Elemental Mercury Contamination in Soil Leading to Vapor Intrusion Impacts on an Occupied Building: A Detailed Case Study

Elemental mercury has been used in the production of bleach since at least 1892 and continues to be utilized in some manufacturing processes today. This case study examines a former bleach manufacturing facility in the western United States, where elemental mercury was utilized as an electrical conductor in the chlor-alkali process to produce chlorine and sodium hydroxide, essential constituents in bleach formulation. The operational practices implemented at the facility led to the discharge of elemental mercury into both soil and groundwater. Subsequent investigations identified the presence of mercury in indoor air at levels surpassing the screening thresholds established by the California Environmental Protection Agency (CalEPA) Department of Toxic Substances Control (DTSC) and the United States Environmental Protection Agency (USEPA) for commercial exposure scenarios. Additionally, these concentrations exceeded the California Office of Environmental Health Hazard Assessment (OEHHA) acute 1 h reference exposure level (REL). The origins of mercury in indoor air have been identified as vapor intrusion associated with subsurface sources, along with a potential secondary indoor air source associated with mercury deposition and adsorption in building materials through the years. In the context of interim vapor intrusion mitigation, air purifiers and fans were deployed to enhance air exchange rates, while a comprehensive assessment led to the identification and sealing of 52 preferential pathways. Even with these interim vapor intrusion mitigation systems in place, elevated concentrations of elemental mercury are still present in the building and may represent the presence of a secondary indoor air source from accumulation of elemental mercury in building materials.