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Vertical Profiling of Soil Vapor Concentrations Using a New Passive Diffusion Sampler at a UST Site
Citation:
PAUL, C. J., J. T. WILSON, D. C. DIGIULIO, AND K. P. JEWELL. Vertical Profiling of Soil Vapor Concentrations Using a New Passive Diffusion Sampler at a UST Site. Presented at 20th Annual National Tanks Conference, Atlanta, GA, March 17 - 19, 2008.
Impact/Purpose:
Present information at the 20th Annual National Tanks Conference
Description:
Understanding the transport of volatile contaminants in soil gas, particularly those associated with underground storage tanks (USTs), requires a detailed knowledge about the depth-dependent distribution of chemical species in the subsurface. Traditional monitoring wells generally provide an average concentration across the screened interval which may not be representative of the concentration at a specific discrete depth where the sampling port is located. A simple and affordable passive diffusion sampler (PDS) was developed that can be used to estimate depth-discrete concentrations of contaminants in soil gas and ground water. The PDS consists of a 40-ml VOA vial fitted with a modified cap where the Teflon-lined septa is replaced by a permeable membrane to allow contaminants to diffuse into the water-filled VOA vial. The PDS is inserted into a holder or “messenger” for deployment down monitoring wells. A field study was conducted to provide three dimensional site characterization of a gasoline plume at an underground storage tank (UST) site to evaluate the effectiveness of the PDS for determining BTEX concentrations in soil gas. PDS concentrations were compared with those obtained with traditional soil gas sampling techniques. A monitoring system was installed at the Hal’s Chevron site in Green River, Utah, consisting of an array of 2-in diameter PVC monitoring wells installed at discrete depths with 2-in screens. The monitoring wells were installed adjacent to existing vapor probes for comparison purposes. The messengers containing the PDS were lowered into each monitoring well so that the cap of the PDS was exposed within the well screened interval. The PDS was left in the monitoring wells for approximately one month. Previous laboratory studies showed that one month was sufficient time for BTEX compounds to diffuse across the membrane and reach equilibration. Results of this study show that the PDS provides a simple and affordable alternative to traditional sampling techniques at UST sites. Additionally, results of discrete depth contaminant concentrations may be used to provide information on whether natural attenuation processes are controlling risk associated with the site.