Release Detection for Underground Storage Tanks (USTs) - External Methods
EPA allows owners and operators to use these external methods – groundwater monitoring and vapor monitoring – for detecting releases from their UST systems.
Monitoring for Liquids on the Groundwater
How does the leak detection method work?
This method monitors the groundwater table near an UST for the presence of released free product on the water table. Monitoring wells near the UST are checked frequently to see if petroleum can be detected. The federal UST regulation describes several requirements for use of this method. For example, this method cannot be used if the water table is more than 20 feet below the surface of the ground.
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.
What are the regulatory requirements?
- Groundwater monitoring can only be used if the stored substance does not easily mix with water and floats on top of water.
- If groundwater monitoring is to be the sole method of leak detection, the groundwater must not be more than 20 feet below the surface, and the soil between the well and the UST must be sand, gravel or other coarse materials.
- Monitoring wells must be properly designed and sealed to keep then from becoming contaminated from outside sources. The wells must also be clearly marked and secured.
- Wells should be placed in the UST backfill so that they can detect a leak as quickly as possible.
- Product detection devices must be able to detect one-eighth inch or less of leaked product on top of the groundwater.
- Beginning on October 13, 2018 you must keep records of a site assessment for as long as you use this method, showing that the monitoring system is set properly. If you do not have a site assessment, you will need to have one conducted.
- Site assessments conducted after October 13, 2015 must be signed by a licensed professional.
- 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 groundwater bailers):
- Check for operability and serviceability
- 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 well probes and sensors for residual build-up
- Test manual electronic devices (such as portable probes)
- 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?
In general, groundwater monitoring works best at UST sites where:
- Monitoring wells are installed in the tank backfill; and
- There are no previous releases of product that would falsely indicate a current release.
A professionally conducted site assessment is critical for determining these site-specific conditions.
Will you be in compliance?
For USTs installed on or before April 11, 2016 owners and operators may use groundwater monitoring as their primary method of release detection. When installed and operated according to the manufacturer's specifications and in accordance with the site assessment, groundwater monitoring meets the federal release detection requirements. Operation of the groundwater monitoring system at least once each month fulfills federal regulatory requirements. Groundwater monitoring can also be used to detect leaks in piping. USTs installed or replaced after April 11, 2016 may no longer use groundwater monitoring as the primary method of release detection. USTs must be secondarily contained and use interstitial monitoring.
Monitoring for Vapors in the Soil
How does the leak detection method work?
This method samples for either: product vapors in the soil gas surrounding the UST (sometimes called passive monitoring) or presence of a tracer compound introduced into the UST system (sometimes called active monitoring). Leaked petroleum produces vapors that can be detected in the soil gas. The federal UST regulation describes several requirements for using this leak detection method. For example, this method requires using porous soils in the backfill and locating the monitoring devices in these porous soils near the UST system.
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.
- The number of wells and their placement is very important. Only an experienced contractor can properly design and construct an effective monitoring well system. Vapor monitoring requires the installation of monitoring wells within the tank backfill. 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.
What are the regulatory requirements?
- The UST backfill must be sand, gravel or another material that will allow the vapors to easily move to the monitor.
- The backfill should be clean enough that previous contamination does not interfere with the detection of a current leak.
- The substance stored in the UST must vaporize easily so that the vapor monitor can detect a release.
- High groundwater, excessive rain, or other sources of moisture must not interfere with the operation of vapor monitoring for more than 30 consecutive days.
- Monitoring wells must be secured and clearly marked.
- Beginning on October 13, 2018, you must keep records of a site assessment, for as long as you use this method, showing that the monitoring system is set properly. If you do not have a site assessment, you will need to have one conducted.
- Site assessments conducted after October 13, 2015 must be signed by a licensed professional.
- 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
- 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
- Test manual electronic devices (such as photoionization detectors)
- 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?
Before installing a vapor monitoring system, a site assessment must be done to determine whether vapor monitoring is appropriate at the site. A site assessment usually includes at least a determination of the groundwater level, background contamination, stored product type, and soil type. This assessment can only be done by a trained professional.
Will you be in compliance?
For USTs installed on or before April 11, 2016, owners and operators may use vapor monitoring as their primary method of release detection. When installed and operated according to the manufacturer's specifications and in accordance with the site assessment, vapor monitoring meets the federal release detection requirements. Vapor monitoring can also be used to detect leaks in piping. Operation of the vapor monitoring system at least once each month fulfills federal release detection requirements. USTs installed or replaced after April 11, 2016 may no longer use vapor monitoring as the primary method of release detection. USTs must be secondarily contained and use interstitial monitoring.