Keywords: partition coefficient, heavy metal modelling, lead modelling, sediment modelling, Mersey estuary.

Modelling heavy metals in estuarine environments is extremely complex for various reasons; one of the primary complicating factors is that metals exist in two phases: dissolved and particulate bound. Dynamic changes in water chemistry, and in particular salinity, affect the partitioning of metals between the two phases and hence make it difficult to determine the relative fractions of each phase. A relatively simple approach was developed to relate variations in partition coefficient for lead to salinity fluctuations in the Mersey Estuary. The functional relationship developed between partition coefficient and salinity departs from the traditional exponential type curve, providing a more realistic relationship. A numerical model was then developed for predicting the transport and distribution of lead about the Mersey Estuary. The model couples transport of metals throughout the water along with incorporating the chemical processes controlling how lead is fractioned between dissolved and particulate phases through the newly developed partition coefficient relationship. Model predictions of dissolved lead along the longitudinal axis of the estuary were compared with measurements of lead for two events; very good correlation was obtained between the model results and the data.

In the paper the authors describe a model that was developed to predict distributions of lead throughout the Mersey Estuary. An extensive water quality monitoring programme was developed for the Mersey Estuary from the mid 1970's. Water samples are collected from up to 23 mid-stream locations and analysed for various constituents including sanitary parameters and trace contaminants. The main difference in approach adopted during this research has been the manner in which the partitioning coefficient has been developed; this has significance for future heavy metal modelling studies. As in many previous studies, KD is expressed as a function of salinity which reflects total water chemistry. The KD-salinity relationship developed for lead is based on direct sampling of the estuary waters and subsequent chemical analysis for dissolved and particulate-bound concentrations of lead. Having developed this relationship it is quite easy to incorporate it into a numerical model, requiring only to compute salinity before calculating the value of KD. The hydrodynamic and solute transport model used was based on the work by Falconer [1] and the sediment transport model was based on work by Wu and Falconer [2].

The authors are aware of the work done by Turner and Millward [3] in considering conceptual aspects of KD-salinity relationships to develop improved predictive bio-geochemical models. Issues addressed by Turner and Millward included the dependency of solid-solution partitioning on particle size and isotope speciation in solution, and the relative contribution and implications of flocculant products to absorbed contaminant concentrations. However, in order to use the approaches outlined by Turner and Millward is it necessary to have considerable more information on estuarine processes such as floc formulation.

1

Falconer, R.A. "An introduction to nearly horizontal flows", In Coastal, Estuarial and Harbor Engineers' Reference Book, Ed., M.B. Abbott & W.A. Price, E and FN Spon, London, 27-36, 1993.

2

Wu, Y. and Falconer, R.A., "A mass conservative 3-D numerical model for predicting solute fluxes in estuarine water", Advances in Water Resources, 23, 531-543, 2000. doi:10.1016/S0309-1708(99)00035-4

3

Turner, A. and Millward, G.E., "Partitioning of trace metals in a macrotidal estuary. Implications of contaminant transport models", Estuarine, Coastal and Shelf Science, 39, 45-58, 1994. doi:10.1006/ecss.1994.1048

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