NJDEP Site Remediation Program - News

[Help] [NJ Home Page][NJDEP Home Page][SRP Home Page][SRP News]

Site Remediation News
January 1997 (Vol 9 No 1) - Article 01

Return to Table of Contents.

How to Avoid Common Problems With Monthly Inventory Reconciliation
By: Sharon P. McLelland
Division of Responsible Party Site Remediation
Industrial Site Evaluation Element
Bureau of Underground Storage Tanks

Monthly inventory reconciliation, combined with periodic tank tightness tests, is a temporary leak detection method, which may be used to evaluate the integrity of underground storage tanks (USTs). This method requires daily measurements and monitoring, which can be extremely useful in detecting a problem early, thereby saving a tank system owner or operator significant costs in terms of product loss, environmental investigation and cleanup.

The inventory reconciliation procedure entails taking a volumetric measurement of the product from a port in the tank (a "stick reading") and comparing this volume with the expected volume, based on the prior end volume measurement, sales and deliveries (a "book reading").

The New Jersey Department of Environmental Protection (DEP) has compiled several booklets to assist the regulated community with understanding the process of monthly inventory reconciliation and the necessary steps in compiling the data. It is recommended that all owners and operators of UST systems obtain the following documents from the DEP:

  • DEP, August 1996, Doing Inventory Control Right;
  • DEP, August 1996, Manual Tank Gauging for Smaller USTs;
  • DEP, January 1996, Straight Talk on Tanks and,
  • DEP, January 1995, Don't Wait Until 1998.

The August 1996 editions are recent upgraded versions of the original EPA documents and provide tear-away master sheets for daily and monthly tabulations. Contact the Bureau of Underground Storage Tanks at (609) 292-8761 to request a copy.

The DEP reviews various forms of inventory reconciliation. Consistent problems are seen amongst the large and small UST system owners/operators. The intent of this article is to discuss what these problems are and how you, the UST owner/operator, can avoid making similar mistakes.

Stick Readings. The data is only as good as the measurement taken. This single measurement can result in overages/shortages which may not in actuality exist. The condition of the stick is most critical. It should be notched or marked to 1/8"ot worn at the base. A varnished stick can reduce the potential for petroleum "creep" and inaccurate readings; however, all that is required is that the stick be of non-sparking material (i.e. wood) and that it is legible to 1/8" markings. Automatic Tank Gauges (ATG) within a tank provide the same reading on actual volume measurement, thus measurement with a "stick" is not always a requirement.

Daily readings are required. Opening and/or closing readings are recommended for more accurate inventory records. It is a requirement that stick readings (or ATG readings) are NOT conducted during fuel dispensing activities, thus it is recommended that readings either be taken while the pumps are shut off (i.e. before the station opens or after it closes) or during a lull in business. Stick or volume measurements are necessary prior to a delivery and following a delivery. Before product is delivered, a stick reading will correlate the book value used in determining how many gallons are needed. This action can help prevent overfilling tanks not yet upgraded. In measuring the volume following a delivery, it is important to wait a minimum of 5 minutes prior to taking a reading. This allows the temperature of the recently delivered fuel to equilibrate with the temperature of the product already in the tank.

In collecting the stick volume readings, it is important to carry a clipboard. The measurements are to be read to the nearest 1/8". Due to time delays in taking the measurements to writing down the numbers in the office, it is best to write down the tank measurement as it is taken. This reduces the potential for confusion between different tanks' measurements when logging the data, especially where several tanks are measured in the same event.

The opportunity for the earliest numbers to be incorrectly tabulated back in the office is great when coordinating measurements of several tanks, possible interruptions with sales and time delays in taking the measurements and writing them into the log book.

Water Measurements. Measuring for water in a tank is only required to be conducted once a month. However, it may be beneficial in the overall program of evaluating the integrity of a tank to collect this measurement more often. When taking this measurement, the water paste is applied to the base of the stick. The stick should be set in the tank for a period of 10 seconds for gasoline and 30 seconds for diesel, for an accurate measurement. It is critical to remember that this stick reading should NOT be used in measuring for the product volume. The long residence time may result in "petroleum creep", a process by which the stick is wetted at a higher level than the actual product level. This can give a false volume loss (shortage). A separate measurement should be taken for product volume.

If more than one inch of water exists in a tank, the tank should be decommissioned and the water pumped from the tank. Due to this expensive and time-consuming process, more frequent measurements can be a cost-effective manner to evaluate potential causes of water accumulation. In noting the weather conditions daily, the potential for water to be infiltrating into the tank during a rain event, following a rain event due to inadequate runoff protection around the port, or for ground water entering the tank can be quickly evaluated, with minimum cost to the owner/operator. Water build-up as a result of condensation on the walls of the tank is a very slow process. If water accumulation is via ground water, daily or frequent measurements can pinpoint rapid accumulation of water in the tank. Care should be taken when measuring for water through a drop tube in the tank, due to potential condensation on the walls of the drop tube.

Petroleum Measurements. In collecting a petroleum volume measurement, care needs to be taken to avoid "petroleum creep". The problem with the creep factor is that the measurements need to be accurate to an 1/8 of an inch, thus the volumes calculated off a measurement reflecting "creep" are excessive and inaccurate. Creep may occur when the stick is left in a tank for as little as 5 seconds. The proper way to stick a tank is to gently place the stick to the base of the tank and then quickly removed the stick. Petroleum creep can be avoided by use of petroleum paste. This product is similar to the water paste used; however, it is applied along a six-inch interval where the product level is expected. The paste turns color where in contact with the petroleum. Care should be taken to allow readings to 1/8" when applying the paste to the stick. More accurate readings may occur with the use of petroleum paste, as it reduces the chance of error when reading a stick in poor lighting conditions or when volatilization is rapid. Petroleum paste also assists inexperienced workers in reading the measurement to 1/8".

Calibration Charts. Each tank should be provided with its own calibration chart. It is important to use only that chart associated with that tank, unless the tanks are all of the same volume and manufactured by the same company. Each chart is to be calibrated to 1/8 inch to gallon conversions. Often, this is not the case and extrapolation is needed. It is recommended that the tank manufacturer be contacted for a new chart, if extrapolation is currently the manner of calculating volumes. If a tank is a steel tank, the Steel Tank Association (847-438-8265) can provide a calibration chart for your size tank. The DEP's August 1996 Doing Inventory Control Right booklet provides for a method for extrapolating the volumes; however, this is a three step procedure and the more mathematical steps used in determining one value, the more chance for errors to make it on the tabulated sheet. If your tank(s) have been retrofitted with a striker plate, this modification will require a recalibrated chart or significant errors (1" or more) in volume measurement will occur. This can result in shortages of up to hundreds of gallons.

Manifolded Tank Systems. Tank systems which are manifolded or which have dispensers which blend fuels are to be considered one system. This requires adding all measurements as one reading. It is recommended that daily readings of the separate tanks be kept, as math errors can occur when combining several fractional readings. This will provide a double-check if the monthly tabulations indicate an overage or shortage, especially if a blending system is employed at the site. Combined readings from tanks where the fuel is blended may result in a masking effect if one of several tanks is leaking.

Combining readings from separate tanks or systems which do not blend fuel from a common inventory cannot be used for meeting the monthly inventory reconciliation records. When combined readings are provided, a statement clarifying the rationale for combined readings should be provided.

Totalizer Calibration. It is important for the totalizers to be properly calibrated, as these readings are used in the comparison of tank volume inventories to what has been sold and delivered. In New Jersey, the meter calibration is regulated by the Bureau of Weights and Measures, which is under the jurisdiction of the county, for systems which sell product. Contact the county officials if your station's meters have not been inspected recently and you suspect the totalizer readings are not consistent with your records. Meters are to be inspected on an annual basis. If you are not regulated by the Bureau of Weights and Measures, an annual calibration is recommended.

The master sheets included in the August 1996 edition of the Doing Inventory Control Right manual includes several blanks for totalizer readings, if one tank dispenses to several totalizers. Each tank's totalizer reading(s) should be entered in the same column to ensure the correct readings are provided for that specific tank.

Deliveries. It is important to conduct a volumetric ("stick") measurement prior to receiving a delivery of product. This additional check of inventory provides an updated amount needed, as well as reduces any errors in over-deliveries (especially important for tanks which have not yet been upgraded with overfill protection devices). Following delivery, it is important to take a second volume measurement, to ensure the volumes delivered are consistent. When tallying the volume delivered, if sales occur during the delivery process, the "Gross Gallons Delivered (Receipt)" volumes should be used. If no sales occurred during the delivery process, the stick reading should be used (after volume minus the before volume). If two volumes are provided to you on the delivery ticket, the "gross" volume should be used, not the "net" volume.

To avoid common inventory reconciliation problems, it is important to be aware of what each step's procedure and purpose is relative to the end result. Careful measurements, keeping equipment in good condition, and daily review of the data will provide the tank owner with a measure of reliability in the monthly end result. It is recommended that an "overage" or "shortage" which has persisted for five or more days, be reviewed for any math errors or other problems discussed above. Daily records are supposed to be retained at the site. This allows the owner or operator to check for any errors in measurement or any anomalies in the data, as well as meet the regulatory requirements. Poor measurements can be costly as the UST Rules (N.J.A.C. 7:14B) require an investigation within 7 days of one month's exceedance of the leak rate (the federal regulations allow two consecutive months before tank system investigation). If you are having any difficulty with the procedures or if you have any questions about monthly inventory reconciliation, the Bureau of Underground Storage Tanks (609-292-8761) can assist you.

[Go to previous article] [Go to Table of Contents] [Go to next article]

| Disclaimers and Notices |


Call the DEP 24 Hour Hotline 609/292-7172 to Report Enviromental Incidents.

[NJ Home Page][NJDEP Home Page][SRP Home Page][SRP News]

Please send your comments regarding the design or function of the NJDEP SRP Web pages to:

srpweb@dep.state.nj.us

Last revision: 2 Sept 97