Bed and suspended sediments as source and transport mechanisms for polychlorinated biphenyls in the Houston Ship Channel Estuary System
Howell, Nathan 1981-
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Polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) are pollutants of global concern. Though their industrial histories differ, they both represent large classes of chemicals (209 PCBs and 210 PCDD/Fs) that are known for their persistence, bioaccumulative, and toxic (PBT) potentials. Both classes of pollutants are hydrophobic (log Kow > 4.5) and therefore exhibit low water solubility. As a corollary to their strong hydrophobicity, they exhibit high sorptive affinity for biolipids and abiotic organic carbon, both critical environmental matrices which affect overall water quality, ecological risk, and human health. PCBs and PCDD/Fs have been monitored semi-continuously in the Houston Ship Channel for the last ten years because of their higher concentrations in many media. This dissertation examines much of the gathered data in this study and prior studies (more for PCBs than PCDD/Fs) in water, fish, and bed sediment in great detail both spatially and temporally. What is most specifically in view through the analysis is a richer understanding of the role of sediments, in both bed and suspended sediment forms, towards contaminant fate-and-transport and an examination of the differing partitioning behavior between PCDD/Fs and PCBs. Bed and suspended sediment-mediated PCB contamination is examined according to a few major themes--water column suspended to dissolved partitioning, truly dissolved concentrations of PCBs in sediment pore water, contaminant flux of PCBs from a large mass repository in the sediment bed, and detailed and relatively short (1-2 years) fate-and-transport modeling using a developed EFDC model of the estuary. Water column partitioning analysis showed that though PCBs and PCDD/Fs have similar hydrophobicities, PCDD/Fs partition quantitatively higher to suspended particles than PCBs. Another significant finding is that PCBs in a freely dissolved phase, considered to be more of a bioavailable measure of contaminants, can be modeled with reasonable accuracy thus increasing our understanding of environmental risk and the ultimate fate of PCBs currently residing in HSC bed sediment. The developed EFDC model, when combined with the characterized partitioning behavior of PCB congeners in the estuary and the prediction of freely dissolved concentrations will be a key decision-making tool for developing remediation and management strategies.