Mass to Concentration Tie-In for Passive Soil Gas Surveys: Improved Technique for Source Area, Spatial Variability and Vapor Intrusion Assessment

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Abstract
The mass to concentration tie-in [MtoC Tie-In] correlates passive soil gas (PSG) data in mass to active soil gas data in concentration determined by EPA Method TO-17. Passive soil gas surveys consist of rapid deployment of hydrophobic sorbents (dozens to several hundred locations typically installed in one day) to a depth of six-inches to three-feet in a grid pattern with exposure in the field from three days to two weeks to target a wide variety of organic compounds. A power function is used on a compound-to-compound basis to correlate spatially varying mass (nanograms) from selected locations within a passive soil gas survey to concentration (ug/m3) at those same locations. The correlation from selected PSG locations is applied to the remainder of the PSG grid. The MtoC Tie-In correlations provides added value to a PSG survey, with the PSG data then used to estimate risk throughout the limits of the investigation for quantitative assessment.

The results from a site in Northern California show the MtoC Tie-In correlations for both Benzene and Total Petroleum Hydrocarbons (TPH). The correlations are applied on a compound-to-compound basis to the remaining locations in the PSG-grid to provide an estimate of concentration that can be used for comparison to risk/screening levels or fate and transport diagnostic tools (partitioning equations, solubility laws, etc.). An example of how the correlations are applied is presented in tabular form. The results from a chlorinated solvent survey show the MtoC Tie-In correlation from a site in Maryland for Tetrachloroethylene (PCE). In this instance, there was a near-perfect relationship between the PSG mass and the active soil gas concentration (R-squared value of 1). The concentration estimated throughout a PSG-grid enables a vast new realm of interpretive power at sites. Several other sites are discussed.

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