State Water Resources Control Board

The purpose of this letter is to clarify the difference between the detectable leak rate used in UST testing equipment performance standards and the leak rate reported as a result of a tank test.

A leak is defined in terms of flow rate in gallons per hour (gph) and can be positive or negative; that is, product can flow out of the tank or ground water can flow into the tank. Once the flow rate has been determined by a tank test, a decision must be made as to whether to declare the tank leaking or nonleaking. This decision is made by comparing the flow rate to the threshold.

Threshold is a predetermined value against which measurements made during a test are compared and which serves as the basis for declaring the presence of a leak. If, for example, measurements indicate that fluid is flowing out of a tank at a rate of 0.1 gph, and the protocol of a test method states that any measurement greater than a threshold of 0.05 gph constitutes a leak, the tank is declared leaking.

Performance is defined by the test method's probability of detection (PD) and probability of false alarm (PFA) for each leak rate that the method claims to be able to detect. The probability of detection refers to the test's chances of correctly identifying a leaking tank compared to its chances of failing to detect a leak that is actually present. The probability of false alarm refers to a test's chances of reporting the presence of a leak when in fact none exists. There are four possible outcomes of a leak detection test: a correctly identified leak, a correctly identified tight tank, a false alarm, and a missed detection.

Both the PD and the PFA are dependent on the criterion for declaring a leak, that is, on the threshold value set by the manufacturer. If the flow rate determined by a tank test exceeds the threshold, it is assumed that a leak is present. Once this threshold value has been selected, the PFA is established; it does not change.

The most commonly used threshold is 0.05 gallons per hour. The 0.05-gallon per-hour threshold is often confused with the leak rate to be detected. If the threshold is equal to the leak rate to be detected, the PD is only 50% against a leak of that size. State and federal regulations requires tank test methods to have a minimum detectable leak rate of 0.1 gph. In order for a test method to meet this requirement, its threshold must be less than 0.1 gph. Threshold values (usually 0.05 gph) for tank testing methods approved for use in California are listed in LG-113. Therefore, a tank test report should declare the tank to be leaking if the leak rate is equal to or greater than the threshold value.

In summary, the performance standard of 0.1 gph set forth in the regulations is not used by local agencies in reviewing tank test reports, but is used by the third-party testing company in its evaluation of the testing method. Local agencies need to know the threshold values in reviewing tank test reports to make sure that the report makes the proper declaration concerning the condition of the tanks.

UST Program - Local Guidance (LG) 118