Organophosphate
esters (OPEs) have been in use as flame retardants
for many decades, with their actual usage varying over time. Knowledge
of the emission history of OPEs is valuable for improving our prediction
of their environmental loadings and associated risks. In this study,
concentrations and compositions of 10 OPEs were measured in three
dated sediment cores from the Palos Verdes Shelf (PVS) off the coast
of Los Angeles, which has been impacted by wastewater treatment plant
(WWTP) effluents for over a century. The total OPE concentrations
varied from 0.68 to 1064 ng/g along the sediment profile, with two
apparent peaks. The first peak occurred in the 1970s, coinciding with
peak emissions from WWTPs. The second peak appeared in the 2000s and
was possibly attributed to increased consumption of OPEs as replacement
flame retardants. Since downward movement of OPEs in the PVS sediment
bed was retarded by their slow desorption, the reconstructed history
likely provided an accurate picture of OPE emissions in Southern California
and North America. These findings suggest that the near-shore marine
sediments affected by WWTP effluents could serve as an environmental
proxy documenting history in OPE use and emissions.
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 530-2,600 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.
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