Leak Detection for Underground Storage Tanks
by Tammy Norman and Diana Saccone
Fall 1996
INTRODUCTION
When people go and fill up their cars with gas at the local
service station, they rarely stop and think about where the gas
is coming from or where it is being stored. Not many people
realize that they are standing over some of the biggest
contributors to groundwater contamination - leaking underground
storage tanks, commonly abbreviated as LUSTs. Underground storage
tanks (USTs) are commonly used by many facilities as a method of
storing gasoline. Slightly less than half of the USTs in the
country are located at retail gasoline stations. If these USTs
leak, they can significantly alter and contaminant local ground
water. Through the process of advection the gasoline contaminant
plume can spread out and move thereby possibly contaminating
hundreds of miles of aquifer. This poses a serious threat to
people who rely on ground water as a drinking water supply. Due
to this fact, federal and state laws require owners and operators
of underground storage tanks to have leak detection and
monitoring systems to check the tanks and surrounding soil for
possible spills or leaks. These systems serve to detect releases
at the earliest possible time and possibly prevent large amounts
of petroleum from contaminating the soil and ground water. This
web page will briefly describe some of the leak detection and
monitoring systems commonly used for USTs, in particular at
automobile fuel stations, according to the Virginia Department of
Environmental Quality regulations. The following methodologies
were taken directly from the Virginia DEQ regulation regarding
leak detection for underground storage tanks.
These are some of the different methods we hope to explore:
INVENTORY
CONTROL
Product inventory control must be conducted monthly to detect
a release of at least 1.0% of flow-through plus 130 gallons on a
monthly basis in the following manner:
- Inventory volume measurements for regulated substance
inputs, withdrawals, and the amount still remaining in
the tank are recorded each operating day;
- The equipment used is capable of measuring the level of
product over the full range of the tank's height to the
nearest one-eight of an inch;
- The regulated substance inputs are reconciled with daily
receipts by the measurement of the tank inventory volume
before and after delivery;
- Deliveries are made through a drop tube that extends to
within one foot of the tank bottom;
- Product dispensing is metered and recorded according to
regulations of the Bureau of Weights and Measures of the
Department of Agriculture and Consumer Services for meter
calibration within their jurisdiction; for all other
product dispensing meter calibration, an accuracy of six
cubic inches for every five gallons of product withdrawn
is required; and
- The measurement of any water level in the bottom of the
tank is made to the nearest one-eighth of an inch at
least once a month.
Daily sticking of an UST at local gas station.
photo by Tammy Norman
MANUAL TANK
GAUGING
Manual Tank Gauging must meet the following requirements:
- Tank liquid level measurements are taken at the beginning
and ending of a period of at least 36 hours during which
no liquid is added or removed from the tank;
- Level measurements are based on an average of two
consecutive stick readings at both the beginning and
ending of the period;
- The equipment used is capable of measuring the level of
product over the full range of the tank's height to the
nearest one-eigth of an inch;
- Only tanks of 500 gallons or less nominal capacity may
use this as a sole method of release detection. Tanks of
551 to 2,000 gallons may use the method in place of
inventory control. Tanks of greater than 2,000 gallons
nominal capacity may not use this method to meet the
requirements of leak detection.
TANK TIGHTNESS
TESTING
Tank tightness testing (or another method of equivalent
performance) must be capable of detecting a 0.1 gallon per hour
leak rate from any portion of the tank that routinely contains
product while accounting for the effects of thermal expansion or
contraction of the product, vapor pockets, tank deformation,
evaporation or condensation, and the location of the water table.
AUTOMATIC TANK
GAUGING
Equipment for automatic tank gauging that tests for the loss
of product and conducts inventory control must meet the following
requirements:
- The automatic product level monitor test can detect a
0.02 gallon per hour leak rate from any portion of the
tank that routinely contains product; and
- Inventory control ( or another test of equivalent
performance) is conducted in accordance with the
requirements mentioned above under INVENTORY CONTROL.
Tidel
automatic tank gauge system at local gas station.
Photo by Tammy Norman
VAPOR
MONITORING
Testing or monitoring for vapors within the soil gas of the
excavation zone must meet the following requirements:
- The materials used as backfill are sufficiently porous
(eg. gravel, sand, or crushed rock) to readily allow
diffusion of vapors from release into the excavation
area;
- The stored regulated substance, or a tracer compound
placed in the tank system, is sufficiently volatile
(e.g., gasoline) to result in a vapor level that is
detectable by the monitoring devices located in the
excavation zone in the event of a release from the tank;
- The measurement of vapor by the monitoring device is not
rendered inoperative by the ground water, rainfall, or
soil moisture or other known interferences so that a
release could go undetected for more than 30 days;
- The vapor monitors are designed and operated to detect
any significant increase in concentration above
background of the regulated substance stored in the tank
system, a component or components of that substance, or a
tracer compound placed in the tank system;
- In the UST excavation zone, the site is assessed to
ensure compliance with the requirements of the this
section and to establish the number and positioning of
monitoring wells that will detect releases within the
excavation zone from any portion of the tank that
routinely contains product; and
- Monitoring wells are clearly marked and secured to avoid
unauthorized access and tampering.
GROUNDWATER
MONITORING
Testing or monitoring for liquids on the ground water must
meet the following requirements:
- The regulated substance stored is not readily miscible in
water and has a specific gravity less than one;
- Ground water is never more than 20 feet from the ground
surface and the hydraulic conductivity of the soils
between the UST system and the monitoring wells or
devices is not less than 0.01 cm/sec (e.g. the soil
should consist of gravels, coarse to medium sands, coarse
slits or other permeable materials);
- The slotted portion of the monitoring well casing must be
designed to prevent migration of natural soils or filter
pack into the well and to allow entry of regulated
substance on the water table into the well under both
high and low groundwater conditions;
- Monitoring wells shall be sealed from the ground surface
to the top of the filter pack;
- Monitoring wells or devices intercept the excavation zone
or areas close to it as is technically feasible;
- The continuous monitoring devices or manual methods used
can detect the presence of at least one-eigth of an inch
of free product on the top of the groundwater in the
monitoring wells;
- Within and immediately below the UST system excavation
zone, the site is assessed to ensure compliance with the
requirements of this section and to establish the number
and positioning of monitoring wells or devices that will
detect releases from any portion of the tank that
routinely contains product; and
- Monitoring wells are clearly marked and secured to avoid
unauthorized access and tampering.
Ground
water Monitoring Well Cover. Photo by Tammy Norman
CONCLUSION
After visiting several of the local gas stations in
Blacksburg, we found that most stations were using at least one
of the aforementioned methods for leak detection and monitoring.
If all gas stations were to use these regulatory methods
correctly then the likelihood of ground water contamination would
be greatly reduced. So the next time you fill up your car at the
gas station, stop and think about the importance of leak
detection and monitoring systems that exist to safeguard our
precious natural resource, ground water.
For more information regarding regulation of USTs or general
information about USTs:
Virginia
Department of Environmental Quality Home page
U.S. Environmental Protection Agency
Texaco, Inc.
Chevron
Homepage
Acknowledgments
- Exxon Service Station located on the corner of Main
Street and Turner Street, Blacksburg
- Robert at Exxon
- Wilco Service Station on North Main Street, Blacksburg
- Rix Johnston, Manager of Wilco
- Texaco Service Station located on North Main Street,
Blacksburg
- Citco Service Station located on Tom's Creek
- Dougie at Citco
- Chevron located on South Main Street, Blacksburg
References
Virginia Department of Environmental Quality. "Virginia
Regulations for USTs." 1989.
Send comments or suggestions to:
Student Authors: Tammy Norman, tnorman@vt.edu
and Diana Saccone, dsaccone@vt.edu
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1996 Daniel Gallagher
Last Modified: 011-22-1996
