The Distribution and Reactivity of Organochlorines in Estuaries
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In February 1997, the Governing Council of the United Nations Environment Programme initiated a protocol to reduce the risks to human health and the environment arising from persistent organic pollutants. The initial list of 12 families of organochlorine pollutants included polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) selected for their persistence, bioaccumulation capacity and toxicity. The aim of this work was to further understanding of the behaviour of these organochlorines (OCs) in estuarine systems. Field surveys were conducted at UK estuarine (Humber, Clyde and Dee) and coastal locations. Sediments were analysed for OCs and a range of chemical and physicochemical properties (trace metals, organic carbon content, specific surface area). Hydrographic measurements were also made to assist in defining the estuarine environment. Concentrations of PCDD/DF on a dry weight basis ranged from non-detectable (n.d.) to 11100 pg g"^ and PCB from n.d. to 3000 ng g Estuarine OC contamination was generally elevated above coastal waters concentrations. The distribution of OCs was studied to determine the extent of source, compound physicochemical, and particulate geochemical influences. In the Clyde estuary some source-related effects were observed but in the Humber estuary, characterised by highly dynamic conditions, these were masked by mixing of sediments. The characteristics of the sorbent were found to be of key importance with the content and nature of particulate organic carbon being the major control. To further understand the sorption behaviour of OCs, partitioning experiments were conducted with ^"C analogues of the compounds under realistic estuarine gradient conditions (K^ range from 2.4 x 10** to 49.4 x 10"* for TCB and 23.1 x 10Mo 313 x 10^ for TCDD). Particle concentration was found to be of key importance and it was concluded that this effect was transferable to environmental situations. Salinity was found to be a minor influence on partitioning behaviour. Development of sorption sub-models and their incorporation into an integrated estuarine contaminant transport model was demonstrated. A particle interaction-based sorption model was shown to only partially account for the observed partitioning behaviour; inclusion of an irreversibly adsorbed fraction may more adequately represent estuarine partitioning. Riverine and outfall discharges were simulated in the integrated contaminant transport model to demonstrate the influence of tidal hydrodynamics, sediment dynamics and variable partitioning applied to the Humber estuary.
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