Cumulative water quality impact study on coastal developments in western waters of Hong Kong

Abstract

The western waters of Hong Kong have long been subjected to pollution from the Pearl River flow, local discharges and contaminated sediment disposal facilities. An increasing number of major coastal development projects including the Hong Kong – Zhuhai – Macao Bridge, has posed a threat and further deteriorated the already poor water quality in the area. The cumulative water quality impact arisen from many concurrent developments is significant and beyond the scope of project-specific environmental impact assessment (EIA) and environmental monitoring and audit (EM&A). In order to provide a comprehensive picture of how the individual developments combined to affect western waters, this study reviewed the recent and future development studies and identified 18 major development projects which could affect water quality in western waters over a 20-year period (2005- 2024). A computer tool making use of Delft3D model was developed to input the collected data for assessing the cumulative water quality impact of on-going projects and predicting the impact of those in the pipeline.

Three timeframes were chosen for this study: baseline condition (2005-2008), recent projects (2009-2015) and future projects (2016-2024). The baseline water quality before the intensive developments took place in the western waters was analyzed. The characteristics of the western body were found to have high concentrations of suspended solids (SS) and total inorganic nitrogen (TIN) with moderate dissolved oxygen (DO) levels. For the three recent projects under the Hong Kong – Zhuhai – Macao Bridge mega project, large sediment plumes were generated and frequently exceeded SS limits, resulting in a substantial decline in water quality. For future works to be carried out in 2016-2024, combined impacts from six reclamation projects and a dumping facility for contaminated mud were determined by the water quality model. During the construction phase, SS and other parameters were projected to be greatly elevated and the extent of sediment plume was immense when all projects are going at the same time. Thus, proper phasing of marine construction works is recommended to minimise the cumulative water quality impact to avoid the adverse impact on human and marine life. During the operation phase of the projects, SS will largely return to normal but TIN will rise further and DO will decrease due to increase in population and activities. A more holistic and strategic planning, reducing pollutants from new projects as well as existing pollution sources, is therefore essential for improving the water quality of western waters.