Underground Storage Tanks: Types, Regulations, Release Detection, Corrosion Protection, and Operation & Maintenance

     While doing this research, I was struck by the variability of methods and the details of managing underground storage tanks (USTs) and leak detection. Something seemingly as simple as containing a fluid in a tank can get complex and detailed, especially when trying to detect and measure leaks. 33 years ago, I applied for a job with an environmental consulting firm doing compliance for USTs. I didn’t get the job. Now, I am doing this research to prepare a cover letter for a job at a state agency doing the same from the regulator's perspective. I am pasting a lot of information from the EPA in this post as it is generally concise and informative.

        According to the EPA:

"Underground storage tank" or "UST" means any one or combination of tanks including connected underground pipes that is used to contain regulated substances, and the volume of which including the volume of underground pipes is 10 percent or more beneath the surface of the ground.”

USTs are regulated by the EPA. According to Wikipedia:

“In 1984, U.S. Congress amended the Resource Conservation Recovery Act to include Subtitle I: Underground Storage Tanks, calling on the U.S. Environmental Protection Agency (EPA) to regulate the tanks. In 1985, when it was launched, there were more than 2 million tanks in the country and more than 750,000 owners and operators. The program was given 90 staff to oversee this responsibility. In September 1988, the EPA published initial underground storage tank regulations, including a 10-year phase-in period that required all operators to upgrade their USTs with spill prevention and leak detection equipment.”

Tank owners are responsible for the cleanup of any leaks or spills detected. They must be bonded or carry UST insurance.

“EPA updated UST and state program approval regulations in 2015, the first major changes since 1988.[6] The revisions increase the emphasis on properly operating and maintaining UST equipment. The revisions will help prevent and detect UST releases, which are a leading source of groundwater contamination.

“In addition, EPA added new operation and maintenance requirements and addressed UST systems deferred in the 1988 UST regulation. The changes:

·        Added secondary containment requirements for new and replaced tanks and piping

·        Added operator training requirements

·        Added periodic operation and maintenance requirements for UST systems

·        Added requirements to ensure UST system compatibility before storing certain biofuel blends

·        Removed past deferrals for emergency generator tanks, field-constructed tanks, and airport hydrant systems

·        Updated codes of practice

·        Made editorial and technical corrections.

 

Types of Underground Storage Tanks

Underground storage tanks fall into four different types:

1)        Steel/aluminum tanks, made by manufacturers in most states and conforming to standards set by the Steel Tank Institute.

2)        Composite overwrapped, a metal tank (aluminum/steel) with filament windings like glass fiber/aramid or carbon fiber or a plastic compound around the metal cylinder for corrosion protection and to form an interstitial space.

3)        Tanks made from composite material, fiberglass/aramid or carbon fiber with a metal liner (aluminum or steel). See metal matrix composite.

4)        Composite tanks such as carbon fiber with a polymer liner (thermoplastic). See rotational molding and fibre-reinforced plastic (FRP).

          The EPA explains the laws and regulations regarding underground storage tanks:

“A complete version of the law that governs underground storage tanks (USTs) is available in the U.S. Code, Title 42, Chapter 82, Subchapter IX. This law incorporates amendments to Subtitle I of the Solid Waste Disposal Act as well as the UST provisions of the Energy Policy Act of 2005 and gives EPA the authority to regulate USTs.”

“The UST provisions of the Energy Policy Act focus on preventing releases. Among other things, it expands eligible uses of the Leaking Underground Storage Tank (LUST) Trust Fund and includes provisions regarding inspections, operator training, delivery prohibition, secondary containment and financial responsibility, and cleanup of releases that contain oxygenated fuel additives.”

“The LUST provision of the American Recovery and Reinvestment Act appropriated $200 million to assess and clean up leaks from underground storage tanks. The vast majority of the money is allocated to states and territories in the form of assistance agreements to address shovel-ready sites within their jurisdictions.”

     Design, construction, installation, and maintenance of USTs must be according to industry codes and standards. Corrosion protection, upgrading, assessing tank integrity, repair, interior lining, piping, filling practices, release detection equipment, and closing USTs must also abide by industry codes and standards.

     Proper documentation and closure of USTs, particularly old ones that have leaked, is very important. Most USTs made before the mid-1980s were made of bare steel, which is likely to corrode over time and lead to releases into the environment. Incorrect installation or inadequate operating and maintenance procedures also can cause releases. The main hazards of LUSTs are contents, often petroleum or related chemicals, leaking into the local soil or groundwater.

     The following types and sizes of USTs are exempt from the regulations.

·        Farm and residential tanks of 1,100 gallons or less capacity holding motor fuel used for noncommercial purposes;

·        Tanks storing heating oil used on the premises where it is stored;

·        Tanks on or above the floor of underground areas, such as basements or tunnels;

·        Septic tanks and systems for collecting storm water and wastewater;

·        Flow-through process tanks;

·        UST systems of 110 gallons or less capacity; and

·        Emergency spill and overfill tanks.

      There are notification requirements for the installation of USTs, documenting existing USTs, for change of ownership, and for closure. Hazardous substances stored in USTs must have secondary containment. This may be achieved by double-walled tanks where a UST is enclosed in another tank. Double-walled piping is also required. Lining the excavation zone around the UST with an impenetrable liner is another way to achieve secondary containment. Hazardous substances must also be outfitted with interstitial monitoring where the space between the inner wall and outer wall is monitored for leaks. All UST systems are required to have spill, overfill, and corrosion protection. O & M of USTs should be documented. Leak detection equipment, corrosion protection, and tightness must be tested. Tightness testing should be performed annually.

     An underground oil tank has a life of just 10-15 years according to Chem Service. They note that after 20 years it is a good idea to have the tank removed. They note the following UST testing methods:

·        Precision Tank Tightening Test: used to measure for gallons per hour (gph) of tank leak to better understand the ullage of a tank.

·        Line and Leak Detector Testing: used to identify potential petroleum ground leaks by gauging tank volumetric pressure via a mechanical or line leak detector.

·        Secondary Containment Testing: used to safeguard the environment and local water supplies from potential leaks.

·        Stage 1-2 vapor recovery testing: a test used to ensure compliance with air quality regulations.

What are UST Testing Standards for Tightness? (from Chem Service)

It is required that a UST pass what’s called a tank tightness test (otherwise known as precision testing or integrity testing). As part of this test, it must be proven that there are no tank leaks that exceed more than .1 gallons per hour (gph). This test can be conducted in one of two ways: a technician manually checking it on-site, or via an automatic tank gauge that’s been pre-certified to perform the test.

When conducting a tank tightness test it’s important to keep in mind a few factors:

·        Allow enough time for the temperature to adjust from one product to the next

·        To identify if the tank is tilted, look for vapor pockets

·        Groundwater may mask a leak by creating counter pressure

·        Bungs on top of the tank are less tight

 

Release Detection

     The EPA recognizes three general methods of UST leak detection, or release detection: interstitial, internal, and external. EPA release detection requirements are shown below.

 

Interstitial Methods

As noted, the interstitial method involves secondary containment with interstitial monitoring or monitoring for leaks in the interstitial space between primary containment (the single-walled tank) and secondary containment (the second outer wall, if applicable, or the space between the tank and the impenetrable liner in the excavation zone). Secondary containment barriers may include the following:

·        Double walled or jacketed tanks, in which an outer wall partially or completely surrounds the primary tank;

·        Internally fitted liners (bladders); and

·        Leakproof excavation liners that partially or completely surround the tank.

·        Clay and other earthen materials cannot be used as barriers.

Interstitial monitors are installed in the interstitial space. Some interstitial monitors measure the presence of the leaked fluid. Others measure changes in pressure, loss of vacuum, or changes in a monitoring liquid, all indicative of a leak. Monitors can also be sophisticated automated devices that monitor for leaks continuously. There are rules for interstitial monitoring such as a requirement to check the monitor at least once every 30 days. Excavation liners must be designed to direct any leak toward the monitor. Monitors must not interfere with the tank's cathodic corrosion protection system. They must be above groundwater or a 25-year floodplain. Newer requirements include the following:

“Beginning on October 13, 2018 you must either test your containment sumps used for interstitial monitoring at least once every three years to ensure the equipment is liquid tight by using vacuum, pressure, or liquid testing or use a double-walled containment sump where the space between the sump is periodically monitored.”

 

Internal Methods

     Internal release detection methods include automatic tank gauging, manual tank gauging, statistical inventory reconciliation, and tank tightness testing with inventory control. Automatic tank gauging (ATG) utilizes a probe installed permanently at the bottom of the tank that measures tank level and temperature. These systems can operate in inventory mode or leak detection mode. Leak tests may be done manually or automatically. Manual leak tests are in-tank static tests and automatic leak tests are continuous in-tank leak detection tests. ATG systems must comply with he following rules:

“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

Tanks should also be periodically checked for water infiltration into the tank, although this is difficult to do. Water around a tank may mask a hole in the tank or distort the data to be analyzed by temporarily preventing a release. After 2016 ATG systems may not be used as the primary leak detection method.

     Manual tank gauging may be used as the primary leak detection for tanks less than 1000 gallons. For tanks between 1000 and 2000 gallons manual tank gauging may be used in combination with periodic tank tightness testing. According to the EPA:

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.

     Statistical inventory reconciliation (SIR) involves software-based statistical analysis of inventory, delivery, and dispensing data, which must be collected and supplied to the vendor on a regular basis. According to the EPA:

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.”

An SIR system 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 all requirements regarding detection and false alarm probabilities. It must be able to detect a leak as small as 0.1 gallons per hour to be considered equivalent to a tank tightness test. SIR has been used on tanks up to 18,000 gallons capacity.

Continuous in-tank leak detection (CITLD) is of two main types: continuous statistical release detection (also referred to as continuous automatic tank gauging methods) and continual reconciliation. Both use sensors for inventory measures and are then processed via the ATG system software. Sensors in the dispensing systems may also be utilized in the reconciliation methods. Regulatory requirements are similar to the other methods. Newer requirements according to the EPA include the following:

“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

CITLD may be used as a primary release detection method.

     Tank tightness testing with monthly inventory control can meet the requirements for tanks, but not for piping. Tightness testing for piping should be performed every three years. The EPA explains tank tightness testing very well below:

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.

“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.”

Tank tightness testing is typically used on tanks of 15,000 gallons or greater that hold gasoline or diesel fuel. EPA notes that procedure and personnel are more key to successful tank tightness testing, rather than equipment. Regulatory agencies may provide training and certification for tank tightness testing.  

Inventory control utilizes frequent, usually daily measurements of tank levels 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.” Like the other methods, inventory control is standardized. It is used in combination with periodic tank tightness testing.

 

External Methods

     There are two external methods for UST release detection. These are groundwater monitoring and soil vapor monitoring. These two methods are also used for environmental monitoring around landfills and other waste disposal sites. Groundwater monitoring wells may only be used around a UST if the water table is less than 20 feet below the surface.  The EPA summarized groundwater monitoring for USTs:

Features of groundwater monitoring are:

·        Groundwater monitoring involves the use of permanent monitoring wells placed close to the UST. The wells are checked at least monthly for the presence of product that has leaked from the UST and is floating on the groundwater surface.

·        The two main components of a groundwater monitoring system are the monitoring well (typically a well of 2-4 inches in diameter) and the monitoring device.

·        Detection devices may be permanently installed in the well for automatic, continuous measurements for leaked product.

·        Detection devices are also available in manual form. Manual devices range from a bailer (used to collect a liquid sample for visual inspection) to a device that can be inserted into the well to electronically indicate the presence of leaked product. Manual devices must be operated at least once a month.

·        Before installation, a site assessment is necessary to determine the soil type, groundwater depth and flow direction, and the general geology of the site. This assessment can only be done by a trained professional.

·        The number of wells and their placement is very important. Only an experienced contractor can properly design and construct an effective monitoring well system. A minimum of two wells is recommended for a single tank excavation. Three or more wells are recommended for an excavation with two or more tanks. Some state and local agencies have developed regulations for monitoring well placement.

UST groundwater monitoring is only used for stored fluids that float on water or otherwise do not mix with water, such as diesel fuel and gasoline. The wells should be installed in the excavation zone of the tank so that they can detect leaks as quickly as possible.

     Soil vapor monitoring involves sampling the vapor in the soil porosity. Passive soil vapor monitoring uses vapors detected in the soil that arise from product leaked from the tank. Active soil vapor monitoring uses chemical tracers added to the UST that can be detected in soil vapor. Porous soil is typically used in the backfill around the UST and probes are installed in that soil. According to the EPA:

Features of vapor monitoring systems are:

·        Passive vapor monitoring senses or measures fumes from leaked product in the soil around the tank to determine if the tank is leaking.

·        Active vapor monitoring senses or measures a tracer compound leaked in the soil around the tank to determine if the tank is leaking.

·        Fully automated vapor monitoring systems have permanently installed equipment to continuously or periodically gather and analyze vapor samples and respond to a release with a visual or audible alarm.

·        Manually operated vapor monitoring systems range from equipment that immediately analyzes a gathered vapor sample to devices that gather a sample that must be sent to a laboratory for analysis. Monitoring results from manual systems are generally less accurate than those from automated systems. Manual systems must be used at least once a month to monitor a site.

·        All vapor monitoring devices should be periodically calibrated according to the manufacturer's instructions to ensure that they are properly responding.

·        Before installation, a site assessment is necessary to determine the soil type, ground water depth and flow direction, and the general geology of the site. This can only be done by a trained professional.

 

Corrosion Protection

     The Defense Logistics Agency (DLA) explains corrosion protection for USTs below:

·        Use tanks of non-corrodible materials, such as fiberglass-reinforced plastic or a steel−fiberglass-reinforced plastic composite

·        Protect steel tanks by coating them with a suitable non-conducting material, and use a cathodic protection system (with impressed current or sacrificial anode, as described below)

·        Get approval from your regulatory agency for another corrosion protection method that is equally safe (new techniques are emerging).

      An impressed current system uses a rectifier to convert alternating current to direct current (see Exhibit 3−1). This current is sent through an insulated wire to the anodes, which are special metal bars buried in the soil near the UST. The current then flows through the soil to the UST system and returns to the rectifier through an insulated wire attached to the UST. The UST system is protected because the current going to the UST system overcomes the corrosion-causing current normally flowing away from it.”

“Another type of cathodic protection is called a sacrificial anode or galvanic anode system (see Exhibit 3−2). The galvanic anode system relies on the natural potential difference between the metallic sacrificial anode (usually aluminum, magnesium, or zinc) and the steel tank to provide a protective flow of current. Metal ions migrate from the more reactive metal anode to the tank, and in the process, the anodes corrode (are sacrificed).”

Corrosion-protection systems must also be tested and evaluated periodically by certified qualified testers.

 

Spill and Overflow Prevention Equipment

     In order to prevent spills and accidental overflows this equipment must be installed. Spill catchment basins, as shown below, are used to prevent spills. These are used in combination with overflow control equipment such as automatic shutoff devices, overfill alarms, and ball float valves (fitted to the vent pipe), as shown in the second figure below.

 

UST Operator Training

     As noted, training is very important in operating and maintaining USTs. There are three levels of training: Class A, Class B, and Class C. The DLA describes the training levels:

·        Class A operators have the knowledge and skills to make informed decisions for UST compliance and the ability to determine if operating staff are operating the system in compliance.

·        Class B operators have the knowledge to implement UST compliance requirements for the site-specific UST system components.

·        Class C operators have the knowledge to take actions in response to emergencies, alarms, or spills resulting from operating the UST system.

    The DLA lists UST reporting requirements in the table below:

 

 

References:

 

Underground Storage Tanks (USTs) Laws and Regulations. U.S. EPA. Underground Storage Tanks (USTs) Laws and Regulations | US EPA

Underground storage tanks. Wikipedia. Underground storage tank - Wikipedia

Release Detection for Underground Storage Tanks (USTs) – Introduction. U.S. EPA. Release Detection for Underground Storage Tanks (USTs) - Introduction | US EPA

Release Detection for Underground Storage Tanks (USTs) - Interstitial Method. U.S. EPA. Release Detection for Underground Storage Tanks (USTs) - Interstitial Method | US EPA

Release Detection for Underground Storage Tanks (USTs) - Internal Methods. U.S. EPA. Release Detection for Underground Storage Tanks (USTs) - Internal Methods | US EPA

Release Detection for Underground Storage Tanks (USTs) - External Methods. U.S. EPA. Release Detection for Underground Storage Tanks (USTs) - External Methods | US EPA

Frequent Questions About Underground Storage Tanks. U.S. EPA. Frequent Questions About Underground Storage Tanks | US EPA

Underground Storage Tanks (USTs). U.S. EPA. Underground Storage Tanks (USTs) | US EPA

Energy Environmental Guide for Fuel Facilities (PDF) (March 2019. CHAPTER 3: Underground Storage Tanks. Defense Logistics Agency. Chapter3_UndergroundStorageTanks_Mar2019.pdf

How do you Test Underground Storage Tank (UST) Tightness using Standards? Boomtown Devs. Chem Service, Inc. October 27, 2020. How do you test Underground storage tank tightness using standards?Chemservice News