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UST Program - Available Local Guidance (LG) Letters

UST Program - Local Guidance (LG) 123-2


LG-123-2: STATISTICAL INVENTORY RECONCILIATION
(REVISED FEBRUARY 9, 1993)


February 9, 1994

Local Implementing Agencies and Interested Parties:

The California State regulations for underground storage tanks (USTs) require owners and operators of existing USTs to check for leaks on a routine basis in accordance with the performance standards listed in the California Code of Regulations, Title 23, Division 3, Chapter 16, Article 4. The requirements of Article 4 do not prescribe specific tank monitoring methods. Any method or procedure is acceptable as long as it meets the performance standards and receives a third-party evaluation which is reviewed by the State Water Board and determined to meet the EPA’s Standard Test Procedures for Leak Detection Methods.

From time to time new monitoring equipment and/or methods are developed, reviewed as discussed above, and placed on the list of leak detection methods (LG-113 series). Several companies have submitted third-party evaluation reports for statistical inventory reconciliation (SIR). The third-party evaluation reports have been reviewed and determined to meet the performance standards for monthly leak detection. The third-party evaluations were performed in accordance with the "Standard Test Procedures for Evaluating Leak Detection Methods: Statistical Inventory Reconciliation Methods" (EPA Document Number EPA/530/UST-90/007 dated June 1990).

SIR methods employ sophisticated statistical software to conduct computerized analysis of inventory data collected by UST owners/operators who either gauge their tanks manually or record data from an automatic tank gauging system. Both SIR and manual inventory reconciliation (MIR) use similar types of flow-through data. The difference between SIR and MIR is the method of analysis. SIR adopts a systematic statistical and engineering analysis whereas MIR is done using a bookkeeping accounting system of inventory to determine if the tank is leaking.

MIR in high ground water areas has been prohibited in California since January 1, 1993 because of the method’s inability to find leaks less than one gallon per hour. This restriction in high ground water areas was put in the State regulations in August 1991, and, at that time, SIR was not in use or approved in California. SIR can find leaks significantly smaller than the MIR method.

The use of SIR is not a new requirement, but an additional leak detection method available to tank owners. SIR providers use different computer software programs, hut all SIR methods listed by the State Water Board in the LG-113 series are acceptable in California and meet minimum state and federal requirements. Further, EPA provides evaluation protocols for both qualitative and quantitative SIR methods. A qualitative SIR method will indicate pass, fail, or inconclusive results whereas a quantitative SIR method will calculate a leak rate for the tank.

Like other monthly monitoring methods, each SIR method has a preset threshold which must be less than 0.2 gph. A tank is declared leaking if the leak rate determined by a monthly test exceeds this preset threshold. Thresholds are listed in the LG-113 series. (For further discussion on threshold values, see LG-118).

SIR may be used on hydraulicallymanifolded UST systems only if a proper third party evaluation has been conducted, and the evaluation demonstrates that the SIR method is capable of meeting the monthly monitoring performance standards when testing manifolded tanks. To date, no SIR methods have demonstrated the ability to test hydraulically-manifolded tanks.

SIR must be conducted by a SIR provider who has no direct or indirect financial interest in the UST system being monitored. The SIR provider can be the developer of the SIR method who received the third-party evaluation; or the provider can be an entity that has been trained and certified by the developer.

Section 2646 of the California UST regulations describe requirements pertaining to SIR. Some responsibilities of UST owners/operators for SIR inventory measurements and recordkeeping are as follows:

  1. Inventory volume measurements for product inputs, withdrawals, and the amount remaining in the tank must be recorded daily. [See section 2646 for the definition of "daily".]
  2. Equipment used to take inventory readings must be in good working order and capable of measuring the product level over the full range of the tank’s height to the nearest one-eighth of an inch.
  3. Product inputs must be reconciled with delivery receipts by measurement of the tank inventory volume before and after delivery.
  4. Deliveries must be made through a drop tube that extends to within 12 inches of the tank bottom.
  5. Petroleum dispensing must be metered and recorded within the local standards for meter calibration. Dispenser meters must be inspected and calibrated on an annual basis as described in the California Business and Professions Code, Division 5, Chapter 5.5. This applies to all meters used for determining withdrawals, including those at non-retail facilities.
  6. Daily measurements of any water level at the bottom of the tank must be made to the nearest one-eighth of an inch. Even if the SIR method does not need daily water level readings to determine the status of the tank, the tank owner or operator must record this information daily to be in compliance with state regulations. If the tank is gauged manually, water-finding paste must be used daily.
  7. UST systems must have tank charts that have been calibrated in one-eighth inch increments. A tank chart must clearly indicate which tank it is calibrated for, and may not be used for more than one tank.
  8. The tank owner must provide the minimum number of data records to the SIR provider monthly. The previous month’s data may be included with the current month’s data to satisfy the required minimum data records.
  9. Monthly data submissions to the SIR provider and subsequent receipt of report from the SIR provider shall be completed within 20 calendar days of the month. Notification to the local implementing agencies of all failed or inconclusive reports shall be completed within 24 hours from receipt of the report from the SIR provider. A copy of the inconclusive or failed SIR report must be provided to the local agency within 10 calendar days. An inconclusive report is one that is not decisive as to whether a release has been detected.
  10. A tank integrity test or piping tightness test is required within 15 calendar days from the date of the second of two consecutive monthly SIR reports that are inconclusive and/or failures. If two consecutive monthly SIR reports are inconclusive and/or failures, then the release reporting requirements of Article 5 of the UST regulations must be met.
  11. The local agency should require a tank integrity test or piping tightness test if frequent inconclusive results are reported for a tank.
  12. SIR shall be supplemented by performing a tank integrity test once every 24 months. The first tank test shall be conducted within the first year.
  13. Single-walled pressurized piping shall be equipped with an automatic line leak detector and shall be precision tested every 12 months. Single-walled suction piping must be precision tested every three years.
  14. Results of inventory readings and test results from the SIR provider must be maintained and readily available for a period of at least three years.

Developing and fulfilling minimum data requirements, implementing improved operating procedures recommended by SIR developers, and establishing a good trend on SIR reports for new SIR subscribers may require several months. It is very likely that during this learning period, SIR results may be "inconclusive". Therefore, tank owners who elect to use SIR will have up to three months before they must follow the reporting and testing requirements listed above. During this time, however, tank owners must continue to fulfill requirements of their old monitoring program.

If you have any questions, please contact David Holtry at (916) 227-4332.

Sincerely,

[ orginal signed ]

Mike McDonald, Manager
Underground Storage Tank Program

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