[Volume 3-1 Index]

Soil Gas Survey Methodology

December 1999


Soil gas surveys were conducted at AOC 37, AOC 41, and AOC 66 in December 1999. A summary of the soil gas survey methods, from determination of sampling locations through sample analysis and quality control procedures, is presented in the following subsections. The quality assurance/quality control (QA/QC) procedures used to insure the integrity of the soil gas data are also discussed. Results of the survey at each site are described behind the site-specific tabs in Volume 3-2.

Soil Gas Collection Methods

A summary of the soil gas survey methods, from determination of sampling locations through sample analysis and quality control procedures, is presented in the following subsections. The quality assurance/quality control (QA/QC) procedures used to insure the integrity of the soil gas data are also discussed. A discussion of the sampling results and their implications for each site is presented in Volume 3-2.

Determination of Sampling Locations

Soil gas sampling locations were based on a grid set up at the site. Grid`` points were established at 100-foot intervals.

Sampling Method

Soil gas samples collected in Tedlar bags were obtained by the following methodology:

  1. Decontaminated 5/8-inch diameter hollow steel sampling probes were driven into the ground by an electric hammer to provide access for sampling soil gases. The probes were driven into the soil to a maximum depth of 6 feet or until refusal. Soils up to 6 feet in thickness were encountered at each AOC. The minimum thickness of soil encountered was 1.5 feet of overburden at AOCs 41 and 66. For the survey areas, the overall soil stratum had an average soil thickness of 4.2 feet. On average, the soil horizon was better developed at AOC 41 than at AOCs 37 and 66.

  2. Next, a three-step process was implemented in preparation for sampling. First, the probes were pulled up slightly to detach the driving tip from the probe and thus allow soil gas to enter the probe. Next, a sampling adapter was placed on top of the probe and a Tygon tube run from the adapter to a vacuum pump used to withdraw soil gases from the ground. Third, the system was purged with 5 to 15 probe volumes prior to sampling to insure that a representative sample of soil gas was be obtained. The variance in purging volumes was dependent upon the inherent permeability of the soil being tested. A consistent amount of time was required to perform all the activities associated with collection of the sample. In those areas were soils tended to be more permeable, an increased amount of soil gas could be purged prior to the actual collection of the sample.

  3. After purging was completed, the desiccator was opened and a new tedlar bag was connected to the line from the sampling probe. The top of the desiccator was put back in place.

  4. The vacuum pump was then used to withdraw soil gas from the ground.

  5. After a sample was collected, the bag was removed from the desiccator, the valve on the bag closed and transported to the gas chromatograph for analysis.

The samples were transported to the on-base field gas chromatograph (GC) located on CSSA for analysis. Samples were analyzed within five hours of collection. After sampling, probes were decontaminated for use at another location. The resulting hole was abandoned with dry, powdered cement.

Soil Gas Sample Screening

An initial screening of the soil gas samples was performed in the field concurrent with sample collection. This was done by scanning the exhaust from the vacuum pump with an explosimeter. The vacuum pump was a rotary vane, oil-less, 1/6 horsepower model equipped with a vacuum regulator. An Industrial Scientific Corporation, Model HMX 271 explosimeter was used to measure the levels of oxygen.

The explosimeter was calibrated daily for oxygen readings by setting the readout to 20.9 percent oxygen when held in ambient air. For oxygen measurements, the explosimeter had a stated accuracy of + 1.2 percent oxygen by volume in the range of five to 30 percent.

Soil Gas Sample Analysis

Analytical Equipment

Soil gas samples were analyzed on a HNu model 321 GC equipped with an electron-capture detector (ECD) and a photoionization detector (PID) with a 10.2 electron volt (eV) light source. A Spectra-Physics model 4400 dual-channel integrator was used to plot the chromatograms, to measure the size of the peaks, and to compute compound concentrations.

The chromatographic column used for analysis was a 12-foot long, 1/8-inch diameter stainless steel packed column containing three percent OV-101 Chromosorb W-HP packing material with a 100/120 mesh particle size. The OV-101 Chromosorb W-HP is the column packing material that performs the actual separation of compounds. This column was selected for use since it is able to separate the compounds targeted for analysis and allows for a relatively rapid analysis time.

Target Compounds and Standard Preparation

Eight compounds were targeted for calibration and analysis. These compounds were the fuel components benzene, toluene, ethylbenzene, m/p/o-xylenes (BTEX), and tetrachloroethene (PCE), cis-1,2-dichloroethene (DCE), and trichloroethene (TCE).

Calibration standards used to calibrate the PID and the ECD were prepared in a two-step procedure. First, a primary standard was prepared by diluting certified pure chemicals into methanol. Second, a quantity of each primary standard was then injected into a 40-milliliter (mL) vial to make the day to day working standards.

The primary standards were prepared by diluting certified pure chemicals into methanol. The primary standards were prepared using compounds with certified purities typically greater than 99 percent and individual samples of each chemical were lot numbered by the manufacturer for quality control. These calibration standards were obtained from Chem Service, Inc. of Westchester, Pennsylvania.

The calculation of the primary standard concentration in a vial filled with methanol and specifically for TCE in a 40 mL VOA vial is as follows:

Equation 1:

(V �L)(D g/cm3) (cm3/1000 �L) (1000mL/L /40 mL) (106 g/g) = C �g/L

V = Volume of pure compound injected in micro liters (�L)

D = Density of Compound in grams per cubic centimeter (g/cm3)

C = Concentration of compound in micrograms per liter (�g/L)

Example - TCE

(2 �L)(1.464 g/cm3) (cm3/1000 �L) (1000mL/L /40 mL) (106 g/g) = 73,210 �g/L

Preparation of Calibration Standard

A quantity of each primary standard is then injected into a 40-mL vial to make the daily calibration standards used in the calibration of the GC. The concentrations of the standards were in micrograms per liter. The concentration of the calibration standard was within the linear range of the detector.

The calibration standard was produced by injecting a portion of the stock solution into a 40-mL VOA vial and then evaporating the methanol solution in the vial. The standard was analyzed while the vial was still hot. An example of the calculation of the concentration of TCE in the daily calibration standard is as follows:

Equation 2:

(C g/L)(Vs L)(1000mL/L / 40mL)(1L/ 106 L) = �L

C = Concentration of compound (g/L) (Equation 1)

Vs = Volume of primary standard injected into vial (L)

Example - TCE

(73,210 g/L) (5 L) (1000 mL/L /40 mL) (1L/ 106 L) = 9.15 g/L

The gas phase standards were directly injected into the GC with injection volumes of 100 �L. The concentration of the standards ranged from five to 10 g/L for PCE and TCE to approximately 55 g/L for BTEX compounds. Halogenated compounds like TCE and PCE have lower standard concentrations because the ECD detector is more sensitive to these compounds.

Conversion of Data from g/L to ppbv

All data are reported in micrograms per liter. Data reported in m g/L can be converted to parts per billion by volume (ppbv) by the following formula:

Equation 3:

g/L = (C/24,450) x (MW) and

C = (g/L x 24,450)/MW

where: C = concentration in ppbv

MW = molecular weight of the compound (g/mole)

24,450 = Conversion factor for standard temperature and pressure

The ppbv number calculated above represents the compound concentration on a volume basis at standard temperature and pressure.

Target Compound Calibration, Identification and Quantification

Calibration standards were run at the beginning of each day to determine the response factor and retention time for each of the target compounds. The standards were injected directly into the gas chromatograph in the same manner as were the soil gas samples.

Soil gas samples were analyzed after the GC had been calibrated and an ambient air sample had been analyzed. The quantity of a target compound in a soil gas sample was determined by dividing the area of the peak registered on the chromatogram by the injection volume and the response factor. The compounds PCE and TCE were quantified and identified using the ECD, while the compounds cis-1,2-DCE, benzene, ethylbenzene, m/p-xylenes, o-xylene and toluene were quantified and identified using the PID. If the concentration of target compounds in a soil gas sample was much beyond the linear range of the detector, the injection volume was decreased or the sample diluted to bring the sample concentration within or near to the linear response range of the detector.

Identification of the target compounds in soil gas samples was based on a single column analysis and a peak on the chromatogram being within three percent of the retention time of a standard compound. To confirm the presence of the target compounds in a soil gas sample, the sample would need to be analyzed on a second column and the results compared to that of the first column. In the case of complex mixtures of hydrocarbons such as weathered fuel vapors, a second column confirmation is usually needed for positive identification of the compounds benzene, toluene, and ortho-xylene due to chromatographic interferences from the numerous other hydrocarbons often present in fuel vapor samples.

Whenever a compound such as a halogenated aromatic or halogenated alkene is detectable on both detectors, the response on the second detector could be used to confirm the presence of the compound in the sample. For example, the compounds TCE, PCE, and cis-1,2-DCE respond to both detectors and, therefore, would be confirmable. A negative response for these compounds on either detector when they would be above the detection limits of both detectors constitutes a nonconfirmed response. In the case of a nonconfirmed response, the compound would be reported as not detected.

Detection Limits

Detection limits for target compounds in soil gas samples can vary depending on the volume injected and chromatographic interferences from adjacent peaks. Detection limits are defined as the minimum discernible peak divided by the typical injection volume and the response factor for the particular compound of interest.

Typically, 300 �L of each soil gas sample was injected into the GC giving detection limits of approximately 0.01 �g/L for halocarbons such as PCE and TCE, and approximately 0.3 �g/L for aromatic hydrocarbons such as benzene and toluene.

Quality Control Procedures

A number of QC procedures were followed to insure that valid data was obtained during sampling.

Soil Gas Sampling Quality Control Procedures

Probes were decontaminated with Alconox and tap water wash, a final rinse of water and air drying before use. Syringes used to analyze samples were decontaminated prior to each use by washing in Alconox and hot tap water, and then rinsing with methanol or isopropyl alcohol or alternatively baking them in the GC oven for a couple of hours. To aid in the identification of contamination problems in the sampling system, syringes were numbered.

Soil Gas Analytical Quality Control Procedures

The QC requirements for soil gas survey samples are considered within the context for which the data is used.

Precision is demonstrated by the internal consistency of the analytical system. Internal consistency is demonstrated in two ways: (1) the consistent response of the detectors over the course of the day coupled with the consistency of the daily calibration for various compounds demonstrate consistency of the detectors and; (2) consistent duplicate soil gas sample results as demonstrated by the results for duplicate samples being within 30 percent of the average of the two analyses.

The QC analyses included analyzing both duplicate and blank samples. For the project, duplicate samples were done on approximately 12 percent of the samples collected. Duplicate samples were analyzed by repeating the analysis done on a particular sample. Blank samples, including system blanks and air blanks, were done at the rate of one per day over the eight days of sampling at all soil gas survey sites.

Determination of Linear Response Range

Three-point curves were constructed to demonstrate the linear response range of the PID and electron capture detector ECD. A measure of the linear response of the detector is the correlation coefficient. The closer the correlation coefficient is to 1 the more linear the detector response. Single point calibration checks performed on a daily basis during the course of soil gas survey were within the linear response range of the detector as determined by the three-point calibration curves (Table SGS-1).

If the concentrations of compounds in a sample were much beyond the linear response range of the 3-point curve, the injection volume was decreased or the sample diluted to bring the sample within or near the linear response range of the detector.

Linear response range curves for TCE and PCE were determined using the ECD. The correlation coefficient (r) was greater than 0.99 for both compounds.

Linear response range curves for the compounds benzene, toluene, m/p-xylenes, o-xylene, and ethylbenzene were determined using the PID. The correlation coefficient (r) was greater than 0.99 for all the above compounds.

Daily Calibration Results

At the beginning of each day, a standard containing the eight compounds targeted for analysis were run through the gas chromatograph to determine a response factor for each compound (Table SGS-2). The response factor is defined as the area units of the standard peak divided by the concentration in the standard. Continuing calibration checks were within QC criteria.

Results of Duplicate Sample Analyses

In Table SGS-3, PCE and TCE were the only compounds evaluated for duplicate analysis because they were the only compounds detected. Some duplicates were run on samples in which all compounds were non-detect, and therefore these duplicates are not shown in Table SGS-3. Duplicate sample results within 30 percent of the mean of the two results are regarded as being within the necessary analytical precision. The relative percent difference (RPD) was calculated from the following equation:

Equation 4:


Rs = Sample result

RD = Duplicate Result.

Results from duplicate samples were generally within + 30 percent and met with the necessary analytical precision (Table SGS-3). In one instance when the sample result was close to the detection limit, the RPD was greater than 30 percent, but this was most likely due to number rounding to two significant digits. For instance, PCE results of 0.013 and 0.017 would round to 0.01 and 0.02 and would produce an RPD of 67 percent. At levels greater than 5 times the detection limit, the rounding of results to two significant numbers typically does not cause a problem with RPD calculation.

Results of Air Sample Blank Analyses

After the instrument had been calibrated, ambient air blank samples were analyzed. An ambient air sample was collected at the beginning of each day by filling a decontaminated syringe with air from the GC room. The sample was analyzed to aid in identifying any contaminants present in the analysis room and to check the effectiveness of the syringe decontamination procedure.

Low concentrations of toluene and on one occasion 0.008 g/L of PCE were detected in air samples collected from the GC room (Table SGS-4). Toluene was detected at concentrations of 0.31 to 0.60 g/L in the air from the GC room and from system blank samples. Data at less than two times the blank value was flagged as non-detect.