Keywords: partition coefficient, heavy metal modelling, sediment modelling, Mersey Estuary.

One of the legacies bequeathed by the Industrial Revolution to the Mersey Estuary is that a large buildup of industrial discharges has left considerable deposits of heavy metals bound to the fine sediments of the estuary. As a result of the neglect of waste management, the estuary earned itself the ignominious title of the most polluted estuary in Europe. During the period 1850-1950 increased industrial output resulted in many industries requiring large volumes of water for cooling and processes such as bleaching, tanning and metalworking. It is likely that during the 1960s the water quality status of the Mersey Estuary was at its lowest being severely contaminated by toxic organic and inorganic compounds from the local industries. At the beginning of the 1970s pollution effects from heavy metals caused serious concerns. Then, UK Government laboratories reported that fish caught in Liverpool Bay contained elevated levels of many heavy metals. These raised levels were attributed to the contaminated waters of the Mersey Estuary due to the wanton discharging of industrial waste.

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 six heavy metals 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. The results also show the different functional relationships between KD and salinity for different metals.

Previous researchers undertook a detailed study of seven trace metals distributed throughout the Mersey Estuary. They describe the estuary as a highly contaminated, organic-rich estuary and they observed an increase in the sediment-water distribution coefficient, with increasing salinity for all metals considered except Cd. These observations were inconsistent with inorganic speciation calculations and empirical modelling studies in other estuaries, predicting an inverse relationship between, partitioning coefficient, KD and salinity. The primary focus of their work was to investigate the factors controlling the salinity distributions of metal data based on defining the chemical processes that control the KD salinity relationship. They developed an empirical model based on salting out of neutral organic chemicals that reflected the salinity distribution of the data.

A numerical model was developed for predicting the transport and distribution of six heavy metals about the Mersey Estuary. 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 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 concentrations of the metals along the longitudinal axis of the estuary were compared with measurements; very good correlation was obtained between the model results and the data.

1

R.A. Falconer, "An introduction to nearly horizontal flows", In "Coastal, Estuarial and Harbor Engineers", M.B. Abbott, W.A. Price, (Editors), E. and F.N. Spon, London, 27-36, 1993.

2

Y. Wu, R.A. Falconer, "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

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