Improving Risk Assessment from Contaminated Soil from Iqaluit, Nunavut
Risk assessment guidelines utilize conservative values when estimating the exposure to humans from contaminated soil. The conservative guidelines include a default bioavailability of 100% and it is assumed that soil inhalation exposure is insignificant compared to the combined exposure from ingestion and dermal absorption. Due to the conservative guidelines, Site Specific Risk Assessments often attempt to determine the relative bioavailability of contaminants, however due to the high cost of in vivo models, in vitro models for polycyclic aromatic hydrocarbon (PAH) bioaccessibility are being developed. One such development for in vitro models is the addition of a lipophilic phase which acts as a sink for organic contaminants such as PAHs. The primary objective of this research project is to determine if exposure to contaminants in soil presents excessive risk to human health specifically to the residents of Iqaluit. I hypothesize that exposure from soil inhalation is a relevant exposure pathway for human health. Furthermore, I hypothesize that in-vitro PAH bioaccessibility models require a lipid sink to accurately represent in-vivo bioavailability of PAHs. In this thesis, soil inhalation was a relevant exposure pathway for both carcinogenic and non-carcinogenic risk for residents in Iqaluit, NU. Using enrichment factors it was found that the soil was the primary source for many airborne contaminants. Vehicle traffic on unpaved roads led to a large amount of soil re-suspended into the atmosphere, leading to an airborne particulate matter with an aerodynamic diameter less than 10 µm (PM10) average concentration of 35 ± 17 µg m-3. Once roads were paved the average PM10 concentration was significantly (p < 0.05) reduced to 6.5 ± 6 µg m-3 and as a result both carcinogenic and non-carcinogenic risk from soil inhalation was reduced. The results demonstrate that soil inhalation is likely a relevant exposure pathway under particular environmental conditions and that paving roads is an effective method of reducing this risk. To better characterize the correlation between in vivo bioavailability and in vitro bioaccessibility models a case study using eight PAH contaminated soils is performed to determine if the addition of a lipid sink improves this correlation. The addition of a lipid sink significantly increases PAH release for in vitro models (P <0.05). In the presence of a lipid sink, results of the In Vitro Digestion model (IVD) closely corresponded with a slope of 0.85 (r2=0.45, P<0.07) to the in vivo results, whereas there was no correspondence in the absence of a lipid sink (r2 = 0.03). The results from the other in vitro digestion model, the Relative Bioaccessibility Leaching Procedure (RBALP), did not correspond to the in vivo results, with or without the addition of lipid sink, but did tightly reflect total soil PAH concentration. The in vivo release and uptake of PAHs is correlated to fugacity capacity, indicating that PAH release and uptake is a function of both matrix and PAH properties. Environmental conditions play a major role in risk assessment, hence the importance of site specific risk assessments. In Iqaluit, vehicle traffic on unpaved roads is the major source of airborne particulate matter and as a result the soil ingestion pathway is relevant in terms of risk assessment. The correlation between in vivo PAH uptake and release to soil fugacity capacity demonstrates environmental conditions, such as soil properties, influencing bioavailability and subsequently risk assessment.
DegreeMaster of Science (M.Sc.)
SupervisorSiciliano, Steven D.; Farrell, Richard E.
CommitteeWeber, Lynn; Koehncke, Niels
Copyright DateDecember 2012
risk assessment, soil contamination, airborne particulate matter, in vitro bioaccessibility