Role of Sponge City development in China's battle against water pollution: a predictive study in the Guangdong-Hong Kong-Macao Greater Bay area

Along with the rapid economic development, water pollution in China has been pervasive. China is now having a battle against its severe water pollution in urban areas, and Sponge City development is one of the major strategies. Sponge City emphasizes low impact development (LID) and promotes construction of green infrastructures, although construction of grey infrastructures is also an important measure. However, few studies have quantitatively addressed how Sponge City development would help improve the water quality in China's urban areas. This study performs a predictive analysis in the Guangdong-Hong Kong-Macao Greater Bay area, the most developed metropolitan area in the southern China. An integrated three-dimensional hydrodynamic and water quality model was developed on the Delft3D suite for the Shenzhen Bay, the transjurisdictionary water body between Shenzhen (often referred to as China's Silicon Valley) and Hong Kong. Watershed load estimation and source attribution were performed for total nitrogen (TotN), total phosphorus (TotP) and BOD. The model was further applied to assess the water quality impact of proposed Sponge City development in Shenzhen (SCP), city's plan of upgrading its waste water treatment plants (WWT), and combination of the two (SCP_WWT). The modeling results show that SCP can reduce a large amount of TotP loading, but only has a small effect on TotN, whereas WWT performs the reverse. The impact SCP_WWT is greater than the sum of the impacts of individual implementations of SCP and WWT, reflecting a synergy impact. The proposed Sponge City development is overall effective in mitigate the water pollution in the bay, while grey infrastructures play a more critical role than green infrastructures. Furthermore, an increase in Chl.a concentration in the bay is found, along with the decrease in TotN, TotP and BOD, which is due to the improvement in light condition in the bay water. The water quality in the Shenzhen Bay is not only determined by its watershed conditions, but also dependent on the water quality in the Pearl River Estuary. The results imply that larger-scale (covering the entire Greater Bay area) pollution prevention activities are needed to further improve the water quality of this important transjurisdictionary water body.