Release Detection for Underground Storage Tanks (USTs) - Internal Methods

EPA allows owners and operators to use these internal methods – automatic tank gauging, manual tank gauging, statistical inventory reconciliation, and tank tightness testing with inventory control – for detecting releases from their UST systems.

On this page:

Automatic tank gauging
Manual tank gauging
Statistical inventory reconciliation (SIR)
Continuous in-tank leak detection
Tank tightness testing with inventory control

Automatic Tank Gauging

How does the leak detection method work?

Overfill alarm — Automatic tank gauging system

This method uses automated processes to monitor product level and inventory control. A probe permanently installed in the tank is connected to a monitor to provide information on product level and temperature. These systems calculate changes in product volume that can indicate a leaking tank. Automatic tank gauging systems (ATG systems) operate in one of two modes: inventory mode and leak detection mode. In the leak detection mode, ATG systems can be set manually or automatically to perform a leak test. Manual leak tests are in-tank static tests and automatic leak tests are continuous in-tank leak detection tests. When we refer to ATG systems we are referring to testing performed in the in-tank static test mode. ATG systems operating in continuous in-tank leak detection test mode are covered under Continuous In-Tank Leak Detection.

Features of ATG systems include:

The product level and temperature in a tank are measured and recorded by a computer, this saves labor and time.
In the inventory mode, the ATG system replaces the use of the gauge stick to measure product level and perform inventory control. This mode records the activities of an in-service tank, including deliveries.
In the leak detection mode (in-tank static test), the tank is taken out of service and the product level and temperature are measured for at least one hour.

Note: Some systems, known as continuous ATG systems, do not require the tank to be taken out of service to perform a test. This is because these systems can gather and analyze data during many short periods when no product is being added to or taken from the tank. These systems are discussed under Continuous In-Tank Leak Detection.

What are the regulatory requirements?

The ATG system must be able to detect a leak no larger than 0.2 gallon per hour with certain probabilities of detection and false alarm. Some ATG systems can also detect a leak of 0.1 gallon per hour with the required probabilities.

Beginning on October 13, 2018 you must perform the following, as applicable, on your release detection equipment annually to make sure it is working properly:

Verify the system configuration
Test alarm operability and battery backup
Inspect probes and sensors for residual build-up
Ensure floats move freely, the shaft is not damaged, and cables are free of kinks and breaks
Keep records of these tests for three years

Testing must be performed in accordance with manufacturer’s requirements; a nationally recognized code of practice; or requirements determined by your implementing agency to be no less protective of human health and the environment.

Will it work at your site?

ATG systems have been used primarily on tanks containing gasoline or diesel. If considering using an ATG system for larger tanks or products other than gasoline or diesel, discuss its applicability with the manufacturer or installer.

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.
The ATG system probe is permanently installed through an opening (not the fill pipe) on the top of the tank. Each tank at a site must be equipped with a separate probe. The ATG system probe is connected to a monitor that displays ongoing product level information and the results of the monthly test. Printers can be connected to the monitor to record this information.
ATG systems are often equipped with alarms for high and low product level, high water level, and theft.
ATG systems can be linked with computers at other locations from which the system can be programmed or read.
For ATG systems that are not of the continuous type, no product should be delivered to the tank or withdrawn from it for at least 6 hours before the monthly test or during the test (which generally takes 1 to 6 hours).
An ATG system can be programmed to perform a test more often than once per month (a recommended practice).

Will you be in compliance?

For USTs installed on or before April 11, 2016 owners and operators may use ATG systems as their primary method of release detection. When installed and operated according to the manufacturer's specifications, ATG systems meet the federal release detection requirements. USTs installed or replaced after April 11, 2016 may no longer use ATG systems (used solely for in-tank release detection) as the primary method of release detection. USTs must be secondarily contained and use interstitial monitoring.

View EPA's publication, Automatic Tank Gauging Systems for Release Detection: Reference Manual for Underground Storage Tank Inspectors, for information on evaluating how well UST owners and operators are using their ATG systems to comply with release detection requirements.

Manual Tank Gauging

How does the leak detection method work?

This method involves keeping the tank undisturbed for at least 36 hours each week, during which the tank's contents are measured, twice at the beginning and twice at the end of the test period. Manual tank gauging can be used as the sole method of leak detection for the life of the tank only for tanks up to 1,000 gallons. Tanks between 1,001 and 2,000 gallons can use this method only in combination with tank tightness testing. This combined method, however, can be used only during the first 10 years following tank installation.

The features of manual tank gauging are:

Four measurements of the tank's contents must be taken weekly, two at the beginning and two at the end of at least a 36-hour period during which nothing is added to or removed from the tank (see Table of Test Standards for Manual Tank Gauging).
The average of the two consecutive ending measurements are subtracted from the average of the two beginning measurements to indicate the change in product volume.
Every week, the calculated change in tank volume is compared to the standards shown in the Table of Test Standards for Manual Tank Gauging at the end of this page. If the calculated change exceeds the weekly standard, the UST may be leaking. Also, monthly averages of the four weekly test results must be compared to the monthly standard in the same way.

What are the regulatory requirements?

Liquid level measurements must be taken with a gauge stick that is marked to measure the liquid to the nearest one-eighth of an inch.
Manual tank gauging may be used as the sole method of leak detection for tanks with a capacity of 1,000 gallons or less for the life of the tank. Tanks between 551 and 1,000 gallons have two testing standards based on their diameter (see table). These tanks may use a combination of manual tank gauging and periodic tank tightness for the life of the tank (see table for testing standards).
For tanks with a capacity of 1,001-2,000 gallons, manual tank gauging must be combined with periodic tightness testing. New tanks must be tightness tested every 5 years for 10 years following installation. Upgraded existing tanks must be tightness tested every 5 years for 10 years following upgrade. (Upgraded tanks have spill, overfill, and corrosion protection.) Existing tanks that have not been upgraded must be tightness tested every year until 1998. See Tank Tightness Testing for details on this method.
Unless the tank is 1,000 gallons or less, this combined method will meet the federal requirements only temporarily (as explained above). You must eventually have another monitoring method that can be performed at least once a month.
Tanks greater than 2,000 gallons in capacity may not use this method of leak detection to meet these regulatory requirements.
Beginning on October 13, 2018 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 (such as tank gauge sticks):

Check for operability and serviceability

Keep records of these checks for one year

Testing must be performed in accordance with manufacturer’s requirements; a nationally recognized code of practice; or requirements determined by your implementing agency to be no less protective of human health and the environment.

Will it work at your site?

Manual tank gauging is inexpensive and can be an effective leak detection method when used as described above with tanks of the appropriate size.

Anything else you should consider?

You can perform manual tank gauging yourself. Correct gauging, recording, and interpretation are the most important factors for successful tank gauging. The accuracy of tank gauging can be greatly increased by spreading product-finding paste on the gauge stick before taking measurements.

Will you be in compliance?

Note: Manual tank gauging can only be used for smaller tanks. Tanks 1,000 gallons or less can use this method alone, but tanks from 1,001-2,000 gallons can only use manual tank gauging when it is combined with tank tightness testing. Manual tank gauging cannot be used for tanks over 2,000 gallons. When performed according to recommended practices, manual tank gauging meets the federal leak detection requirements for USTs with a capacity of 1,000 gallons or less for the life of the tank.

USTs installed or replaced after April 11, 2016 may no longer use manual tank gauging as the primary method of release detection. USTs must be secondarily contained and use interstitial monitoring.

More Information On Manual Tank Gauging

EPA's Publication, Manual Tank Gauging: For Small Underground Storage Tanks, clearly explains how to do manual tank gauging with simple step-by-step directions. The booklet also includes standard forms used to record inventory data.

Table of Test Standards for Manual Tank Gauging

Tank Size	Minimum Duration of Test	Weekly Standard (1 test)	Monthly Standard (4-test average)
up to 550 gallons	36 hours	10 gallons	5 gallons
551-1,000 gallons (when tank diameter is 64")	44 hours	9 gallons	4 gallons
551-1,000 gallons (when tank diameter is 48")	58 hours	12 gallons	6 gallons
551-1,000 gallons (also requires periodic tank tightness testing)	36 hours	13 gallons	7 gallons
1,001-2,000 gallons (also requires periodic tank tightness testing)	36 hours	26 gallons	13 gallons

Statistical Inventory Reconciliation (SIR)

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.

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 ongoing basis occurring in an uninterrupted or nearly uninterrupted manner.

The features of SIR are:

SIR analyzes inventory, delivery, and dispensing data collected over 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 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 or a release of 150 gallons within a month and meet the federal UST 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 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 verify 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 valid for your UST system.
Beginning on October 13, 2018, 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

Testing must be performed in accordance with manufacturer’s requirements; a nationally recognized code of practice; or requirements determined by your implementing agency to be no less protective of human health and the environment.

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 April 11, 2016 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 April 11, 2016 may no longer use SIR as the primary method of release detection. USTs must be secondarily contained and use interstitial monitoring.

Additional Information on SIR

EPA developed two publications that more fully explain SIR:

Introduction to Statistical Inventory Reconciliation: For Underground Storage Tanks (EPA 510-B-95-009)
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)
March 1990.

Continuous In-Tank Leak Detection

How does the release detection method work?

Continuous in-tank leak detection (CITLD) encompasses all statistically based methods where the system incrementally gathers measurements on an uninterrupted or nearly uninterrupted basis to determine a tank’s leak status.

There are two major groups that fit into this category: continuous statistical release detection (also referred to as continuous automatic tank gauging methods) and continual reconciliation. Both groups typically use sensors permanently installed in the tank to obtain inventory measurements. They are combined with a microprocessor in the ATG system or other control console that processes the data. Continual reconciliation methods are further distinguished by their connection to dispensing meters that allow for automatic recording and use of dispensing data in analyzing tanks’ leak status.

What are the regulatory requirements?

CITLD operates on an uninterrupted basis or operates within a process that allows the system to gather incremental measurements to determine the release status of the tank at least once every 30 days.
CITLD must be able to detect a leak at least 0.2 gallon per hour with a probability of detection of at least 95 percent and a probability of false alarm of no more than 5 percent. Some CITLD methods can also detect a leak of 0.1 gallon per hour with the probabilities listed above.
Beginning on October 13, 2018, you must perform the following, as applicable, on your release detection equipment annually to make sure it is working properly:
- Verify the system configuration of the controller
- Test alarm operability and battery backup
- Inspect probes and sensors for residual build-up
- Ensure floats move freely, the shaft is not damaged, and cables are free of kinks and breaks
- Keep records of these tests for three years

Will it work at your site?

Continuous in-tank leak detection methods may allow for monitoring larger tank capacities and higher system throughputs. However, these methods have limitations as well. Check the capabilities of any method you use to ensure your system meets all regulatory performance requirements.

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.

Will you be in compliance?

For USTs installed on or before April 11, 2016 owners and operators may use CITLD as their primary method of release detection. When installed and operated according to the manufacturer's specifications, CITLD meets the federal release detection requirements. CITLD can also be used to detect leaks in piping. USTs installed or replaced after April 11, 2016 may no longer use CITLD as the primary method of release detection. USTs must be secondarily contained and use interstitial monitoring.

Tank Tightness Testing with Inventory Control

When performed according to the manufacturer's specifications, periodic tank tightness testing combined with monthly inventory control can temporarily meet the federal leak detection requirements for tanks (this method does not detect piping leaks). Inventory control alone does not meet the federal requirements for leak detection for tanks.

Tightness testing (if conducted at least every 3 years) is also an option for underground piping.

Tank Tightness Testing

How does tank tightness testing work?

Tightness tests include a wide variety of methods. Other terms used for these methods include precision, volumetric, and nonvolumetric testing.

The features of tank tightness testing are:

Many tightness test methods are volumetric methods in which the change in product level in a tank over several hours is measured very precisely (in milliliters or thousandths of an inch).
Other methods use acoustics or tracer chemicals to determine the presence of a hole in the tank. With such methods, all of the factors in the following bullets may not apply.
For most methods, changes in product temperature also must be measured very precisely (thousandths of a degree) at the same time as level measurements, because temperature changes cause volume changes that interfere with finding a leak.
For most methods, a net decrease in product volume (subtracting out volume changes caused by temperature) over the time of the test indicates a leak.
The testing equipment is temporarily installed in the tank, usually through the fill pipe.
The tank must be taken out of service for the test, generally for several hours, depending on the method.
Many test methods require that the product in the tank be a certain level before testing, which often requires adding product from another tank on-site or purchasing additional product.
Some tightness test methods require all of the measurements and calculations to be made by hand by the tester. Other tightness test methods are highly automated. After the tester sets up the equipment, a computer controls the measurements and analysis.
A few methods measure properties of the product that are independent of temperature, such as the mass of the product, and so do not need to measure product temperature.
Some automatic tank gauging systems are capable of meeting the regulatory requirements for tank tightness testing and can be considered as an equivalent method. Check with your implementing agency.

What are the regulatory requirements for tank tightness testing?

The tightness test method must be able to detect a leak at least as small as 0.1 gallon per hour with certain probabilities of detection and of false alarm.
Tightness tests must be performed periodically. UST systems installed on or before April 11, 2016 must have tank tightness tests every 5 years for 10 years following installation. After the applicable time period noted above, you must have a monitoring method that can be performed at least once every 30 days.
Beginning on October 13, 2018, you must test your release detection equipment annually to make sure it is working properly:
- Tank tightness testing is typically performed by a qualified testing company. Therefore, this requirement may not be applicable. If your implementing agency allows use of ATG systems for tank tightness testing, you must follow the testing procedures required for ATG systems.

Anything else about tank tightness testing that you should consider?

For most methods, the test is performed by a testing company. You just observe the test.
Tank tightness testing has been used primarily on tanks no more than 15,000 gallons in capacity containing gasoline and diesel. If you are considering using tightness testing for larger tanks or products other than gasoline or diesel, discuss the method's applicability with the manufacturer's representative.
Manifolded tanks generally should be disconnected and tested separately.
Procedure and personnel, not equipment, are usually the most important factors in a successful tightness test. Therefore, well-trained and experienced testers are very important. Some implementing agencies have tester certification programs.

Inventory Control

How does inventory control work?

Inventory control requires daily measurements of tank contents and mathematical calculations that let you compare your stick inventory (what you've measured) to your book inventory (what your recordkeeping indicates you should have). If the difference between your stick and book inventory is too large, your tank may be leaking.

EPA's booklet, Doing Inventory Control Right, explains how to do inventory control with simple step-by-step directions. The booklet also includes standard forms used to record inventory data.

The features of inventory control are:

UST inventories are determined daily by using a gauge stick and the data is recorded on a form. The level on the gauge stick is converted to a volume of product in the tank using a calibration chart, which is often furnished by the UST manufacturer.
The amounts of product delivered to and withdrawn from the UST each day are also recorded. At least once every 30 days, the gauge stick data and the sales and delivery data are reconciled and the month's overage or shortage is determined. If the overage or shortage is greater than or equal to 1.0 percent of the tank's flow-through volume plus 130 gallons of product, the UST may be leaking.

What are the regulatory requirements for inventory control?

Inventory control must be used in conjunction with periodic tank tightness tests.
The gauge stick should be long enough to reach the bottom of the tank and marked so that the product level can be determined to the nearest one-eighth of an inch. A monthly measurement must be taken to identify any water at the bottom of the tank.
Product dispensers must be calibrated to the local weights and measures standards.
Beginning on October 13, 2018, 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 (such as tank gauge sticks):

Check for operability and serviceability

Keep records of these checks for one year

Testing must be performed in accordance with manufacturer’s requirements; a nationally recognized code of practice; or requirements determined by your implementing agency to be no less protective of human health and the environment.

Anything else about inventory control that you should consider?

Inventory control is a practical, commonly used management tool that does not require closing down the tank operation for long periods.
The accuracy of tank gauging can be greatly increased by spreading product-finding paste on the gauge stick before taking measurements (or by using in-tank product level monitoring devices).
If your tank is not level, inventory control may need to be modified. You will need to get a corrected tank chart.

Are there time restrictions on the use of this combined method?

USTs installed on or before April 11, 2016 may use this method for 10 years after the date the tank was installed or upgraded with corrosion protection. Note that the end date is based on the compliance status of the tank only, not the entire UST system. As a result, some USTs may not be able to use this combined method for as long as 10 years. At the end of the valid period, you must use one of the monthly monitoring release detection choices described herein.

USTs installed or replaced after April 11, 2016 may no longer use inventory control combined with periodic tank tightness testing as the primary method of release detection. USTs must be secondarily contained and use interstitial monitoring.