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IMPROVING THE CALIBRATION OF MODELS TO EVALUATE VAPOR MOVEMENT AT UST SITES BY VERTICAL PROFILING OF CONTAMINANTS IN GROUND WATER
Citation:
PAUL, C. J. IMPROVING THE CALIBRATION OF MODELS TO EVALUATE VAPOR MOVEMENT AT UST SITES BY VERTICAL PROFILING OF CONTAMINANTS IN GROUND WATER. Presented at 18th Annual National Tanks Conference and Expo, Memphis, TN, March 20 - 22, 2006.
Impact/Purpose:
To inform the public
Description:
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 model. The concentration of volatile organic contaminants in ground water is needed to calibrate these models. EPA recommends that ground water samples should be obtained from the uppermost portions of the ground water and/or capillary fringe (Appendix G of EPA OSWER Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway, November, 2002). Historically, samples have not been obtained from these zones. Generally, water samples have been acquired from conventional wells screened across the water table. A conventional well produces a composite sample. The average concentration across the screened interval may not be representative of the concentration at the top of the aquifer or in the capillary fringe. Recent studies were conducted to evaluate different sampling techniques for determining VOC concentrations in ground water. Samples were obtained using multi-level and traditional sampling techniques in three monitoring wells at the Raymark Superfund site in Stratford, CT. Vertical profile devices evaluated include polyethylene passive diffusion bags (PDBs) and a discrete multi-level sampler (DMLS®). Traditional sampling devices include bladder, peristaltic, and submersible pumps. Low-flow purging and sampling techniques were used with these devices. Direct-push technology (Geoprobe®) was used adjacent to monitoring wells for comparison purposes. Samples were analyzed for 1,1,1-trichloroethane (TCA), trichloroethene (TCE), and 1,1-dichloroethene (DCE). Distinct concentration gradients were seen with all three vertical profile methods, although differences were seen between concentrations obtained within monitoring wells and those obtained using the Geoprobe® in a presumably undisturbed geologic zone adjacent to monitoring wells Comparison between vertical profile sampling and traditional sampling with pumps showed values obtained with traditional techniques tend to provide an average concentration over the entire screened interval or water column within the screened interval. Vertical profile sampling has proven to be a useful tool for site characterization, particularly where discrete vertical contaminant distribution is needed to aide in evaluating contaminant transport in the subsurface.