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CONSTRUCTION AND INSTALLATION OF A NEW PASSIVE DIFFUSION SAMPLER CAPABLE OF MONITORING BENZENE IN EITHER SOIL GAS OR GROUND WATER
Citation:
JEWELL, K., C. PAUL, AND J. T. WILSON. CONSTRUCTION AND INSTALLATION OF A NEW PASSIVE DIFFUSION SAMPLER CAPABLE OF MONITORING BENZENE IN EITHER SOIL GAS OR GROUND WATER. Presented at The 19th Annual National Tanks Conference and Expo, San Antonio, TX, March 05 - 07, 2007.
Impact/Purpose:
To inform the public.
Description:
Conventional practice to estimate intrusion of fuel vapors from ground water to buildings measures the concentration of BTEX in ground water beneath the building using a conventional well screened across the water table. This practice assumes that the concentration of contaminants in the capillary fringe that are available to diffuse upward into the building is the average concentration of contaminants produced from the monitoring well. Conventional practice uses screening models to estimate the affect of biodegradation on removal of hydrocarbon vapors. Our approach replaces the estimates of a mathematical model with discrete in-situ passive diffusion monitoring to see if there are vapors in the soil gas immediately above the capillary fringe that can diffuse into buildings. To minimize averaging of concentrations along sharp diffusion gradients, we used a 2” screen interval with a passive diffusion sampler. The sampler is a 40 mL VOA vial with a passive diffusion membrane. The vial is filled with DI water and trisodium phosphate as a preservative. To suspend the VOA at the screen interval and minimize influence from the rest of the well, a messenger was constructed from HDPE. The messenger has vanes to seal the passive diffusion sampler from the space above the sampler. Twelve passive diffusion monitoring wells were installed at different depths at each transect in a gasoline plume using a Geoprobe. For comparison, a conventional monitoring well with 10 feet of screen was also installed in each transect. Core samples were acquired from each transect to determine the vertical distribution of TPHg, and of oxidized and reduced iron minerals in the sediment (based on color of the sediment samples).