You are here:
METHOD AND LOCATION OF GROUND WATER SAMPLING: IMPACT ON ATTENUATION FACTORS FOR ASSESSING IMPACT ON VAPOR INTRUSION
Citation:
Paul*, C J. AND D C. DiGiulio*. METHOD AND LOCATION OF GROUND WATER SAMPLING: IMPACT ON ATTENUATION FACTORS FOR ASSESSING IMPACT ON VAPOR INTRUSION. Presented at 14th Annual West Coast Conf. on Soils, Sediments, and Water, San Diego, CA, March 15 - 18, 2004.
Impact/Purpose:
To inform the public.
Description:
The Draft EPA Subsurface Vapor Intrusion Guidance Document was established to "address the incremental increases in exposures and risks from subsurface contaminants that my be intruding into indoor air". The document utilizes attenuation factors based on indoor air/soil gas or indoor air/ground water concentration ratios in order to calculate vapor intrusion action levels. The information provided by attenuation factors is open to interpretation and varies from site to site and even from house to house. Attenuation factors are a function of the method(s) used to obtain soil gas and/or ground water samples, the depth of the sample and distance from the impacted house/building, and the time frame in which different samples are obtained.
The Guidance Document states that ground water samples should be obtained from the uppermost portions of the ground water and/or capillary fringe. The capillary fringe is a dynamic zone, which includes both water filled and air filled pore spaces and is considered responsible for transport of contaminants near the water table. How this impacts VOC (volatile organic compounds) concentrations in the unsaturated zone and resulting vapor intrusion into homes or buildings has not been well documented. Recently, a discrete interval vertical profile study was conducted at the Raymark Superfund Site in Stratford, CT using passive diffusion bags (PDBs), discrete multi-level sampler (DMLS?), and a Geoprobe?. Ground water samples were also obtained using pumps and traditional low-flow purging and sampling techniques for comparison purposes. Data for trichloroethene (TCE), 1,1,1-tricholoroethane (1,1,1-TCA), and 1,1-dichloroethene (1,1-DCE) were used in the study. 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 traditional low-flow purging and sampling techniques. Samples obtained with pumps generally provided an average concentration over the entire screened interval when compared with discrete vertical concentrations. Indoor air sampling results from nearby houses showed vapor intrusion of one or more of the compounds evaluated.
Results of this study indicate that using ground water concentrations to determine attenuation factors may not provide adequate information to accurately assess risk from vapor intrusion. Location, depth, and method of sample collection, and spatial variability affect attenuation factors. These and other variables must be considered if ground water data are used in the risk assessment process.