Underground Storage Tanks

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 regulations describe several requirements for the 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.

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?

When installed and operated according to the manufacturer's instructions, a groundwater monitoring system meets the federal leak detection requirements for new and existing USTs. Operation of a groundwater monitoring system at least once each month fulfills the requirements for the life of a tank. Groundwater monitoring can also be used to detect leaks in piping.

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