Preliminary evaluation of soil vapor intrusion at manufactured gas plant sites
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The potential for soil vapor intrusion (SVI) of volatile contaminants has drawn increasing recent attention. In October 2006, the New York State Department of Health (NYSDOH) issued its Guidance for Evaluating Soil Vapor Intrusion in the State of New York which indicates that a SVI evaluation is required whenever a potential subsurface source of volatile chemicals exists at a site. However, the guidance fails to recognize the unique conditions that characterize former manufactured gas plant (MGP) sites, and may result in unnecessary SVI investigations at sites where the potential for significant impacts is small. This study is a preliminary evaluation of pilot data at MGP sites in New York State, focusing on a subset of the available MGP data. The analysis focuses on initial observations, an investigation into potential research questions, and the formulation of statistical objectives. The pilot data indicated that the indoor air at former MGP sites shows no greater contamination of "MGP compounds" than background levels, and therefore, no concerns to the indoor air quality or human exposure. However, full nonparametric comparisons of the sample data with the background levels could not be performed as intended because of severe censoring of the data in the range of the recommended screening levels. As a result, the analysis was performed using proportional comparisons. This resulted in a loss of statistical power and less confidence and substantiation in this conclusion. In future expansions of this study, there is a need for either improved censored data methods or lower detection limits so there is better substantiation for a statistical conclusion regarding a reduction of sampling at MGP sites. An evaluation of the NYSDOH's recommended background screening level was also performed. It was determined that a weakness of utilizing this screening level was its combination of the winter/ summer sampling data. The relationship between seasons is complex, varies by compound, and is still not well understood.