3.5.3 Volatile Hydrocarbon Detectors
Volatile petroleum hydrocarbon concentrations are screened in the field using a
handheld hydrocarbon detector. Appendix C lists several types of detectors that are
particularly suited for jet fuel a nd gasoline vapors. These instruments use a platinum based catalyst detector and most are equipped with 100 ppmv, 1 ,000 ppmv, and 10,000
ppmv range settings to improve accuracy over a variety of concentrations. If gasoline or
JP- 4 free product is found at the test site, the detector should be calibrated using a 4 ,000
ppmv hexane standard. If JP- 8 or heavier diesels/heating oils are present ,a 500 ppmv
hexane standard is more appropriate. If available, Mylar® bags are reco mmended for
holding h exane calibration gases during the calibration process to minimize sorption to
the bag.
When hydrocarbon concentrations are below 100 ppmv, a flame ionization detector
(FID) provides good accuracy. A photoionization detector (PID) is not acceptable for
detecting fuel hydrocarbons. PIDs are appropriate for low levels of chlorinated solvents.
NOTE: Extracting air directly with direct reading instrumentation should be
avoided as these instruments are not designed to pull a vacuum or calibrated
against a vacuum. Direct reading instrumentation will generally use a side stream
of air (at atmospheric pressure) discharged from a vacuum pump.
3.5.4 Air - Tight Chamber
One of the challenges of bioventing tests is the prevention of atmospheric oxygen
leaks i nto the soil gas sample. This is particularly true when soil gas is extracted from
low - permeability soils at higher vacuums. A standard 12- inch diameter vacuum
desiccator can be modified to collect soil gas samples in a manner that minimizes the
potentia l for air leaks. Section 4.1.2 and Figure 4.2 provide additional details on how to
modify and use the desiccator for collection of soil gas samples.
3.5.5 Temperature Monitoring
For arctic sites, in - situ soil temperature can be monitored using Omega Type J or K
thermo - couples (or equivalent). The thermocouples can be connected to an Omega OM -400 Thermo - couple The rmometer (or equivalent). Each thermocouple should be
calibrated in ice water before field installation.
3.5.6 Pressure/Vacuum Monitoring
Changes in soil gas pressure during the air injection testing will be measured at
monitoring points using Magnehelic
®
or equivalent high - quality gauges. Tygon
®
or
equivalent tubing can be used to connect the hand - held pressure/vacuum gauge to the
hose barb on the top of each monitoring point. A valve and hose barb can also be
installed at the top of the vent well tocheck injection pressure at the vent well. Pressure
ga uges are available in a variety of pressure ranges, and the same gauge can be used to
measure either positive or negative (vacuum) pressure by simply switching inlet ports.
Gauges are sealed and calibrated at the factory and should be re- zeroedbefore each test.
The following pressure ranges (inches H
2
O) should be available for bioventing pilot tests:
3-16
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