Vertical water renewal in a large estuary and implications for water quality

Estuaries are a special transition zone subject to both riverine and marine processes, where environmental issues, e.g. water pollution, eutrophication and hypoxia, have become an increasing cause of concern in recent decades. The vertical transport of water and material is an intrinsic process in estuarine environments, with the atmosphere and seabed being the upper and lower boundaries. However, vertical water renewal in estuaries is not fully understood despite its significance to the estuarine environment being widely recognized. In the present study, the vertical water renewal process in a large estuary is investigated using the concept of water age. A three-dimensional water age model is built based on a hydrodynamic model, in which the age of a water parcel is defined as the time interval since it last touched the air-water interface, and thus indicates the renewal duration from the free surface. Water renewal durations, especially when relatively long, can provide insight into environmental and water quality issues, e.g. a low dissolved oxygen (DO), that can have a significant impact on ecosystem functioning. Results showed that the water age in the Pearl River Estuary (PRE) was characterized by significant spatial distributions and seasonal variations, which depends heavily on the water density stratification, as indicated by the Richardson number. During the wet season, the bottom water age was large at the lower reach of the estuary, up to 8 days, whereas the maximum bottom age during the dry season was ~1 day at the upper reach. Based on the quantification of vertical renewal, a new approach was proposed, and used to successfully evaluating DO depletion. The data and method would benefit for future environmental management, eco-biological restoration and related policy-making, especially when oxygen-based pollution is considered.