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A New Passive Diffusion Sampler for Improving the Calibration of Models to Evaluate Vapor Movement at UST Sites
Citation:
PAUL, C., D. C. DIGIULIO, J. T. WILSON, K. JEWELL, J. Menatti, AND R. Davis. A New Passive Diffusion Sampler for Improving the Calibration of Models to Evaluate Vapor Movement at UST Sites. Presented at The 2010 North American Environmental Field Conference and Exposition, Tampa, FL, January 12 - 14, 2010.
Impact/Purpose:
Understanding transport of volatile contaminants in soil gas and ground water, particularly those associated with underground storage tanks (USTs)
Description:
Understanding transport of volatile contaminants in soil gas and ground water, particularly those associated with underground storage tanks (USTs), requires a detailed knowledge about the depthdependent distribution of chemical species in the subsurface. A risk assessment of the movement of vapors of volatile organic contaminants (VOCs) from ground water through the unsaturated zone and into living spaces usually involves a transport and fate model such as the Johnson and Ettinger (J&E) model. Conventional practice assumes that the concentration of contaminants in the capillary fringe that are available to diffuse upward into a building is the concentration of the contaminants predicted from their measured concentration in the shallow ground water and their Henry’s Law Constant for partitioning between water and air. A field study was conducted at a former gasoline service station located in Green River, Utah to provide three dimensional site characterization. Core samples were extracted and analyzed for TPH and individual petroleum hydrocarbons concentrations. Samples were also collected from shallow ground water monitoring wells. Soil gas samples were obtained from conventional vapor monitoring probes and a new passive diffusion sampler (PDS) was deployed that can estimate depth-discrete concentrations of contaminants in soil gas. Generally, screening models, such as the J&E Model , are used to estimate the affect of biodegradation on removal of hydrocarbon vapors. 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. This approach replaces the estimates of a mathematical model with monitoring to see if there are vapors in the soil gas immediately above the capillary fringe that can diffuse into buildings. 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. 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. The PDS provides an effective method of sampling both ground water and soil gas in low permeability formations. 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.
