Theme: 09. Vapor Intrusion

Topic Area: 9d. Vapor Intrusion Risk Assessment and Site Management

Formatted Title:

Human Health Risk Assessment Considering Biodegradation of Petroleum Hydrocarbon Vapors in Brazil

Background/Objectives:

In the State of São Paulo (Brazil), human health risk assessments (HHRAs) are based on the model developed by CETESB (State Environmental Agency). CETESBs’ model, however, does not consider the effects of contaminant biodegradation in the vadose zone, which can significantly reduce vapor intrusion potential for petroleum hydrocarbons (PHC) that readily biodegrade under aerobic conditions. The objective of this presentation is to summarize the HHRA methodology used for a PHC-contaminated site, considering the effects of biodegradation to calculate vapor inhalation risks and define site-specific target levels (SSTLs) for soil and groundwater.

Approach/Activities:

The site is a former industrial plant, located in the city of São Paulo (Brazil), with BTEX, trimethylbenzenes and TPH as chemicals of concern (CoCs). Soil contamination started in the 1980s due to leaks in underground tanks and pipes. Investigations concluded that soil impacts extend from 3 mbgs to the water table at 12 mbgs. In 2017, a HHRA using the CETESB model identified risks associated with vapor inhalation by commercial/industrial receptors in outdoor and indoor environments. The SSTLs calculated by CETESB model were extremely conservative and would require considerable financial and environmental resources to restore the area. Therefore, in 2020, the HHRA was updated considering the effects of biodegradation within the vadose zone.

The methodology included:

a) Installation and sampling of multilevel soil vapor wells: wells were installed at four soil stratigraphic levels (0.5, 4.0, 8.0, and 10.0 mbgs) to verify vapor concentrations and confirm empirical bioattenuation in the vadose zone, and

b) Bioattenuation Factors, Vapor Inhalation Risk Assessment, and Calculation of SSTLs: site soil data, vapor concentration and oxygen levels were used as input to BioVapor (model developed by API) to determine atte

tivities: The site is a former industrial plant, located in the city of São Paulo (Brazil), with BTEX, trimethylbenzenes e TPH as chemicals of concern (CoCs). Soil contamination started in the 1980s due to leaks in underground tanks and pipes. Investigations concluded that soil impacts extend from 3 mbgs to the water table at 12 mbgs. In 2017, a HHRA using the CETESB model identified risks associated with vapor inhalation by commercial/industrial receptors in outdoor and indoor environments. The SSTLs calculated by CETESB model were extremelynuation factors considering aerobic biodegradation. Bioattenuation factors obtained from BioVapor were entered in Risk Based Corrective Action model (RBCA) (developed by GSI Environmental Inc), and more realistic SSTLs for soil and groundwater were developed.

Results/Lessons Learned:

Aerobic biodegradation of BTEX, trimethylbenzenes, and TPH reduced vapor concentrations along the vertical soil profile by up to 8 orders of magnitude. Vapor concentrations were higher near the source of contamination (up to 5 orders of magnitude above the refence values), accompanied by oxygen depletion and carbon-dioxide and methane enrichment. Above the contaminated zone (clean soil), oxygen and carbon dioxide rebounded to near-atmospheric conditions. Field data (vapor wells) and BioVapor/RBCA modeling did not confirmed CETESBs’ estimated risks. Based on the significant bioattenuation that was observed, SSTLs developed by RBCA were set at the CoC solubility limit in water and saturation limit in soil, which is 1 to 3 orders of magnitude higher than what was developed in 2017 using CETESB model. The risk assessment approach and new SSTLs were accepted by CETESB, and the remediation systems present in the area were discontinued. Additional water and soil vapor sampling campaigns were performed in 2022/2023 and confirmed the results. The case is in process of being closed.

This proposed HHRA methodology had never previously been approved by CETESB and should be used as a model for site-specific evaluations of PHC VI risk worldwide.

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Karin Guiguer