High Resolution Soil Gas Sampling

Traditional soil gas screening methods typically utilize various containers such as Tedlar®  bags, disposable syringes, etc., and rely on direct injection into a GC or, at best, injection of a larger sample into a standard purge and trap unit designed for soil and water analysis.  These methods generally have reporting limits well above indoor air screening or action levels.  Vista GeoScience has developed a soil gas sampling method, based on EPA Methods TO-17 and SW846-8260, using multi-bed sorbent tubes similar to those designed for use indoor air analysis.  These sorbent tubes contain multiple packed layers of adsorbent materials that are designed to adsorb compounds over a wide range of boiling points.  The main advantage is that larger volumes of air or soil gas can be pumped through the tube concentrating the analytes resulting in lower detection limits.

The sorbent tubes are thermally activated, batch tested, and sealed with brass swage-lock fittings prior to shipment to the field.  The tubes remain sealed until readied for sampling, as the activated adsorbents will readily adsorb trace amounts of VOCs from the ambient air, and are quickly re-sealed after sampling is completed.  As a control, unused and unopened sorbent tubes are preserved as trip blanks.

After the samples are collected on the sorbent tubes, they are shipped to the laboratory where they are loaded onto an automatic thermal desorption (ATD) unit that is connected to a GC/MS and configured to run EPA Methods TO-17 and SW846-8260.  Method EPA Method TO-17 is the air VOC method equivalent to EPA Method SW846-8260, which is for water and solid wastes.  This method is up to 1,000 times or more sensitive in detecting compounds of interest than conventional soil gas sampling and analysis methods.  The detection limit is lowered by simply concentrating a larger volume on the sorbent tube.  Indoor air screening and action levels can be achieved for most compounds that may have previously gone undetected in other soil gas surveys.  The VOC compounds are desorbed from the tubes at high temperature and are concentrated onto a “cold trap,” then desorbed again onto the GC/MS for separation and analysis of up to 70 target compounds by EPA SW846 method TO-17/8260B.  The results are reported in nanograms (ng), and the reporting limit (RL) for individual VOCs is 5 ng for all the compounds in the target list. The lowest calibration point is set at 5 ng which sets the official method reporting limit. However, values detected between 1 and 5 ng are also reported and flagged as estimated if deemed reliable detections on the GC/MS by the analytical chemist.  These low estimated values can be useful for mapping the trends or detecting deeper or low concentration contaminant plumes.  Detections below 1 ng are rejected and not reported.  The volume collected on the sorbent tube determines the reporting limit concentration.  For example, if 1 liter of air/soil gas is collected on a tube, the reporting limit is 5 ng/L of air, or 5 µg/m3 of air.  Trip blanks, pre-ship blank checks, method blanks, surrogates, and spikes are run according to the method specifications.

Soil gas samples are collected using Vista GeoScience SOP NFSV101 “Standard Operating Procedure for Active Soil Gas Vapor Sampling Using Direct Push Probe and Post-Run Tubing (PRT) Adapter”.  After driving the PRT tool, with an expendable drive-point, to the target sample depth, the probe rod is retracted to drop the drive point and expose the formation.  A Teflon® lined tube and threaded adaptor is inserted into the rod and is threaded into the PRT tool at the bottom of the hole.  This tubing is then connected to a portable gas meter (Landtech GEM2000) to measure CO2, O2, and CH4 during purging.  At least 3 system volumes are removed and purging continues until CO2 and O2 readings are stable, indicating that ambient air in the system has been removed and only soil gas was entering the sampling system.  At that point, the flow is switched over to the sampling apparatus where the soil gas is pulled through the loaded sorbent tube using a vacuum pump.  The flow rate and volume is monitored with a flow meter and kept below 200cc/minute.  After 1 liter of soil gas has flowed through the sorbent tube, the flow is shut off and the sorbent tube is removed, immediately resealed and tagged with the sample ID.  All of the meter and flow parameters are recorded on a sampling log form.

A Thermo Foxboro-TVA1000B FID-PID vapor meter can also be used to screen the soil gas.  The FID (Flame Ionization Detector) responds to all hydrocarbons, including methane. The PID (Photo Ionization Detector) responds selectively to aromatics, alkene hydrocarbons, and solvents and is less sensitive to alkane compounds.