Report No FR/D0021

MARCH 1995

OBJECTIVES

1. To quantify the processes affecting the export of metals (eg. cadmium, mercury, aluminium and zinc) from major east coast estuaries.
   
2. To understand the subsequent transport processes that affect the fate of these metals, with particular reference to the appropriate biological factors.
   
3. To test the validity of transport and water quality models developed for estuarine plumes.

SUMMARY

The distribution of trace metals (Fe, Mn, Zn, Cu, Cd, Pb, Co, Ni, Al, Hg) has been determined in dissolved and particulate phases, along with relevant hydrographic and geochemical parameters, in the plume of the Humber Estuary during winter, spring and summer. Fluxes of dissolved trace metals across the sediment-water interface have also been measured. Fluxes of dissolved and particulate metals through the Humber plume region have been calculated using these observed metal data together with modelled tidal data. Partition coefficients for trace metal transfers between soluble and adsorbed phases have been used to investigate the causes of variations in chemical speciation within the plume.

The results show that fluxes of both dissolved and particulate metals through the Humber plume region are dominated by the coastal flow from the north. Riverine inputs superimpose a plume of lower salinity, higher metal contents on this net southward-moving flow. Seasonal variations in trace metal fluxes are dominated by variation in the freshwater inputs of trace metals and fine suspended sediments. The adsorption of trace metals onto fine suspended sediment is a major sink for dissolved metals in the region. The presence of manganese oxides from estuarine sources enhances these processes for Zn, Cd, Co and Ni. Observations and model calculations suggest that biological activity makes a major contribution to the removal of trace metals from the system during periods of sustained plankton growth.

Existing 2-dimensional depth-averaged hydrodynamic and advection/diffusion models of the Humber plume have been modified. Sediment transport models have been investigated. A seasonally varying depth-averaged single-point phytoplankton growth model has b een developed. This has been encoded into the advection/diffusion model to allow the prediction of the seasonal variations in primary production. These models have been used to provide building blocks for the development of metal sediment and metal biota process models. Two basic types of model have been developed. The first assumes that trace metals are at chemical equilibrium with the fine sediment and biota, and thereby calculates speciation using partition coefficients. The second uses simple chemical and biological kinetics to calculate the concentration of metals in the dissolved, particulate and biological phases. Both approaches show considerable promise for examples where appropriate data are available to parameterise the models. The development of water quality models for estuarine plume is currently hampered by the dearth of data on the time dependence of biogeochemical processes involving trace metals. A lack of suitable data upon freshwater inputs to initiate models and the poor spatial and temporal coverage of trace metal data sets for validation compounds this. Water quality models with a capability for prediction for trace metal behaviour are perfectly feasible given our understanding of the processes involved. A concerted effort is needed to develop a suitable kinetic data set which can be used to parameterise such models.

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