Conceptual Site Model
Mearns evaluated potential exposure for current commercial workers, future residents, and construction workers (including trench workers).
Soil exposure pathways including ingestion, dermal contact, and inhalation, were evaluated for all three groups.
Inhalation exposure to contaminants in soil gas was evaluated indoors for residents and commercial workers, and outdoors for construction (trench) workers.
Exposure parameters were based on DTSC (2019).
Exposure point concentrations (EPCs)
Soil EPCs were the 95% upper confidence limit (UCL) of the mean when this statistic could be calculated, otherwise the maximum detected concentration was used as the soil EPC.
Soil gas EPCs were the UCL when Pro-UCL was able to calculate a reliable UCL, otherwise the maximum detected concentration was used as the EPC.
OEHHA typically recommends point-by-point results for single-family residences.
However, UCLs may be more realistic for multi-family housing.
Risk Assessment
Mearns’ estimated risks and hazard indices (HI) are summarized as follows:
Exposure scenario Risk Hazard index
Resident 3.0E-3 506
Commercial worker 6.9E-4 117
Construction worker 2.0E-6 6
All risks and hazard indices except the construction worker risk exceeded typical thresholds. OEHHA agrees with these estimates.
The risk drivers for residents were naphthalene, benzene, and methyl tert-butyl ether (MTBE) in soil gas and n-nitroso di-n-propylamine and 2,6-dinitrotoluene in soil. The hazard drivers were gasoline range organics, benzene, and naphthalene in soil gas and C13-C22 hydrocarbons in soil.
The risk drivers for commercial workers were naphthalene, benzene, and methyl tert-butyl ether (MTBE) in soil gas. The hazard drivers were gasoline range organics, benzene, and naphthalene in soil gas.
Elise McCaleb
August 26, 2021
The hazard drivers for construction workers were C4-C12 and C13-C22 hydrocarbons in soil and gasoline range organics in soil gas.
Risk and Hazard Mitigation
Mearns proposes installing a vapor barrier with a passive subsurface depressurization system to mitigate vapor intrusion. OEHHA recommends that care be used to ensure that preferential pathways connecting soil gas to occupied portions of the building are not created during construction. Indoor air sampling after construction of the new building and prior to being occupied by the tenants would help to determine whether indoor air is clean.
Mearns proposes a soil management plan to deal with discolored or odiferous soils discovered during excavation and grading. OEHHA recommends that soil contamination in excess of risk and hazard thresholds be addressed prior to excavation and grading.
Conclusions
All risks and hazard indices except the construction worker risk exceeded typical thresholds. OEHHA agrees with these estimates.
Given these exceedances, OEHHA agrees that soil and soil vapor remediation and/or mitigation will be needed for all scenarios.
Reviewed by
Amanda Palumbo, PhD
Staff Toxicologist
Carmen Milanes, M.P.H.
Chief, Climate Indicators and Site Assessment Section
References
DTSC (California Department of Toxic Substances Control). 2020. Human Health Risk Assessment (HHRA) Note Number 11. Southern California Ambient Arsenic Screening Level, Human and Ecological Risk Office (HERO). December 28, 2020.
DTSC 2019. Human Health Risk Assessment (HHRA) Note. HERO Note #1.
Recommended DTSC Default Exposure Factors for Use in Risk Assessment at California Hazardous Waste Sites and Permitted Facilities. April 9.