Modelling diagnosis of heavy metal (copper) transport in an estuary

A vertical (laterally-averaged) two-dimensional heavy metal transport model was developed to simulate the fluxes of copper in the Danshuei River estuarine system. The toxic model was incorporated into the hydrodynamic, the salt transport and the sediment transport modules. The model has been validated with observed time series data of water surface elevation, current, salinity, and suspended sediment concentrations in 2001 and 2002. An exponential relationship was established to relate variation in partition coefficient of copper to salinity and suspended sediment concentrations in the Danshuei River estuary. Comparisons of the total, dissolved, and particulate copper concentrations calculated by the numerical model and field data along the Danshuei River-Tahan Stream show good agreement. It is noteworthy that the model requires a point source of total copper with the strength of 250 kg/day to account for the observed persistently high total copper concentration near Hsin-Hai Bridge in the Tahan Stream, about 24 km upriver from the river mouth. The huge garbage dump site near the Hsin-Hai Bridge is likely the source of heavy metal pollution. The validated model was then applied to investigate the tidally averaged salinity distributions, residual circulation, suspended sediment, and total copper concentrations under low flow condition. The residual circulation is characterized by the upriver movement of denser saline water in the lower layer and the downriver movement of fresher water in the upper layer. The circulation pattern is driven by the longitudinal gradient of salinity distribution. The residual circulation occurs in the deep channel of Kuan-Du Bridge and Taipei Bridge. Under low flow condition, the limits of salt intrusion are located at Hsin-Hai Bridge. The tidally-averaged sediment concentration distribution exhibits a local maximum concentration around the null point in the Danshuei River-Tahan Stream. A local maximum of total copper concentration, which is nearly as high as that near the point source, occurs around the null point, which is potentially a hot spot of heavy metal accumulation.