Underground Storage Tanks

How does the leak detection method work?

For statistical inventory reconciliation (SIR), a trained professional uses sophisticated computer software to conduct a statistical analysis of inventory, delivery, and dispensing data, which you must both collect and supply to the vendor on a regular basis.

The features of SIR are:

• SIR analyzes inventory, delivery, and dispensing data collected over a period of time to determine whether or not a tank system is leaking.
• Each operating day, the product level is measured using a gauge stick or other tank level monitor. You also keep complete records of all withdrawals from the UST and all deliveries to the UST. After data have been collected for the period of time required by the SIR vendor, you provide the data to the SIR vendor.
• The SIR vendor uses sophisticated computer software to conduct a statistical analysis of the data to determine whether or not your UST may be leaking. The SIR vendor provides you with a test report of the analysis.

What are the regulatory requirements?

• To be allowable as monthly monitoring, a SIR method must be able to detect a leak at least as small as 0.2 gallons per hour and meet the federal regulatory requirements regarding probabilities of detection and of false alarm. Data must be submitted at least monthly.
• To be allowable as an equivalent to tank tightness testing, a SIR method must be able to detect a leak at least as small 0.1 gallons per hour and meet the federal regulatory requirements regarding probabilities of detection and of false alarm.
• The individual SIR method must have been evaluated with a test procedure to certify that it can detect leaks at the required level and with the appropriate probabilities of detection and of false alarm.
• If the test report is not conclusive, you must take the steps necessary to find out conclusively whether your tank is leaking. Because SIR requires multiple days of data, you will probably have to use another method.
• You must keep on file both the test reports and the documentation that the SIR method used is certified as valid for your UST system.

Will it work at your site?

• Generally, few product or site restrictions apply to the use of SIR.
• SIR has been used primarily on tanks no more than 18,000 gallons in capacity. If you are considering using a SIR method for larger tanks, discuss its applicability with the vendor.
• Water around a tank may hide a hole in the tank or distort the data to be analyzed by temporarily preventing a leak. To detect a leak in this situation, you must check for water at least once a month.

Anything else you should consider?

• Data, including product level measurements, dispensing data, and delivery data, should all be carefully collected according to the SIR vendor's specifications. Poor data collection produces inconclusive results and noncompliance.
• The SIR vendor will generally provide forms for recording data, a calibrated chart converting liquid level to volume, and detailed instructions on conducting measurements.
• SIR should not be confused with other release detection methods that also rely on periodic reconciliation of inventory, withdrawal, and delivery data. Unlike manual tank gauging or inventory control, SIR uses a sophisticated statistical analysis of data to detect releases. This analysis can only be done by competent, trained practitioners.

Will you be in compliance?

Statistical inventory reconciliation, when performed according to the vendor's specifications, meets federal leak detection requirements for new and existing USTs as follows:

• SIR with a 0.2 gallon per hour leak detection capability meets the federal requirements for monthly monitoring for the life of the tank and piping.
• SIR with a 0.1 gallon per hour leak detection capability meets the federal requirements as an equivalent to tank tightness testing.
• SIR can, if it has the capability of detecting even smaller leaks, meet the federal requirements for line tightness testing as well.

Additional Information on SIR

OUST has two publications that more fully explain SIR:

• Introduction to Statistical Inventory Reconciliation: For Underground Storage Tanks (EPA 510-B-95-009) (PDF) (16 pp, 426K, About PDF)
  September 1995. This 12-page booklet provides basic information on this leak detection method.
• Standard Test Procedures for Evaluating Leak Detection Methods: Statistical Inventory Reconciliation Methods (SIR) (EPA/530/UST-90/007) (PDF) (59 pp, 3K, About PDF)
  March 1990

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