Indoor air inhalation risk assessment for volatiles emanating from light nonaqueous phase liquids

The indoor air inhalation pathway for volatile contaminants in soil and gro undwater has received much attention recently. The risk of exposure may be higher when volatile organic compounds (VOCs) reside as constituents of a f ree product plume below residential or commercial structures than when diss olved in groundwater or adsorbed on soil. A methodology was developed for a ssessing the potential for vapor phase migration-and associated risk of ind oor air inhalation-of volatile constituents from a light nonaqueous phase l iquid (LNAPL) plume on top of the water table. The potential risk from inha lation of VOCs in indoor air emanating from a subsurface Jet Fuel 4 (JP-4) plume by hypothetical residential receptors was assessed at a site. Chemica ls of concern (COCs) were identified and evaluated using data from the comp osition of JP-4 mixtures and published chemical, physical, and toxicologica l data. The method estimates the equilibrium vapor concentrations of JP-4 c onstituents using Raoult's Law for partial vapor pressure of mixtures based on assumptions about the mixture composition of JP-4. The maximum allowabl e vapor concentration at the source (immediately above the LNAPL) correspon ding to an indoor air target concentration based on acceptable risk levels are calculated using the Johnson and Ettinger model. The model calculates t he attenuation factor caused by the migration of the vapor phase VOCs throu gh the soil column above the JP-4 plume and through subsurface foundation s labs. Finally, the maximum allowable soil gas concentrations above the LNAP L for individual constituents were calculated using this methodology and co mpared to the calculated equilibrium vapor concentrations of each COC to as sess the likelihood of potential risk from the indoor air inhalation pathwa y.