Statistical Inventory Reconciliation (SIR)
How does the release detection method work?
For statistical inventory reconciliation (SIR), typically, 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.
SIR methods are distinguished from continuous in-tank leak detection methods by how inventory, delivery, and dispensing data are processed and provide a determination of the release status of the tank (or piping). SIR data are processed on a periodic basis involving a separate analysis that is performed either by a SIR vendor or SIR software. Continuous statistically based in-tank release detection methods process data on an on-going basis occurring in an uninterrupted or nearly uninterrupted manner.
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 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 gallon per hour or a release of 150 gallons within a month and meet the federal regulatory requirements regarding probabilities of detection and of false alarm. SIR methods must use a threshold that does not exceed one-half the minimum detectible leak rate. 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 as 0.1 gallon per hour and meet the federal regulatory requirements regarding probabilities of detection and false alarm.
- The individual SIR method must have been evaluated with a test procedure to verify that it can detect leaks at the required level and with the appropriate probabilities of detection and 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 valid for your UST system.
- Beginning on [three years after effective date], you must perform the following, as applicable, on your release detection equipment annually to make sure it is working properly:
For hand held non-electronic equipment:
- Check for operability and serviceability (includes dipsticks)
- Keep records of these checks for one year
For other equipment:
- Verify the system configuration of the controller
- Test alarm operability and battery backup
- Inspect sensors for residual build-up
- Ensure sensor communication with controller
- Keep records of these tests for three years
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.
Anything else you should consider?
- Detecting water in the tank is important. Water around a tank may mask a hole in the tank or distort the data to be analyzed by temporarily preventing a release. To detect a release in this situation, check for water at least once a month. Depending upon the product in the tank, detecting water may be difficult to do, but not impossible. Products such as ethanol-based fuels may not form a water bottom. An unexplained presence of water in the tank is considered an unusual operating condition. If you find water in your tank you must investigate and correct the source of the water. Suspected releases must be reported to the implementing agency within 24 hours, or period specified by the implementing agency.
- 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?
For USTs installed on or before [180 days after effective date], owners and operators may use SIR as their primary method of release detection. When installed and operated according to the manufacturer's specifications, SIR meets the federal release detection requirements 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.
USTs installed or replaced after [180 days after effective date] may no longer use SIR as the primary method of release detection. USTs must be secondarily contained and use interstitial monitoring.
Additional Information on SIRPublications 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)