Human exposure to many environmental contaminants such as DDT and metabolites (DDTs), PCBs, PAHs, and dioxins occurs primarily through food chain transfer and biomagnification. Due to their affinity for natural organic matter, these contaminants are distributed predominantly in the soil or sediment compartment. The initial trophic transfer involves uptake by sediment or soil-dwelling organisms, which dictates the ultimate human health risk of a contamination site. An often-neglected fact is that contamination at most Superfund sites is a result of historical events, and that contaminants at these sites have resided in the same media for many years or decades, a biogeochemical process loosely termed aging. Aging generally leads to lower contaminant bioavailability, i.e., a reduced potential for entering the food chain. However, at present, there lacks a good understanding of aging effects and its value is not used or exploited in current risk assessment and remediation practices. In this project, we will first develop a method to measure aging effects of DDTs and PCBs in field-aged sediments by quantifying their relative bioavailability. We will use concurrent bioaccumulation tests to validate the method. We will then apply the method to sediment samples collected as a function of depth (reflecting deposition at different historical times) and spatial location (reflecting different sediment properties) at the Palos Verdes (PV) Shelf Superfund site off the Los Angeles coast. The PV Shelf sediments contain high levels (up to 200 mg/kg) of DDTs (especially p,p'-DDE) and PCBs deposited from as far back as 60 years ago. Correlation analysis will reveal the relationship between age and bioavailability, and elucidate the influence of sediment properties. The aging-induced bioavailability changes will be incorporated into trophic food chain models to improve the estimation of hazards to ecological receptors and humans. Black carbon sorbents (e.g., activated carbon, charcoal, biochar) are increasingly used in remediation, as they immediately immobilize HOCs in soil or sediment. We will further carry out long-term laboratory incubation experiments to understand the effect of such amendment on contaminant aging or permanent sequestration. Through this project, we will obtain a holistic understanding of aging effects and its role in regulating contaminant bioavailability, and capture opportunities for their application to achieve improved risk assessment and mitigation.
A biogeochemical process common to contamination at nearly all Superfund sites is that the contaminants have aged for many years or even decades. Aging diminishes the bioavailability of contaminants and hence potential human exposure, but its role is not well understood or used. The proposed research will provide a simple method for measuring aging effects, improve our knowledge on the mechanisms of aging and conditions that control aging effects, and expand our ability to consider aging in risk assessment and remediation.