An owner or operator of an existing aboveground storage tank system or tank that does not meet the secondary containment requirements for new tank systems in subsection 3 of section 33.1-13-01-02, shall provide release detection for tanks in accordance with at least one of the following methods:

1. 1. a. Statistical inventory reconciliation. Statistical inventory reconciliation must be conducted on a monthly basis by a statistical inventory reconciliation vendor to detect a release rate no greater than two-tenths gallon [76 liter] per hour; or

2. b. Inventory control. If statistical inventory reconciliation is not able to be performed by a statistical inventory reconciliation vendor due to configuration of the aboveground storage tank system, the department may approve inventory control. Liquid fuel inventory control (or another test of equivalent performance) must be conducted monthly to detect a release of at least one percent of flowthrough plus one hundred thirty gallons [492.10 liters] in the following manner:

1. (1) Inventory volume measurements for liquid fuel inputs, withdrawals, and the amount remaining in the tank must be recorded each operating day;

2. (2) The equipment used must be capable of measuring the level of product over the full range of the tank's height to the nearest one-eighth of an inch [3.18 millimeters];

3. (3) The liquid fuel inputs must be reconciled with delivery receipts by measurement of the tank inventory volume before and after delivery; and

4. (4) Liquid fuel dispensing from an aboveground storage tank system must be metered and recorded within the local standards for meter calibration or an accuracy of six cubic inches [98.32 milliliters] for every five gallons [18.93 liters] of product withdrawn.

3. 2. Manual tank gauging method. Only tanks of two thousand gallons [7570.82 liters] or less nominal capacity may use the manual tank gauging method as the sole method of release detection. Manual tank gauging must be performed at least every thirty days and meet the following requirements:

1. a. Tank liquid level measurements must be taken at the beginning and end of a period of at least thirty-six hours during which no liquid is added to, or removed from, the tank;

2. b. Level measurements must be based on an average of two consecutive stick readings at both the beginning and ending of the period;

3. c. The equipment used must be capable of measuring the level of product over the full range of the tank's height to the nearest one-eighth of an inch [3.18 millimeters]; or

4. d. A suspected leak is subject to the requirements of sections 33.1-13-01-10 through 33.1-13-01-12 if the variation between beginning and ending measurements exceeds the weekly or monthly standards as outlined below: 1. (1) Nominal standard tank capacity of five hundred gallons [1892.71 liters] or less. 1. (a) Weekly standard (one test) - variance of ten gallons [37.85 liters]. 2. (b) Monthly standard (average of four tests) - variance of five gallons [18.93 liters].

(2) Nominal standard tank capacity of five hundred fifty-one to one thousand gallons [2085.76 to 3785.41 liters]. (a) Weekly Standard (one test) - variance of thirteen gallons [49.21 liters]. (b) Monthly Standard (average of four tests) - variance of seven gallons [26.50 liters].

3. Automatic tank gauging method. Equipment for automatic tank gauging must meet the following requirements: a. The automatic product level monitor test must be able to detect a two-tenths gallon [.76 liter] per hour leak rate from any portion of the tank that routinely contains liquid fuel; and b. Automatic tank gauging must perform a leak test at least once every thirty days with the system operating with continuous in-tank leak detection on an uninterrupted basis or within a process that allows the system to gather incremental measurements to determine the leak status of the tank.

4. Other methods. Use of any other release detection method, or combination of methods, may be approved by the department if: a. It can detect a two-tenths gallon [.76 liter] per hour leak rate or a release of one hundred fifty gallons [567.81 liters] within a month with a probability of detection of ninety-five hundredths and a probability of false alarm of five one-hundredths; b. Owner or operator can demonstrate to the department that the method can detect a release as effectively as any of the methods in subsection 1 or 2. In comparing methods, the department shall consider the size of release the method can detect and the frequency and reliability with which it can be detected. If the method is approved, the owner or operator shall comply with any conditions imposed by the department on its use to ensure the protection of human health and the environment; or c. Appropriate tank gauging technology is not available to meet the performance standards required to use one of the methods outlined in subsections 1, 2, and 3.