You are here:
RESULTS FROM EPA FUNDED RESEARCH PROGRAMS ON THE IMPORTANCE OF PURGE VOLUME, SAMPLE VOLUME, SAMPLE FLOW RATE AND TEMPORAL VARIATIONS ON SOIL GAS CONCENTRATIONS
Citation:
SCHUMACHER, B. A., B. HARTMAN, J. H. ZIMMERMAN, D. SPRINGER, J. ELLIOT, AND M. RIGBY. RESULTS FROM EPA FUNDED RESEARCH PROGRAMS ON THE IMPORTANCE OF PURGE VOLUME, SAMPLE VOLUME, SAMPLE FLOW RATE AND TEMPORAL VARIATIONS ON SOIL GAS CONCENTRATIONS. Presented at Air & Waste Management Association Conference, Vapor Intrusion: Learning from the Challenges, Providence, RI, September 26 - 28, 2007.
Impact/Purpose:
Soil gas data are widely used in site investigation and remediation projects to delineate volatile organic compound (VOC) contamination, as a screening tool to refine soil and groundwater sampling efforts, to track the progress of soil remediation, and to identify potential risks from vapor intrusion. Collection of representative samples; that is, samples that are representative of the environmental, chemical, and physical conditions present during the time of sample collection, is imperative for making responsible decisions. However, there currently exists doubt over the effect of numerous environmental and sampling variables on measured soil gas concentrations. To address this uncertainty, Environmental Sciences Division of the National Exposure Research Laboratory (NERL), a division of EPA’s Office of Research and Development (ORD), funded a research program in the fall of 2006 to quantitatively assess the effect of certain sampling parameters on soil gas sample results, and a second program in the spring of 2007 to study temporal variations in soil gas concentrations due to changes in meteorological variables. The research program was designed and implemented by Tetra Tech, Inc. and H&P Mobile Geochemistry
Description:
Two research studies funded and overseen by EPA have been conducted since October 2006 on soil gas sampling methods and variations in shallow soil gas concentrations with the purpose of improving our understanding of soil gas methods and data for vapor intrusion applications. All studies consisted of a team of researchers. The EPA studies were conducted at a site with chlorinated solvent contamination at Vandenberg Air Force Base (CA). The first study consisted of controlled experiments on soil gas collection methods including purge volume, sample volume, and sample flow rate. Sample flow rate was found to not have a significant effect on soil gas concentrations for flows ranging from 100 cc/min to 5000 cc/min. There was a statistically significant positive correlation between the measured trichloroethylene (TCE) concentrations and purge volume ranging from 1 to 20 dead-space (system) volumes. The effect of purge volume on the measured volatile organic compound (VOC) concentrations was more pronounced than the effect of purge rate; however, this variability may not be significant in terms of site characterization. Measured VOC concentrations were observed to increase with increasing sample volume from 25 to 1,000 ml, but then drop off in the 6,000 ml samples. This observation is significant as the 6,000 ml sample size is commonly used to achieve very low detection levels with EPA method TO-15.
The second study investigated the temporal variation in shallow soil gas concentrations. In this study, soil gas concentrations were measured continuously around the clock for a period of 6 weeks at twelve sampling points in an uncovered field ranging in depth from 3' below ground surface (bgs) to 17' bgs. The contamination source was TCE in groundwater. Over 11,000 analyses were collected by the automated continuous analyzer resulting in approximately 750 analyses per probe. Meteorological data were also collected. Observed TCE maximum and average concentration variations were less than 27% and 20%, respectively for all of the probes over the entire time period. Hence, meteorological variations had little effect on soil gas concentrations even as shallow as 3' bgs in a sandy soil with no surface covering.