Evaluation of a Carbon Adsorption Method for Sampling Gasoline Vapors in the Subsurface |
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| Authors: | Edward D Wallingford Francis A DiGiano Cass T Miller |
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| Institution: | Edward D. Wallingford is a research assistant in the Department of Environmental Sciences and Engineering at The University of North Carolina at Chapel Hill (Rosenau Hall, Chapel Hill, NC 27514). His research in developing vapor-phase sampling methodology is part of a thesis research requirement as a master's degree candidate in environmental chemistry and biology. He received a B.S. in agronomy from the University of Kentucky in 1983.;Dr. Francis A. DiGiano is professor of Water Resources Engineering in the Department of Environmental Sciences and Engineering at The University of North Carolina (Rosenau Hall 201H, Chapel Hill, NC 27514). He specializes in the research on the adsorption process for use in water and waste water treatment and in analytical applications for concentrating organic contaminants. He is also interested in mathematical modeling of pollutant mass transport in design of adsorption systems. Professor DiGiano received a Ph.D. in environmental engineering from The University of Michigan in 1969.;Cass T. Miller is assistant professor of Water Resources Engineering in the Department of Environmental Sciences and Engineering at The University of North Carolina at Chapel Hill (Rosenau Hall, Chapel Hill, NC 27514). His teaching and research are focused on mathematical modeling the fate and transport of contaminants in ground water systems. Miller received a Ph.D. in environmental engineering from The University of Michigan in 1984. |
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| Abstract: | Monitoring of the vapor phase has emerged as a very convenient method for detecting volatile organic contaminants in the subsurface. It can provide a reliable way of placing ground water monitoring and recovery wells. The most common method uses a driveable ground probe (DGP) to extract a vapor-phase sample followed by direct injection of the vapor into a portable gas chromatograph (GC). However, many regional offices of regulatory agencies and consultants do not have ready access to such equipment. This research explores an alternative–the carbon adsorption method—in which the vapor is withdrawn by the DGP but concentrated on a small activated carbon trap (150mg). The carbon traps can be returned to a central laboratory for solvent extraction and GC analysis. This provides the advantages of increased sensitivity, reduction in field equipment and convenience of in-lab analyses (multiple GC injections are possible). A simple DGP and carbon trap system was constructed and tested at a field site. Vapor-phase concentrations of target compounds present in gasoline were mapped quite conveniently, ranging from 10,000μg/liter (vapor phase) to less than 10μg/L. These concentrations were also shown to decrease in the direction of the ground surface, as expected. Measurements of target compounds in soil showed that the vapor phase contributed a large fraction of the total contaminant burden where a non-aqueous-phase layer (NAPL) had been identified; as important, however, is the rather uniform contamination of the soil outside the NAPL region. Finally, the concentrations of target compounds in the vapor phase and ground water could be related in a manner roughly described by a simple equilibrium model, although exceptions were noted. |
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