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The Effect of Equilibration Time and Tubing Material on Soil Gas Measurements
Schumacher, B., JohnH Zimmerman, J. Elliot, AND G. Swanson. The Effect of Equilibration Time and Tubing Material on Soil Gas Measurements. SOIL AND SEDIMENT CONTAMINATION: AN INTERNATIONAL JOURNAL. CRC Press LLC, Boca Raton, FL, 25(2):151-163, (2016).
Soil vapor data are widely used in site investigation and remediation projects to delineate volatile organic compound (VOC) vapor plumes, as a screening tool to refine soil and groundwater sampling efforts, to track the progress of soil remediation, and to assess the vapor intrusion pathway. A critical facet of any sampling program is the collection of representative data: that is, data that accurately reflect the in-situ conditions of the media being sampled. The collection of environmental samples necessarily disturbs the sampled media, so care must be taken to reduce the disturbance to the extent possible. The collection of vadose zone soil vapor samples requires that a sampling tool be advanced to the target sample depth, typically using direct-push drilling equipment (e.g., a GeoProbe® rig), or less commonly with other drilling methodologies, such as hollow-stem augers, hand-augers, or sonic drilling. All of these techniques result in some disturbance to the subsurface. After advancing the drill rod to the target sampling depth, the drill rod is then withdrawn a small amount to create a void space and vapor samples can be collected through the drill rod (e.g., “post-run-tubing”). Semi-permanent vapor probes can be constructed in the borehole after completely removing the drill rod, which further disturbs the subsurface and exposes the vapor column to the atmosphere. Therefore, it is important to know how long the vapor probe must be allowed to equilibrate in the subsurface in order to obtain a representative sample. Various agencies have provided recommendations on equilibration time including: the California Environmental Protection Agency (Cal/EPA) – 2 hours for probes installed using direct push drilling; American Society for Testing and Materials (ASTM) – 30 minutes for probes installed using direct-push and 2 hours for probes installed using hollow-stem auger; Interstate Technology & Regulatory Council (ITRC) – “a few hours”; and the New York State Department of Health – 24 hours after the installation of permanent probes and shortly after the installation of temporary probes. However, little empirical data are available to support these recommendations. A further consideration during the collection of soil vapor samples is the impact of the sampling apparatus on the measurement. Most soil vapor sampling systems consist of a length of tubing that is either open or connected to a permeable tip at the target sampling depth, and a surface termination that facilitates the extraction of vapor. The largest surface area that the vapors come into contact with during sampling is the tubing interior and the vapor may interact with the tubing material during sample collection. The objective of this investigation was to assess the time required following installation for vapor probes to equilibrate with the in-situ environment and yield representative samples (i.e. the equilibration time) and to compare the performance of an assortment of tubing materials commonly used during soil vapor sample collection.
The collection of soil vapor samples representative of in-situ conditions presents challenges associated with the unavoidable disturbance of the subsurface and potential losses to the atmosphere. This article evaluates the effects of two variables that influence the concentration of volatile organic compounds in soil vapor samples: equilibration time and tubing material. The time for three types of soil vapor probes (i.e., macro-purge, mini-purge, and post-run tubing probes [PRT]) to equilibrate with subsurface conditions was assessed by installing probes and collecting multiple samples over a 72-hour period. The effect of tubing material was evaluated by collocating soil vapor probes constructed with different tubing material and collecting samples over several months. We recommend that soil vapor probes constructed with a sand filter-pack and bentonite seal (i.e., macro-purge probe) equilibrate for 24 to 48 hours prior to sample collection. Post-run tubing (PRT) probes equilibrated within one to two hours while a new probe design, (i.e., mini-purge probe) equilibrated and could be sampled after only 30 minutes for screening assessments. Nylaflow, Teflon®, polyetheretherketone (PEEK), and stainless-steel tubing had comparable trichloroethene (TCE) concentrations over all sampling time frames. We recommend that copper tubing be avoided and polyethylene only be used for screening assessments.