Groundwater Plume, Source and Risk Identification Using Passive Soil Gas


The results from sixty passive soil gas (PSG) samplers placed at a grain silo and mixed-use industrial facility with a known chlorinated solvent release aided with the rapid investigation time-schedule. The samplers were used to determine potential sources, plume extent and vapor risks in areas beyond the previous discrete soil sampling and permanent groundwater monitoring well network. Passive soil gas testing involves the installation of sorbents in a one-inch diameter hole, three-foot deep and the samplers are then placed at a depth of approximately six inches. The samplers were installed in grid pattern at a spacing of 25 to 50 feet at the site and across the street. The depth to groundwater was approximately ten to twelve feet below grade.
The installation of numerous monitoring wells in areas where Trichloroethene (TCE) and Tetrachloroethene (PCE) were discovered using the PSG samplers revealed that there were indeed two separate sources and groundwater plumes at the site, which became co-mingled downgradient. In this previously uninvestigated portion of the site, TCE was found at 600 ng in a PSG sampler and a permanent groundwater monitoring well was subsequently installed in that vicinity. The groundwater concentration of TCE in that well was 400 ug/L. The PSG sampling network was wide enough to identify the approximate size of this new plume. In a region approximately 75 feet downgradient from the previously discussed location, 294 ng of TCE was discovered in a PSG sampler. The groundwater monitoring well installed at this location revealed 110 ug/L of TCE. Further yet, a cross-gradient PSG sampler revealed 107 ng of TCE and the groundwater monitoring well placed at the location revealed 74 ug/L of TCE. The PSG investigation was completed from start to finish in a two-week period of time. The paper will focus on the importance of PSG sampling for source identification, groundwater investigation and complying with regulatory directives for the assessment of the vapor intrusion pathway for risk assessment purposes.

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