Light hydrocarbon analyses (C1-C4) measure the lightest, most volatile
compounds present in natural gas and other petroleum based products. These
light hydrocarbon compounds tend to dissipate rapidly with time and/or
distance from the point(s) at which petroleum products are introduced
into the subsurface environment. The light hydrocarbon analyses allow
for the identification and differentiation of natural gas leaks, biogenic
methane and refined petroleum product contaminants. Light hydrocarbon
compound analyses are also utilized to determine areas of natural gas
leakage spills, the presence of gas seepage from deep seated oil and gas
reservoirs and/or the leakage from oil and gas well casings.
C5+ (pentane-xylenes+) hydrocarbons analyses which yield a quantitative
measure of the actual volume of "gasoline type " vapors present
in near surface soils and/or water are conducted on a 30-meter capillary
column. Gasoline range hydrocarbons dissipate more slowly than the lighter
fraction (C1-C4) compounds and are very useful for finding contaminated
soils. A capillary analysis of the gasoline range hydrocarbons also allows
individual quantification of the benzene, toluene, ethylbenzene, m &p-xylene
and o-xylene aromatic hydrocarbons along with other significant components.
Soil gas assessments for chlorinated hydrocarbons include analyses for
PCE, DCE, TCE, vinyl chloride and other chlorinated solvents using two
gas chromatographs: one instrument is equipped with a dry electrolytic
conductivity detector (DELCD) and the other with an electron capture detector
(ECD). Since the detection limit is relatively high (a few ppm) on the
DELCD, the ECD is also used since it has high sensitivity for quantifying
The two instruments and methods of detection serve as independent confirmation
and quality control. These instruments are excellent for quantifying various
chlorinated solvents, and their daughter products, commonly found in subsurface
soils and groundwater. Light (C1-C4) hydrocarbon analyses also provide
valuable information when used in tandem with chlorinated hydrocarbon
constituent analyses. Elevated concentrations of methane, ethane and ethylene
(ethene), in areas where elevated concentrations of chlorinated hydrocarbon
compounds are present, are indicative of anaerobic biodegradation of chlorinated
solvents. Most chlorinated solvents contain either ethane or ethene in
their molecular structures, and these more volatile compounds are very
helpful in delineating the full areal extent of the subsurface contamination.
If petroleum and/or chlorinated hydrocarbon products are present in subsurface
soils or groundwater for a period of time, significant biodegradation
of the hydrocarbon compounds occurs. The degradation of hydrocarbon compounds
by aerobic and anaerobic bacteria generate significant concentrations
of carbon dioxide and methane, respectively, in the subsurface environment.
The biodegradation of hydrocarbons by both aerobic and anaerobic bacteria
can occur within very close proximity in nature, and can yield elevated
concentrations of both carbon dioxide and methane in the subsurface environment.
These biochemical reactions are quite common in soils and groundwater
containing products such as gasoline, diesel, chlorinated solvents, etc.
since such products are excellent food sources for the indigenous bacteria.
Carbon dioxide analyses, as well as oxygen and nitrogen, are also performed
using a gas chromatograph equipped with a thermal conductivity detector
(TCD). The results of these analyses used in tandem with the petroleum
and chlorinated hydrocarbon analyses aid in the delineation of the subsurface
High resolution detailed fingerprints are utilized to correlate product
accumulations with source of contamination and areal distribution of vapor
and dissolved phase constituents in soil and water. High resolution GC
of hydrocarbon extracts from soil provides delineation of the areal and
vertical extent and type product contamination. These analyses are available
with individual component quantification.
Soil gas samples are typically analyzed for a variety of natural gas constituents
to accurately measure the seep magnitudes and compositions. Due to the
very low seepage levels often encountered in the geologic environment
it is imperative that only the most accurate gas chromatographs are employed
for sample analysis.
- Methane through Butane Light Hydrocarbons
Methane through butane light hydrocarbon content of soil gases are measured
using a computer controlled flame ionization detector (FID) gas chromatograph.
The system was designed specifically for trace light hydrocarbon analysis
of soil gas samples and has a detection limit of about 0.010 parts per
million (ppm) for individual light hydrocarbon components. Results are
computed as parts per million hydrocarbons by volume as compared to an
Samples with methane concentrations greater than about 10 ppm, which is
often too high to allow adequate separation of methane and ethane components,
are also analyzed on a second GC with a longer run time. This method allows
for a much more accurate measurement of the ethane and ethylene concentrations.
- Non Hydrocarbon Gases
Helium and hydrogen analyses are performed by computer controlled thermal
conductivity detector (TCD) gas chromatography by flow through methods
simultaneously with the light hydrocarbon analysis on a duel FID/TCD gas
chromatograph. The detection limit for helium is between 10-20 ppm, and
hydrogen is about 2 ppm by volume. Ambient air analysis by this method
generally yields helium values in the 10-20 ppm range because of interference
by neon, even though atmospheric helium values are only 5.24 ppm. Atmospheric
hydrogen values are only 0.5 ppm, and are below the detection limit using
a thermal conductivity detector.
- Quality Control Procedures
Several levels of quality control checks are employed during field and
laboratory analysis programs to help insure that final analytical results
are as accurate as possible.
are cleaned, heated, flushed
with laboratory grade nitrogen and sealed with a butyl-rubber septum and
aluminum seal prior to shipping. About 3% of all sample bottles are tested
for light hydrocarbon contamination as trip blanks prior to leaving Houston.
Trip blanks are also analyzed on return to Houston to insure that the
sample bottles have not been exposed to contamination during handling
are collected through the soil gas sample probe at
about 5% - 10% of the sample locations to insure that there has been no
carry-over contamination between samples and as a measure of the background
atmospheric hydrocarbon content of the field areas.
of each gas chromatograph with an external standard of known concentration
and a check standard to insure accuracy. Each instrument's calibration
is rechecked at the end of each shift and an additional check standard
analyzed to confirm that there has been no loss of sensitivity during
procedures includes a multi-step
process which includes review of individual chromatograms by the analyst
and further compilation of results and review by the data processing technician
to confirm that samples have been properly analyzed. After compilation
into a summary data table, results are further reviewed for accuracy by
the project manager. If any questions or discrepancies are identified,
the sample will be reanalyzed to insure accurate and reliable results.
After final review and approval, results are compiled into a series of
data listings and plot files for statistical evaluation and map generation.