; or

1. B. Continuous in-tank leak detection

operating on an uninterrupted basis or operating within a process that allows the system to gather incremental measurements to determine the leak status of the tank at least once every thirty (30) days; and

1. 3. Inventory control (or equivalent test)

meeting the requirements in 10 CSR 26- 2.043(1)(A) is conducted;

1. (F) Vapor Monitoring. Testing or monitoring for vapors within the soil gas of the excavation zone must meet the following requirements:

   1. 1. The materials used as backfill are suf-

ficiently porous and permeable (for example, gravel, sand, or crushed rock) to readily allow diffusion of vapors from releases into the excavation area;

1. 2. The stored regulated substance, or a

tracer compound placed in the tank system, is sufficiently volatile (for example, gasoline) 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;

1. 3. The measurement of vapors by the

monitoring device is not rendered inoperative by the groundwater, rainfall, or soil moisture or other known interferences so that a release could go undetected for more than thirty (30) days;

1. 4. The level of background contamina-

tion in the excavation zone will not interfere with the method used to detect releases from the tank;

1. 5. The vapor monitors are designed and

operated to detect any significant increase in concentration above background of the regulated substance stored in the tank system, a component(s) of that substance, or a tracer compound placed in the tank system;

1. 6. In the UST excavation zone, the site

is assessed to ensure compliance with the requirements in paragraphs (1)(F)1.–4. of this rule and to establish the number and positioning of monitoring wells that will detect releases within the excavation zone from any portion of the tank that routinely contains a regulated substance;

1. 7. Monitoring wells are clearly marked

and secured to avoid unauthorized access and tampering; and

1. 8. After July 1, 2020, use a tracer chem-

ical and the method is listed by the National Work Group on Leak Detection Evaluations as a tank tightness test;

1. (G) Groundwater Monitoring. Testing or monitoring for liquids on the groundwater may only be used as a release detection method until July 1, 2020, and must meet the following requirements:

   1. 1. The regulated substance stored is

immiscible in water and has a specific gravity of less than one (1);

1. 2. The groundwater is within twenty feet

(20') from the ground surface and the hydraulic conductivity of the soil(s) between the UST system and the monitoring wells or devices is at least one hundredth centimeter per second (0.01 cm/sec) (for example, the soil should consist of gravels, coarse to medium sands, coarse silts, or other permeable materials);

1. 3. The slotted portion of the monitoring

well casing must be 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;

1. 4. Monitoring wells shall be sealed from

the ground surface to the top of the filter pack;

1. 5. Monitoring wells or devices shall

intercept the excavation zone or are as close to it as is technically feasible;

1. 6. The continuous monitoring devices or

manual methods used can detect the presence of at least one-eighth inch (1/8") of free product on top of the groundwater in the monitoring wells;

1. 7. The site is assessed within and imme-

diately below the UST system excavation zone to ensure compliance with the requirements in paragraphs (1)(G)1.–5. of this rule. The site assessment also establishes the number and positioning of monitoring wells or devices that will detect releases from any portion of the tank that routinely contains a regulated substance; and

1. 8. Monitoring wells are clearly marked

and secured to avoid unauthorized access and tampering;

1. (H) Interstitial Monitoring. Interstitial monitoring must monitor between the walls of a double-walled tank or, for piping, is designed to detect a release from the primary piping, including all fittings, and contain it until it can be detected. The entire piping secondary containment must be leak tight. Interstitial monitoring must be installed to detect a leak from any portion of the tank that routinely contains a regulated substance and also meets the following requirements:

   1. 1. The groundwater, soil moisture, or

rainfall will not render the testing or sampling method used inoperative so that a release could go undetected for more than thirty (30) days;

1. 2. For new UST systems installed after

July 1, 2017, interstitial monitoring must be conducted electronically by a system with a report-generating capability; and 10 CSR 26-2

1. 3. For UST systems using continuous

vacuum, pressure, or liquid-filled methods of interstitial monitoring, the method must be capable of detecting a breach in both the inner and outer walls of the tank and/or piping; and

1. (I) Other Methods. Any other type of release detection method, or combination of methods, can be used if—

   1. 1. It can detect a two-tenths- (0.2-) gal-

lon-per-hour leak rate or a release of one hundred fifty (150) gallons within a month with a probability of detection of ninety-five percent (95%) and a probability of false alarm of five percent (5%); or

1. 2. The department may approve another

method if the owner and operator can demonstrate that the method can detect a release as effectively as any of the methods allowed in subsections (1)(C)–(H) of this rule. In comparing methods, the department shall consider the size of release that the method can detect and the frequency and reliability with which it can be detected. If the method is approved, the owner and operator must comply with any conditions imposed by the department on its use to ensure the protection of human health and the environment.

1. (2) Owners and operators of field-constructed or airport hydrant fuel distribution system tanks may not use vapor monitoring or groundwater monitoring, described in subsections (F) and (G) of this rule as their sole method of detection, but may use them in conjunction with 10 CSR 26-2.046.

AUTHORITY: sections 319.105, 319.107, and 319.137, RSMo 2016.* This rule originally filed as 10 CSR 20-10.043. Original rule filed April 2, 1990, effective Sept. 28, 1990. Amended: Filed Aug. 3, 1993, effective April 9, 1994. Moved and amended: Filed April 15, 2011, effective Dec. 30, 2011. Amended: Filed Aug. 15, 2016, effective May 30, 2017. *Original authority: 319.105, RSMo 1989; 319.107, RSMo 1989, amended 1994; and 319.137, RSMo 1989, amended 1993, 1995, 2004.