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GROUND WATER SAMPLING OF VOCS IN THE WATER/CAPILLARY FRINGE AREA FOR VAPOR INTRUSION ASSESSMENT
Citation:
PAUL, C. J. GROUND WATER SAMPLING OF VOCS IN THE WATER/CAPILLARY FRINGE AREA FOR VAPOR INTRUSION ASSESSMENT. Presented at GRA's Symposium on Subsurface Vapor Intrusion to Indoor Air, San Jose, CA, May 25, 2005.
Impact/Purpose:
To inform the public.
Description:
Vapor intrusion has recently been considered a major pathway for increased indoor air contamination from certain volatile organic contaminants (VOCs). The recent Draft EPA Subsurface Vapor Intrusion Guidance Document states that ground water samples should be obtained from the uppermost portions of the ground water and/or capillary fringe. Obtaining representative samples from the water table/capillary fringe area for VOCs can be difficult, particularly when sampling monitoring wells. Vertical profiling of contaminant concentrations can provide a better estimate of VOC transport through the groundwater/capillary fringe zone and into the unsaturated zone. A recent field study was conducted at the Raymark Superfund Site in Stratford, CT which evaluated several sampling techniques for collecting groundwater. Vertical profile samples were obtained using a Geoprobe®, a discrete multi-level sampler (DMLS™), and polyethylene passive diffusion bags (PDBs). For comparison purposes, samples were also obtained using traditional methods (i.e., bladder, peristaltic, and Grundfos® pumps) using low-flow purging and sampling techniques. An inflatable packer was used to ascertain potential impacts from the air/water interface within a monitoring well. Data for trichloroethene (TCE), 1,1,1-trichloroethane (1,1,1-TCA), and 1,1-dichloroethene (1-1-DCE) were evaluated. Discrete interval data from two sampling events showed VOC concentrations at the air/water interface were very low, however, concentrations significantly increased with depth. In most cases, discrete interval concentrations at the air/water interface were much lower than concentrations obtained with the traditional sampling techniques, which provide more of an average concentration over the screened interval. Indoor air sampling results from nearby houses showed vapor intrusion of one or more of the compounds evaluated. Vertical profile sampling is a useful tool for site characterization, particularly where discrete vertical contaminant distribution is needed to aide in evaluating partitioning of VOCs from the ground water to the vapor phase. However, results of this study show that VOC concentrations from the upper water/capillary fringe area in monitoring wells may not necessarily be reflective of potential risk for vapor intrusion in adjacent residences and/or buildings.