Collections > Master's Papers > Gillings School of Public Health > Evaluation of a Carbon Sorption Method for Sampling Gasoline Vapors in the Subsurface
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Monitoring of contaminant vapors in the unsaturated zone is a convenient method for detecting volatile organic contaminants in the subsurface. This technique can be used for optimizing the placement of monitoring and recovery wells. The most common method uses a driveable ground probe (DGP) to extract vapor sample followed by direct injection into a portable gas chromatograph (GC). However, many regional offices of regulatory agencies do not have ready access to such equipment. This research evaluates an alternative method- the carbon adsorption method- in which vapor is withdrawn by the DGP but concentrated on a small activated carbon trap (150 mg). The carbon traps can be returned to a central laboratory for solvent extraction and GC analysis. This provides the advantage of a reduction in field equipment and convenience of in-lab analyses (multiple GC injections are possible) and also increased sensitivity because any volume of soil vapor may be collected. A simple DGP and carbon trap system was constructed and tested at a field site. Soil vapor concentrations of target compounds present in gasoline were conveniently mapped; the concentrations ranging from 10,000 ug/liter of soil vapor to less than 10 ug/L. These concentrations were also shown to to decrease in the direction of the ground surface as expected. Measurements of target compounds in soil indicated that the soil vapor contributed a large fraction of the total contaminant mass where a non aqueous phase layer (NAPL) had been identified. As important is the rather uniform contamination of soil outside the NAPL region. Finally, the concentrations of target compounds in the soil vapor and ground water could be related in a manner roughly described by a simple equilibrium model, although exceptions were noted.