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MACRO- MICRO-PURGE SOIL GAS SAMPLING METHODS FOR THE COLLECTION OF CONTAMINANT VAPORS
Citation:
SCHUMACHER, B. A., J. H. ZIMMERMAN, C. R. SIBERT, K. E. VARNER, AND L. A. RIDDICK. MACRO- MICRO-PURGE SOIL GAS SAMPLING METHODS FOR THE COLLECTION OF CONTAMINANT VAPORS. GROUNDWATER MONITORING AND REMEDIATION. National Ground Water Association, Westerville, OH, 29(1):138-143, (2009).
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:
Purging influence on soil gas concentrations for volatile organic compounds (VOCs), as affected by sampling tube inner diameter and sampling depth (i.e., dead-space purge volume), was evaluated at different field sites. A macro-purge sampling system consisted of a standard hollow 3.2 cm outer diameter (OD) drive probe with a retractable sampling point attached to an appropriate length of 0.48 cm inner diameter (ID) Teflon tubing. The macro-purge sampling system had a purge line volume of 24.5 mL at a 1 m depth. In contrast, the micro-purge sampling systems consisted of a 1.27 cm OD drive rod with a 0.10 ID stainless steel tube or a 3.2 cm OD drive rod with a 0.0254 cm inner diameter stainless steel tubing resulting in purge line volumes of 1.2 mL and 7.05 mL at 1 m depths, respectively. At each site and location within the site, with a few exceptions, the same contaminants were identified in the same relative order of abundances indicating the sampling of the same general soil atmosphere. However, marked differences in VOC concentrations were identified between the sampling systems with micro-purge samples having between 2 and 27 times greater concentrations than their corresponding macro-purge samples. The higher concentrations are the result of a minimal disturbance of the ambient soil atmosphere during purging. The minimal soil gas atmospheric disturbance of the micro-purge sampling system allowed for the collection of a sample that is more representative of the soil atmosphere surrounding the sampling point. That is, a sample that is not reliant upon the recharge from some unknown zone around the sampling point, or residual atmosphere, and which the soil atmosphere is potentially more representative of the equilibrium soil gas concentrations. It is, thus, recommended that when soil gas sampling is required, that the sampling system uses the smallest practical ID soil gas tubing to obtain the soil gas sample so that proper decisions, based on more representative soil gas concentrations, about the site can be made.