LIGHT HYDROCARBON ANALYSES
HIGH RESOLUTION GC C5+ (PENTANE-XYLENES) HYDROCARBON ANALYSES/SOIL VAPOR AND/OR WATER
CHLORINATED HYDROCARBONS
ANALYSIS FOR FIXED GAS (CO2, O2, N2)
HIGH RESOLUTION GC OF PRODUCT AND SOIL EXTRACTS

LIGHT HYDROCARBON ANALYSES

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.

HIGH RESOLUTION GC C5+ (PENTANE-XYLENES) HYDROCARBON ANALYSES/SOIL VAPOR AND/OR WATER

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.

CHLORINATED HYDROCARBONS

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 chlorinated compounds. 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.

ANALYSIS FOR FIXED GAS (CO2, O2, N2)

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 contaminant plume(s).

HIGH RESOLUTION GC OF PRODUCT AND SOIL EXTRACTS

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.

Sample Analysis

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 external standard.
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.

Sample bottles 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 and storage.

Field Blanks 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.

Instrument Calibration includes calibration 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 the shift.

Data Validation 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.