Effect of aging on bioaccessibility of DDTs and PCBs in marine sediment

Hydrophobic legacy contaminants like dichlorodiphenyltrichloroethane (DDT) and polychlorinated biphenyls (PCBs) were banned almost half a century ago. While their residues still remain in many environmental compartments, they have undergone extensive aging and likely have lower bioaccessibility (the available fraction) compared to fresh residues. However, risk assessment relies heavily on the use of total chemical concentration, rather than accounting for age-diminished bioaccessibility, likely leading to overestimated risks. In this study, we used 24 h Tenax desorption to measure the potential bioaccessibility of DDTs and PCBs in two sediment cores taken from the Palos Verdes Shelf Superfund site in the Pacific Ocean. The total concentrations of DDTs and PCBs from the core located at the sewage outfall (8C) were as high as 41,000-15,700 μg/kg (dry weight, dw) and 530-2600 μg/kg dw, respectively, while those from a location 7 km northeast of the outfall (3C) were 2-3 orders of magnitude lower. Bioaccessibility estimated by 24-h Tenax-aided desorption (F_24h) decreased in the order of DDD > DDE > DDT for DDT derivatives, and PCB 52 > PCB 70 > PCB 153 for PCB congeners, showing a negative correlation with their log K_ow. Due to the extensive aging, F_24h values were <20% of the total chemical concentration for most contaminants and <5% for DDT, DDE and PCB 153, suggesting that aging greatly diminished their bioavailability. However, a quantitative relationship between F_24h and sediment age along the vertical profile was not found, likely because the contaminant residues had undergone aging before their offsite transport and deposition onto the ocean floor. As the use of man-made chemicals such as DDT and PCBs was discontinued in the U.S. many decades ago, the reduction in their bioavailability due to aging may be universal and should be taken into consideration to avoid overly conservative risk predictions or unnecessary mitigation interventions.