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Comparison of Sampling Methods to Determine the Impact of Aerobic Biodegradation on Benzene Concentrations at UST Sites
Citation:
PAUL, C. J., JOHN T. WILSON, K. JEWELL, R. Davis, AND J. Menatti. Comparison of Sampling Methods to Determine the Impact of Aerobic Biodegradation on Benzene Concentrations at UST Sites. Presented at North American Environmental Field Conference, San Diego, CA, January 10 - 13, 2011.
Impact/Purpose:
Presentation for the North American Environmental Field Conference - (January 10-13, 2011)
Description:
This material will be interesting to regulators and contractors who collect samples of soil gas to estimate the potential for vapor intrusion of buildings. In the absence of biodegradation, transport of vapors through the unsaturated zone is expected to be by diffusion, and the distribution of concentrations will follow a diffusion gradient. As a first approximation, the distribution of concentration will increase linearly with depth below slab or the ground surface. Petroleum vapors are readily degraded in subsurface sediment whenever the concentration of oxygen is adequate. When vapors degrade, the distribution is decidedly non-linear with depth and the achieved concentrations are much lower than would be expected from a diffusion gradient. To properly evaluate and document the contribution of biodegradation, it is necessary to determine vapor concentrations over a relatively narrow vertical depth interval. A simple and affordable passive diffusion sampler (PDS) was developed that can be used to estimate depth-discrete concentrations of contaminants in soil gas. A field study was conducted at an UST site in Green River, Utah to compare the passive samplers and conventional good practice for active sampling of vapors to determine BTEX concentrations in soil gas. Concentrations of TPH (GRO) and concentrations of benzene in soil core extracts were used to calculate “expected” benzene concentrations in pore water in the vadose zone above the ground water. These “expected” values were used as a bench mark to compare and evaluate concentrations in samples acquired from active soil gas probes and by the passive diffusion samplers. There was general agreement between the expected concentration of benzene in the soil gas and the concentrations produced from active sampling of vapor probes. However, active sampling tended to over-estimate benzene concentrations in the more shallow depth intervals. This is most likely due to pulling vapors up from more highly contaminated intervals below the screen of the vapor probe. There was adequate oxygen in the shallow probes, and biodegradation would be expected. However, stable carbon isotope ratios revealed that benzene from the shallow probes had the same isotopic signature as benzene from lower intervals. Data obtained with the PDS seems to be less biased. Passive sampling approaches may provide a viable option for obtaining representative soil gas samples. For active sampling, short screen lengths, smaller sample volumes and low intake velocity are recommended. This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.