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METAL POLLUTANT PATHWAYS IN COHESIVE COASTAL CATCHMENTS: INFLUENCE OF FLOCCULATION ON PARTITIONING AND FLUX

Potentially toxic metals (PTMs) dispersed within catchments from land-based sources pose serious, long-term threats to aquatic ecology and human health. Their chemical state or form affects the potential for transportation and bioavailability and ultimate environmental fate. PTMs are transported either as (1) particulates adsorbed onto sediments, or 2) solutes in groundwater and open channel flow. Cohesive sediment occupies a major part of the world’s coastlines. PTMs are readily sorbed onto clay/silt and consequently particulate-borne PTMs dominate in estuaries and coastal waters. Sediments also represent a considerable ‘sink’ of contaminants which can be periodically remobilized. The role of suspended particulates in the uptake, release, and transport of heavy metals is thus a crucial link in understanding PTM dispersion in these environments. Cohesive sediment is subject to flocculation which dictates the behaviour of suspended sediment. PTM partitioning, flocculation and particulate-borne PTM dynamics are spatially and temporally variable in response to a complex array of inter-related physical and chemical factors exhibited within tidal catchments. However, knowledge of the dispersion and accumulation of both particulate and soluble forms of PTMs within cohesive coastal catchments is limited by little understanding of the association of PTMs with flocculated sediments and their subsequent deposition. This study investigates the influence of changing hydrodynamics and salinities to reveal the partitioning coefficients (Kp) and PTM settling flux (PTMSF) for different spatial and temporal locations within an idealized mesotidal catchment. The data show that the ratio of soluble and particulate-borne PTMs are dependent on salinity and flocculation, and that PTMSF is dependent upon partitioning and flocculation dynamics. Kp is largely dictated by salinity, but floc size and suspended particulate matter concentration (SPMC) are also influential, particular for PTMs with low chloride complexation and in freshwater. PTMSF is a function of Kp, floc size and settling velocity and varies by up to 3 orders of magnitude in response to changing environmental conditions. Findings will improve our ability to predict and monitor contaminant transport for PTMs generated by industries such as agriculture, mining, fisheries, aquaculture & marine engineers. They can be incorporated in existing decision making tools, and help improve numerical modelling parameteristion, to maintain environmental quality standards and limit the impacts of bioavailability of metals in aquatic environment.