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A CONCEPTUAL UNDERSTANDING OF LEAKAGE DURING SOIL-GAS SAMPLING
Citation:
DIGIULIO, D. C. A CONCEPTUAL UNDERSTANDING OF LEAKAGE DURING SOIL-GAS SAMPLING. Presented at The 17th Annual Association for Environmental Health and Sciences, San Diego, CA, March 21 - 22, 2007.
Impact/Purpose:
To inform the public.
Description:
A heuristic model is developed to develop a conceptual understanding of leakage during soil-gas sampling. Leakage is shown to be simply a function of the permeability contrast between the formation and borehole and geometric factors. As the ratio of formation to borehole permeability decreases, the potential for leakage increases. Thus, the need for leak testing is greatest when sampling in low permeability soil. Leakage can only be minimized by properly sealing a borehole. Applied flow and vacuum are irrelevant. If a tracer concentration is held constant in a chamber above a probe and there is no source of the tracer in the sub-surface, leakage can be quantified simply as the ratio of tracer concentration in a sampling vessel to tracer concentration in a chamber and adjusted for injection time if desired. Injection of helium or a fluorocarbon in a chamber appears to be the cleanest and most robust method of leak testing.