An Assessment of Saltwater Intrusion in the Changjiang (Yangtze) River Estuary, China

This study, on the basis of ca. 50-year hydrological data (discharge, salinity, and tides level), assesses the spatiotemporal distribution of freshwater sources for Shanghai, with a population of 23 million in the Changjiang River estuary. These are threatened by saltwater intrusion, a natural process that is however exacerbated by water transfer projects, the Three Gorges dam in the upper basin, and the sea level rise in the estuary. The historical relationships among discharge, salinity, and the duration of saltwater intrusion were established quantitatively and demonstrate that (1) the lower is the river discharge, the higher is the salinity and the longer is the residence time of saltwater in the estuary, and (2) the threshold value of the river discharge for salinity intrusion impacting the freshwater supply is 15,000m³s^-1 discharge. Based on this, we estimate that freshwater will be unavailable from the Qingcaosha reservoir every 10 years from about 2040, that is, in the near future, during the extremely low flow season for periods of up to 80-126 days, which would dwarf against the remediation plans for the freshwater supply in the reservoir. Remedial measures are necessary to prevent this possible event.