Investigation of the Source Relationship between Benzene Vapor and Alkylate-Containing Light Non-Aqueous Phase Liquid at Adjacent Industrial Facilities

Abstract

This study describes the investigations undertaken at two adjacent properties for known contaminants of concern and for resolution of other “unexpected” contaminant sources. One property is a manufacturing facility with impacts from chlorinated solvent compounds releases at the site. During site investigations, benzene was found present in subslab and multi-depth soil vapor probes installed beneath the basement floor of on-site buildings. Additional investigations at the manufacturing site were initiated in response to a recent, unexpected appearance of an unknown light non-aqueous phase liquid (LNAPL) in three on-site perched water zone wells as observed during sequential quarterly monitoring events and in a soil vapor extraction well. The other property is an adjacent petroleum facility that has ongoing LNAPL recovery operations from past releases at its site. A claim was made that benzene vapor at the manufacturing property was solely related to the LNAPL that migrated onto the facility. Multiple lines of evidence, including LNAPL migration pathways and chemical fingerprints, were used to show that the LNAPL was sourced from the petroleum facility. Further, benzene vapor present in subslab vapor probes installed for vapor intrusion purposes was believed to be sourced from the LNAPL and not from historical operations at the manufacturing facility. However, detailed analysis of vapor distributions, the subsurface stratigraphy, and LNAPL characteristics indicate that spills during past operations at the manufacturing facility itself were likely the source of the benzene vapor found in the subslab vapor probes.