You are here:
EVALUATION OF VAPOR EQUILIBRATION AND IMPACT OF PURGE VOLUME ON SOIL-GAS SAMPLING RESULTS
Citation:
DIGIULIO, D. C. EVALUATION OF VAPOR EQUILIBRATION AND IMPACT OF PURGE VOLUME ON SOIL-GAS SAMPLING RESULTS. Presented at The 17th Annual Association for Environmental Health and Sciences Meeting, San Diego, CA, March 21 - 22, 2007.
Impact/Purpose:
To inform the public.
Description:
Sequential sampling was utilized at the Raymark Superfund site to evaluate attainment of vapor equilibration and the impact of purge volume on soil-gas sample results. A simple mass-balance equation indicates that removal of three to five internal volumes of a sample system should ensure that vapor concentration entering a sampling vessel is 95% to greater than 99%, respectively, of vapor concentration in surrounding soil. Only one internal volume exchange, excluding the gas-filled porosity of the sandpack, was necessary for stabilization of vapor concentration at a dedicated vapor probe that had been sealed for at least three months prior to sampling. This indicates likely attainment of equilibration in the sandpack and surrounding soil. However, stabilization of vapor concentration at a direct-push location sampled immediately after probing required 8 to 12 internal volume exchanges which included the gas-filled volume of the open cylindrical hole. This indicates likely non-attainment of equilibrium and the need for more extensive purging when sampling soon after probing. The potential impact of excessive purging on sample concentration was evaluated at three sub-slab vapor probes and one dedicated vapor probe. Air flow modeling conducted on the sub-slab probes indicated little potential for impact at extraction of up to 12 liters. This finding was confirmed using sequential sampling. Sequential sampling at the dedicated vapor probe indicated little impact on vapor concentration after removal of 103 liters of gas.