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 Ni 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 Ni about the Mersey Estuary. The model couples transport of metals throughout the water along with incorporating the chemical processes controlling how nickel is fractioned between dissolved and particulate phases through the newly developed partition coefficient relationship. Model predictions of dissolved Ni along the longitudinal axis of the estuary were compared with measurements of Ni for two events; very good correlation was obtained between the model results and the data.