UST Interstitial Monitoring FAQ
petroleum underground storage tanks (USTs) and their associated piping. These regulations were implemented due to the continued detriment
of the groundwater quality and to prevent future petroleum releases from contaminating soils and groundwater. These rules require that owners
and operators have release (leak) detection, corrosion protection, and spill/overflow protection upon installation. USTs installed prior to December 22,
1988 are required to have leak detection, and by December 22, 1998 must have corrosion protection and spill/overflow protection in place.
What
is
Interstitial
Monitoring?
Interstitial monitoring systems come in various forms. The key concept
behind this method is that it provides a second layer of protection.
This
second layer enables the leak to be detected and repaired without
coming
into contact with the environment. Some double walled UST systems are
fitted
with special ports for inserting hydrocarbon sensors periodically into
the annular space to check for leaks. Other systems utilize sensors
that
continually monitor for leaks and are wired to alarms inside the
facility.
Still other types incorporate designs that use a liner or secondary
barrier
to detect a leak. Interstitial monitoring can detect leaks in
tanks and
piping.
Requirements
for Interstitial
Monitoring:
Interstitial monitoring is defined as monitoring between the UST system
and a secondary barrier immediately around or beneath, but only if the
system
is designed, constructed, and installed to detect a leak from
any
portion of the tank routinely contains product and also meets one of
the
following
required options:
Option
#1: For
double-walled UST
systems, the sampling or
testing method
must be able to detect a leak through the inner wall in any portion of
the
tank that routinely contains product.
Option
#2: For
UST systems with
secondary barrier within the
excavation
zone, the sampling or testing method must be able to detect a leak
between
the UST system and the secondary barrier. If a system is using a
secondary
barrier, the following requirements must be meet:
- The secondary
barrier around
or beneath the UST system consists of
artificially-constructed
material that is sufficiently thick and impermeable
- (at least 10-6 cm/sec) and sloped to direct a leak of the regulated substance stored to the monitoring point and to permit its detection;
- The barrier must be compatible with the regulated substance stored so that a leak will not cause a deterioration of the barrier that may allow a release to pass through undetected;
- For cathodically protected tanks, the secondary barrier must be installed so that it does not interfere with the proper operation of the cathodic protection system;
- The groundwater, spoil moisture, or rainfall can not render the testing or sampling method used inoperative so that a leak could go undetected for more than 30 days;
- The site is assessed to ensure that the secondary barrier is always above the groundwater and is above the 25 year flood plain' unless the barrier and monitoring device are fir use under such conditions; and
- Monitoring wells are clearly marked and secured to avoid unauthorized access and tampering.
and the liner must be compatible with the substance stored.
What
records
are Required?
It is the owner/operators' responsibility to ensure that the
interstitial
space is monitored monthly and the sensors (continual or automatic
sensing
devices) are serviced regularly and are operative. The inspector will
be
looking for the following:
- Third- party evaluation for all types of leak detection sensors or probes used in the system;
- The last 12 months of your leak detection records which demonstrate that the interstitial space was checked every 30 days;
- If any fails or suspected leaks have occurred and if so, how they were investigated; and
- The maintenance and calibration documents and records are available and indicate that the appropriate maintenance procedures have been completed.
When using interstitial monitoring as a monthly indicator, a leak should be suspected if any indication of hydrocarbon vapor or free product is found during the automatic, continuous, or monthly testing. Your DWM Regional Office must be contacted within 24 hours of receiving information regarding a leak. A leak investigation, in accordance with 15A NCAC 2N.0603, must be conducted within seven days.
Hazardous
Substance Tanks and
Interstitial Monitoring
In accordance with 40 CFR 280.42, as incorporated by NCAC 15A 2N
Section
.0503all USTs containing a hazardous substance (not a
petroleum
product)
must use a form of secondary containment and check for releases within
the secondary containment every 30 days. If you are not sure
if the
product
you are storing is considered a hazardous substance, refer to section
101(14)
of the Comprehensive Environmental Response, Compensation, and
Liability
Act of 1980.
Key Elements of an Interstitial Monitoring System
Interstitial/Annular Space
(Secondary
Barrier)
This method gets its name from the space between the layers of
containment.
What makes this method effective is the secondary barrier
that prevents
the release of leaking product from spreading into the environment.
When
installed and operated properly this method is highly effective.
Three
(3) options available for interstitial systems are described above.
30
Day Testing
This method is only effective if the interstitial space is checked
every 30 days. This is necessary to find the release and react to it
before
the leak I out of control.
Manual/Automatic
Hydrocarbon
Probes
Hydrocarbon probes (sensors) come in various forms, some hard -wired
into the UST system, others are manual type probes which require them
to
be inserted into testing ports to look for leaked product. A
third-party
evaluation is required of all sensors or probes used for the purpose of
leak detection.
Common Questions
I use Interstitial Monitoring on a monthly basis. What records do I need for a compliance inspection?
- Last 12 months of interstitial monitoring well test results indicating that they were checked monthly
- Third-party evaluation for hydrocarbon sensor (contact sensor vendor if you need one).
- Maintenance and calibration records indicating appropriate maintenance procedures for the system.
No, because interstitial monitoring is an applicable monthly monitor for piping (this holds true for suction piping as well). It should be noted however that an automatic leak detector is also required for pressurized piping and it must be tested yearly.
When
do I
have a leak?
You have a suspected leak when there is an indication of a leak in
the interstitial space (i.e., detection of petroleum hydrocarbons,
detection
a pressure drop within the interstitial
space, etc.). Once a leak has
been
identified, follow the notification requirements described above.
Additional
Policy Notes:
A double-walled pressurized piping system that slopes back to a sump
with an alarm detector in it must also have an automatic line leak
detector
(ALLD). For more information about this topic, contact a member of the
UST Compliance Group or you local Regional Office.
Contact your regional
(DWM office (see map)
Or
Contact the DWM Central Office at:
NCDENR
Division of Waste Management
UST Section
1637 Mail Service Center
Raleigh NC 27699-1637
(919) 733-8486
The purpose of this page is for general guidance. More specific information on UST regulations can be found in 15A NCAC 2N.
1646 Mail Service Center, Raleigh, NC 27699-1646
(919)508-8400
