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The Effect of Equilibration Time and Tubing Material on Soil Gas Measurements

Schumacher, Brian A., Zimmerman, John H., Elliot, R. James, Swanson, Greg R.
Soil & sediment contamination 2016 v.25 no.2 pp. 151-163
bentonite, polyethylene, sand, screening, soil air, soil types, stainless steel, vapors, volatile organic compounds
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, TeflonĀ®, 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.