The main focus of the project is the development of a sound framework for assessing and managing river sediment quality. A robust assessment tool (with hydrodynamic and water quality models for the sediment system) to track the fate and transport of heavy metals in rivers and sediments has been developed to support the assessment analysis. Deriving the experience from other countries,the results of the field work and model simulations in this study formed the basis of a comprehensive assessment and management framework, which was then used to evaluate a range of sediment restoration scenarios for the select river system. The assessment outcome was then incorporated into the management strategies and recommendations for the sediment clean-up effort. The project work started with data and information gathering and field program development for collecting hydrographic data of the river system and geophysical surveys of the sediment system. The water quality modeling framework, WASP from US EPA was adopted as the platform for developing the fate and transport model of the Dahan River in Taipei, selected as the target river system for this project. Subsequent field work conducted in the Dahan River provided the necessary data for model development, calibration, and verification of the model. Results from the field program revealed that existing levels of copper, zinc,and nickel in the Dahan River sediments exceed the water quality criteria with copper the metal most frequently violating the standard. The river portion between the Fu-Chou Bridge and Xinhai Bridge is found having the highest metal concentrations in the sediment as a result of the significant metal loads from sources in the watershed. Geophysical survey data indicated typhoons have a massive impact on the morphology of the river channel with significant erosion resulted from these events. In addition, results from the ground-penetrating radar and sonar readings suggest that they can be a useful screening tool to quantifying sediment transport (i.e. shoaling) in the Dahan River. The WASP model simulation results indicated that metal levels in the Dahan River sediments will continue to rise if no actions were taken to reduce the metal input. On-site removal of the contaminated sediments would only be a temporary measure to lower the metal contents in the sediment before the return of the contamination if the external inputs are not curbed. On the other hand, complete removal of the external metal input would significantly lower the metal levels in the river water followed by subsequent reductions of the metal contents in the sediment even without on-site dredging. Further, removing contaminated sediments along with control of the external inputs would be the preferred strategy to gradually restore the river sediment system to within the metal thresholds in 30-40 years. Sediment studies reported in the literature mostly suggest that sediment quality criteria be the basis for the development of control strategies and remediation actions to restore the sediment systems. Other contributing factors such as ecological and health risk considerations could also be incorporated into the assessment. The field data and model results from the Dahan River study indicated that the sediment quality is closely related to the water quality in the river with the external metal input as the major source of contamination. Controlling external metal inputs is therefore viewed as the top priority in the effort to restore the sediment system.

river;transport model;management strategies and recommendations