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Evaluation of a New Passive Diffusion Sampler for Monitoring for Soil Gas and Ground Water at Two UST Sites
Citation:
PAUL, C. J., J. T. WILSON, D. C. DIGIULIO, K. P. JEWELL, AND R. Sharma. Evaluation of a New Passive Diffusion Sampler for Monitoring for Soil Gas and Ground Water at Two UST Sites. Presented at EPA Region III States LUST Technical Workshop, Gettysburg, PA, October 22 - 24, 2007.
Impact/Purpose:
To present information at a LUST Technical Workshop
Description:
Historically, conventional practice to estimate intrusion of fuel vapors from soil and ground water to buildings measures the concentration of BTEX beneath the building using vapor probes or monitoring wells screened across the water table. Standard 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. If oxygen is available at the capillary fringe, biodegradation may remove the fuel vapors before they have a chance to diffuse upward into buildings. Screening models, such as the Johnson and Ettinger Model, are generally used to estimate the affect of biodegradation on removal of hydrocarbon vapors. A new passive diffusion sampler (PDS) was developed which 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. These PDS were evaluated at two UST sites, the A1 Movers in Madison, WI and Hal’s Chevron in Green River Utah. At the A1 Movers site, ground water was obtained with conventional sampling techniques for comparison with PDS data. At the Hal’s Chevron site, soil gas concentrations were obtained from both monitoring wells and vapor probes and collected into Summa canisters for comparison with the PDS Data. There are several advantages to using this PDS. Vertical gradients BTEX concentrations in ground water and in the unsaturated zone can be very sharp, changing by orders of magnitude in a few feet. Passive diffusion sampling allows a more discrete sample from a smaller volume of subsurface material. Conventional methods acquire samples over several vertical feet of subsurface material. There is less averaging of concentrations from a sharp diffusion gradient. The PDS averages concentrations over several days. The PDS does not extract fluids. As a result, there is minimal affect on the concentration gradients during the act of sampling, and there is less question of “where the sample really came from.” The PDS is indifferent to the position of the water table. It will provide a sample from either ground water or soil gas. Samples are acquired from a permanent well, however, there is no water produced from purging the wells. There is no need to mobilize push tools for a new penetration for each sample. The PDS samples are water samples in a 40 ml VOA vial, therefore analysis is low cost compared to the analysis of vapor samples collected into canisters. The greater volume of sample compared to many other passive diffusion samplers provides more sample, allowing a lower analytical detection limit in the chemical analysis. There are some limitations of using the PDS. It requires installation of several discrete wells to sample along a vertical concentration gradient. The sampler may not come to equilibrium with short term exposures. They must be emplaced during one site visit and recovered on a second visit. And, the samples can’t be duplicated.