Naphthalene in Soil Gas Observed at Lower Concentrations During Heating Season



Naphthalene in Soil Gas Observed at Lower Concentrations During Heating Season

Presenter:  Brian G Hennings (NRT, an OBG Company)
Authors:  Brian G. Hennings and Kendall L. Simon (NRT, an OBG Company)


Soil gas samples collected from former MGP sites in the upper Midwest indicate lower naphthalene concentrations tend to occur during the heating season. This is significant when assessing risk because the heating season is commonly assumed to be the period when soil gas has the maximum potential for risk to receptors; and, naphthalene is commonly a driver for evaluating risk from vapor intrusion at former MGP sites. Approximately 650 soil gas samples, collected between 2009 and 2017, from a dozen former MGP facilities located in the upper Midwest were compiled into two populations: (1) samples collected during the heating season from November 1 through March 15; and, (2) samples collected during the warmer months, March 16 through October 31. The Kaplan Meier method was used to estimate means for each population and significant differences were identified using a Peto & Peto modification of the Gehan-Wilcoxon test. The results of the analysis indicate the mean concentrations of naphthalene along with 1,3,5 – trimethylbenzene, and toluene were lower during the heating season. Results from 75 sub-slab soil gas samples were also evaluated using the same techniques, and did not result in any significant differences in observed mean concentrations of MGP vapor intrusion (VI) contaminants of potential concern (COPC). These findings appear to be contradictory to the rationale that collecting soil gas, and other soil vapor samples, during the heating season will produce more conservative (higher concentration) results for comparison to screening levels and evaluating risk. Prior to presentation, the data will continue to be evaluated to test the reproducibility of these trends and potential site characteristics that may be responsible for them.


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