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EFFECTS OF THE VARIATION OF SELECT SAMPLING PARAMETERS ON SOIL VAPOR CONCENTRATIONS
Citation:
SCHUMACHER, B. A. AND J. H. ZIMMERMAN. EFFECTS OF THE VARIATION OF SELECT SAMPLING PARAMETERS ON SOIL VAPOR CONCENTRATIONS. Presented at Remediation of Chlorinated and Recalcitrant Compounds: The Sixth International Conference, Monterey, CA, May 19 - 22, 2008.
Impact/Purpose:
Land Preservation and Restoration - by providing improved scientific knowledge and developing and applying more cost effective tools, models and methods to inform decisions on land restoration.
Description:
Currently soil vapor surveys are commonly used as a screening technique to delineate subsurface volatile organic compound (VOC) contaminant plumes and to provide information for vapor intrusion and contaminated site evaluations. To improve our understanding of the fate and transport of soil vapor containing VOCs, the U.S. EPA funded two research studies conducted at a site on Vandenberg Air Force Base (CA) contaminated with the chlorinated solvent TCE.
The first study evaluated the effect of variations in purge volume, sample flow rate, and sample volume during soil vapor sample collection. Sample purge volumes were varied from 1 to 4,400 system dead-space volumes. The effect of purge volume on the measured VOC concentrations was more pronounced than the effect of sample flow rate; however, this variability may not be significant in terms of site characterization. Sample flow rates were varied from 100 to 5000 cc/min with no significant effect on soil vapor concentrations. Sample volume was varied from 25 to 6,000 mL. The increase in purge volume from 25 to 1,000 mL resulted in an increase in VOC concentrations followed by a decrease in VOC concentrations in sample volumes greater than 1000 mL. The common use of the 6,000 mL sample size to decrease detection limits by EPA method TO-15 may result in artificially low sample analyte concentrations or non-detects.
The second study evaluated the correlation of temporal and shallow soil gas concentrations (4' to 17' bgs) variations over a six week period. Over 11,000 analyses were completed from thirteen sampling points by an automated analytical system. An on-site mobile weather station collected meteorological data during the sampling period. Analyte concentrations varied less than 20% during the sampling period resulting in no correlation for this set of data. No precipitation events occurred during this study.