United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas, NV 89193
Research and Development
EPA/600/S4-91/009 July 1991
EPA Project Summary
Background Hydrocarbon
Vapor Concentration Study
for Underground
Fuel Storage Tanks
Claude A.J. Schleyer
This project was initiated to investi-
gate the effectiveness of soil gas sam-
pling in leak detection. Soil gas sur-
veys were performed at 27 active gaso-
line service stations in three diverse
geographic regions. Hydrocarbon va-
por concentrations in the backfill sur-
rounding the underground storage
tanks were sampled and analyzed. The
27 gasoline service stations were se-
lected as non-leaking sites and the
three regions were selected for their
active underground storage tank regu-
latory programs, as well as their differ-
ences in geology, hydrology and cli-
mate.
A comparison was made with con-
taminated site data obtained from
Tracer Research Corporation's histori-
cal records and significant differences
can be seen between the two distribu-
tions. It was determined that the best
approximation of total hydrocarbon
(less light aliphatics) concentrations,
based on available calibration data, was
achieved using an average response
factor calculated from the daily re-
sponse factors of benzene, toluene,
ethylbenzene, and ortho-xylene.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Las Vegas, NV, to
announce key findings of the research
project that Is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
A field sampling program was under-
taken around underground storage tanks
(USTs) to establish a baseline data set of
hydrocarbon vapor concentrations. Data
were collected from 27 gasoline service
stations selected as non-leaking sites, in
three diverse geographic regions: Central
Texas (Austin, Texas); areas surrounding
Long Island Sound (Suffolk County, New
York; Providence, Rhode Island; Storrs,
Connecticut); and Southern California (San
Diego, California). The three regions were
selected for their active UST regulatory
programs, as well as their differences in
geology, hydrology, and climate.
Procedures
A site was considered to be non-leak-
ing if it had good inventory and mainte-
nance records, or had recently passed a
tank tightness test. The non-leaking data-
base consists of 279 soil vapor samples
from 25 service stations. At the other two
stations, observed or suspected leaks pre-
vented their data from being used in the
non-leaking database.
At each location, soil was sampled at
varying distances and depths from UST
appurtenances (such as submersible
pumps, vents, and product flow lines) to
determine if a particular pattern of hydro-
carbon concentration existed. Samples
were collected by driving a hollow steel
probe into the ground, and evacuating 5
to 10 liters of soil vapors with a vacuum
pump. Volatile hydrocarbon species were
Printed on Recycled Paper
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Identified and quantified at the site by
utilizing gas chromatograph/flame ioniza-
tion detection (GC/FID) equipment. Ten to
fifteen samples were collected and ana-
lyzed at each site.
The types of compound groups that
were studied were aliphatics, aromatics,
and total hydrocarbons. The concentra-
tions of volatile aliphatics that elute from
the GC column before benzene were re-
ported as a group called "light aliphatics."
At 18 of the sites, the "light aliphatics"
represent aliphatic compounds such as
methane, ethane, propanes, butanes, and
pentanes. At seven of the sites where
butanes and pentanes could be quantified
and reported, the concentration of "light
aliphatics" represent only methane, ethane,
and propanes. The aromatics reported
were benzene, toluene, ethylbenzene, and
the xylenes.
Hydrocarbon concentrations in soil gas
are reported in micrograms per liter (mg/
L). These concentrations were calculated
directly from the GC/FID using calibration
gas response factors and sample volumes.
The concentration of total hydrocarbons
(less light aliphatics) were estimated us-
ing an average response factor from the
gas standards; benzene, toluene,
ethylbenzene and ortho-xylene (BTEX).
The concentrations in mg/L were converted
to parts per million by volume (ppmv),
using average molecular weights of BTEX
at each site, and the ambient tempera-
tures and pressures.
Hydrocarbon vapor concentrations from
the non-leaking sites range from detection
limit levels of 0.02 micrograms per liter
(Hg/L) to maximum values of 1,500,000
pg/L of light aliphatics, 110,000 u.g/L of
benzene, 160,000 ug/L of toluene, 25,000
ug/L of ethylbenzene, and 110,000 ug/L
of xylenes. The maximum concentration
of total hydrocarbons (less light aliphatics)
1,100,000 ug/L. Determination of total hy-
drocarbon concentrations exclude the light
aliphatic peaks in order to elevate the
compounds most representative of gaso-
line. Additionally, subtraction of the light
aliphatics peaks precludes the inclusion
of methane concentrations caused by natu-
rally occurring organic decomposition.
Results and Discussion
The statistical distribution of total hydro-
carbons (less light aliphatics) indicates that
a majority of the concentration values are
in the lower concentration ranges. The
relative frequency distribution shows 53.2
percent of the samples below 1,500 ug/L
(500 ppm by volume) and 93.1 percent
below 100,000 ug/L (27,000 ppm by vol-
ume). The median is 800 ug/L and the
mean is 23,300 ug/L.
Contamination site data were obtained
from Tracer Research Corporation's his-
torical records. The contaminated site data
consists of 60 soil vapor samples taken
from nine sites having known contamina-
tion from petroleum fuel leak or spill. These
sites were all active gasoline service sta-
tions or fueling facilities. The contaminated
site data also shows much variability. The
statistical distribution of total hydrocarbons
(less light aliphatics) shows a majority of
sample values to be in the lower concen-
tration ranges. The relative frequency dis-
tribution shows 35 percent of the samples
below 1,500 ug/L (500 ppm by volume)
and 66.7 percent below 100,000 ug/L
(27,000 ppm by volume). The median is
9,000 ug/L and the mean is 160,000 ug/L.
Although much variability exists in both
the non-leaking and contaminated site
data, significant differences could be seen
between the two distributions. A 10-fold
difference exists between the numbers of
samples with concentrations above
100,000 ug/L (3,000 ppmv) for the two
data sets. For example, 29.6 percent of
the non-leaking samples occurred in the
range of 10,000 ug/L to 100,000 ug/L while
33.3 percent of the contaminated samples
concentrations occurred in the range of
100,00 ug/L to 1,000,000 ug/L.
Statistical data patterns associated with
site location and sample depth were de-
lineated using non-parametric statistical
methods. Statistically significant differences
were found to exist between the total hy-
drocarbon (less light aliphatics) vapor con-
centrations among the five locations stud-
ied for steel tank systems, whereas these
differences were not significant for fiber-
glass tank systems. Statistically significant
differences also occurred between the to-
tal hydrocarbon (less light aliphatics) va-
por concentrations among the sample
depths of 2,6, and 10 feet for both steel
and fiberglass tank systems. Higher con-
centrations were found at the lower depths.
A fresh spill at one station in Austin
provided an opportunity to add butane to
the list of analytes under study. The bu-
tane concentration in 15 soil gas samples
taken during the first four days after the
spill occurred ranged from 530 ug/L to
300,000 ug/L. Butane was also sampled
at sites in Storrs, Connecticut, and Provi-
dence, Rhode Island, both of which had
no evidence of recent leaks or spills. At
these two sites, butane concentrations in
65 soil gas samples ranged from the
mimimum detection limit of 0.02 ug/L to
930 ug/L. The large difference between
the butane concentrations at the fresh spill
site in Austin and the non-leaking sites in
Connecticut and Rhode Island suggest that
butane may be a good indicator of a fresh
spill or leak.
Conclusion
Because there are no standard proce-
dures for estimating and reporting total
hydrocarbon concentration data, Geo-
science Consultants, Ltd. evaluated differ-
ent estimation methods. It was determined
that the best approximation of total hydro-
carbon (less light aliphatics) concentra-
tions, based on available calibration data,
was achieved using an average response
factor calculated from the daily response
factors of benzene, toluene, ethylbenzene,
and ortho-xylene.
•ifV.S. GOVERNMENT PRINTING OFFICE: 1991 - 548-028/40039
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Claude A. J. Schleyerls with Geoscience Consultants, LTD., Albuquerque, NM 87102.
Katrlna E. Varner is the EPA Project Officer (see below).
ThB complete report, entitled "Background Hydrocarbon Vapor Concentration Study for
Underground Fuel Storage Tanks," (Order No. PB91-191353/AS; Cost: $31.00,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Las Vegas, NV 89193
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S4-91/009
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