Owners and operators using inventory control (or another test of equivalent performance) must comply with the requirements of this section.

(1) Standards.

(a) Inventory control must be performed in a manner that is able to detect leaks of at least one percent of the monthly flow-through plus one hundred thirty gallons.

(b) Inventory control must be performed using a gauge stick or an automatic tank gauge system that is able to measure the following:

(i) Tank liquid levels over the full range of the tank's height to the nearest one-eighth of an inch; and

(ii) Water levels in the bottom of the tank to the nearest one-eighth of an inch.

(c) Dispensing meters must be calibrated to local standards or an accuracy of at least six cubic inches for every five gallons of regulated substances that is withdrawn.

(d) The fill pipe through which regulated substances are delivered into the tank must have a drop tube that extends to within one foot of the bottom of the tank.

(2) Performance. Inventory control must be performed in accordance with the requirements of this subsection. Automatic tank gauge systems must be operated in accordance with the manufacturer's instructions. The following code of practice may be used, where applicable, as guidance in meeting the requirements of this subsection: American Petroleum Institute, Recommended Practice 1621, "Bulk Liquid Stock Control at Retail Outlets."

(a) Each day regulated substances are removed from or added to the tank, the following inventory volume measurements and calculations must be performed:

(i) Measure the number of gallons of regulated substances removed from the tank within the local standards for meter calibration or an accuracy of at least six cubic inches for every five gallons of regulated substances that is withdrawn;

(ii) Measure the tank liquid level to the nearest one-eighth of an inch before and after any delivery of regulated substances, convert the two measurements into gallons, calculate the difference between the two measurements, and reconcile the change in inventory volume with delivery receipts;

(iii) Measure the tank liquid level at the end of the day (ending inventory) to the nearest one-eighth of an inch and convert the measurement into gallons. The measurement taken after a delivery of regulated substances may be used to meet this requirement;

(iv) Calculate the book inventory by adding the starting inventory and the number of gallons delivered and then subtracting the number of gallons dispensed; and

(v) Calculate the daily inventory imbalance (overage or shortage) by subtracting the book inventory from the ending inventory.

(b) At least once each month, the level of any water in the tank must be measured to the nearest one-eighth of an inch.

(c) At the end of each monitoring period, calculate the monthly imbalance (overage or shortage) by adding together all of the daily imbalances.

(3) Suspected release. A release is suspected based on inventory control if:

(a) The monthly inventory imbalance is greater than one percent of the monthly flow-through plus one hundred thirty gallons; or

(b) The presence of water in the tank is unexplained.