Modeling of polychlorinated biphenyls (PCBs) in the Baltimore Harbor

Source, fate, transport and modelling of selected emerging contaminants in the aquatic environment: Current status and future perspectives

The occurrence of emerging contaminants (ECs), such as pharmaceuticals and personal care products (PPCPs), perfluoroalkyl and polyfluoroalkyl substances (PFASs) and endocrine-disrupting chemicals (EDCs) in aquatic environments represent a major threat to water resources due to their potential risks to the ecosystem and humans even at trace levels. Mathematical modelling can be a useful tool as a comprehensive approach to study their fate and transport in natural waters. However, modelling studies of the occurrence, fate and transport of ECs in aquatic environments have generally received far less attention than the more widespread field and laboratory studies. In this study, we reviewed the current status of modelling ECs based on selected representative ECs, including their sources, fate and various mechanisms as well as their interactions with the surrounding environments in aquatic ecosystems, and explore future development and perspectives in this area. Most importantly, the principles, mathematical derivations, ongoing development and applications of various ECs models in different geographical regions are critically reviewed and discussed. The recommendations for improving data quality, monitoring planning, model development and applications were also suggested. The outcomes of this review can lay down a future framework in developing a comprehensive ECs modelling approach to help researchers and policymakers effectively manage water resources impacted by rising levels of ECs.

Development of a fate and transport model for biodegradation of PBDE congeners in sediments

Polybrominated diphenyl ethers (PBDEs) are a family where each congener possesses different physicochemical properties, persistence and/or toxicity. Biodegradation can selectively change the abundance of congeners. These warrant modeling of individual congeners by considering biodegradation pathways together with fate and transport (F&T) mechanisms. Accordingly, this study aims to develop a F&T model (Fate and Transport model for Hydrophobic Pollutants - FTHP) that integrates congener specific biodegradation of PBDEs in sediments. The model is tested using sediment data from a location representing the Lower South Bay of San Francisco. Results demonstrated settling, resuspension, and biodegradation as important mechanisms. FTHP is then used to predict congener concentrations in a period of 20 years for two cases (constant and time-dependent water column concentrations) and four alternative scenarios: no intervention (i.e., natural attenuation, also serves as the base case), no degradation, dredging and biostimulation. The greatest impact on the reduction of total PBDE concentrations was achieved by a reduction in water column concentrations, i.e. source control, and dredging. On the other hand, biostimulation coupled with source control was the most effective in reducing bioaccumulative PBDE congener concentrations and almost as effective as dredging for the rest of congeners. Proposed FTHP model can distinguish between congeners and help devise informed management plans which focus on decreasing risks associated with persistent and bioaccumulative compounds in contaminated sediments.

Fate and Transport Modelling of Emerging Pollutants from Watersheds to Oceans: A Review

This chapter provides a review of the fate and transport modelling of emerging pollutants (EPs) and discusses the major research challenges. The overwhelming limitation of the past modelling studies has been the lack of data necessary for model validation, thus calling for large-scale field data sampling. The identification and understanding of fate and transport processes and their interactions of the target EPs and the corresponding selection of appropriate parameter values were also challenging. Such limitations and challenges were evidenced by the elaboration of the representative models in the field. The review also reveales that the model parameter values varied significantly with the EPs (and chemical compositions) and media of concerns. Sensitivity analysis was found to be necessary for modelling of those EPs with limited references in the literature. In comparison with traditional water pollutants, the concentrations of many EPs in water bodies are usually low and even at a trace level, leading to uncertainties or inaccuracy in measured data. This could further challenge model calibration and validation, and especially the determination of parameter values when lacking sufficient data support. How to improve the existing models to address such an issue special for EPs is an urgent task for researchers to ensure the accuracy and reliability of modelling results.

Spatial-temporal variability and transportation mechanism of polychlorinated biphenyls in the Yangtze River Estuary

Although the variability of polychlorinated biphenyls (PCBs) is strongly dependent on the hydro-sediment pattern, the quantification of this interaction is still not well described, especially for estuary areas. In this study, both chemical analyses and numerical simulation were conducted to explain the temporal-spatial variability and transportation mechanism of PCBs in the Yangtze River Estuary (YRE). The impacts of the upstream Three Gorges Dam (TGD) on estuarine PCBs were also addressed with a simulated scenario. The results showed that the PCBs levels in the YRE were relatively low or moderate and the highest levels were related to the maximum turbidity zone. The spatial variability of PCBs is strongly dependent on the hydrological circulation, which resulted in a declining trend from the inner YRE to the adjacent sea. The seasonal variability of PCBs could be due to the joint influence of the current and the erosion/deposition environment. The opposite temporal trends of the overlaying water and sediment are driven by the seasonal characteristics of hydro-sediment patterns. The simulated results also indicated that the distribution, fluxes and transport ability of PCBs in the South Branch changed as a result of the sediment discharge reduction after construction of the TGD.

The atmosphere as a source/sink of polychlorinated biphenyls to/from the Lower Duwamish Waterway Superfund site

Waterbodies polluted with polychlorinated biphenyls (PCBs) may cause the air in the surrounding area to become PCB-contaminated. Conversely, when a waterbody is located in or near an urban area, the deposition of atmospheric PCBs may act as a low-level, ongoing source of PCB contamination to that water. Distinguishing these situations is necessary to be protective of human populations and to guide efforts seeking to cleanup such aquatic ecosystems. To assess the situation at the Lower Duwamish Waterway (LDW) Superfund site, low-density polyethylene passive samplers were deployed in the summer of 2015 to quantify freely dissolved water and gaseous air concentrations of PCBs thereby enabling estimates of the direction and magnitude of air-water exchange of PCB congeners. For the sum of the 27 PCB congeners, average concentrations were 220 pg/m³ (95% C.I.: 80-610) in the air and 320 pg/L (95% C.I.: 110-960) in the water. The sum of air-water exchange fluxes of these PCB congeners was estimated to be 68 ng/m²/day (95% C.I.: 30-148) into the lower atmosphere, contrasting with the reported wet and dry depositional flux of only 5.5 ng/m²/day (95% C.I.: 1-38) from the air into the water. Therefore, the atmosphere was ultimately a sink of PCBs from the LDW Superfund site, at least under 2015 summertime conditions. However, we conclude that air-water exchange of PCBs is likely only a minor sink of PCBs from the LDW and only a minor source of contamination to the region's local atmosphere.