Abstract of the proposal

Submarine groundwater discharge (SGD) has been increasingly recognized as an important source of nutrients to coastal seawater, which has lead to numerous studies on the estimations of SGD and SGD-derived nutrients crossing the aquifer-seawater interface. The most popular approach to estimate SGD is a budget analysis of a geochemical tracer in the coastal seawater. In this analysis, the total tracer flux from SGD is estimated first and then the SGD rate is estimated by dividing the flux with the concentration of the tracer entering the seawater along the aquifer-seawater interface. Because the tracer concentration in the SGD at the interface is difficult to estimate, usually the tracer concentration in groundwater samples from existing wells within a certain distance of the shoreline is taken approximately as the tracer concentration at the interface. Some studies further estimate the fluxes of SGD-derived nutrients by simply multiplying the SGD rates by the nutrient concentrations in the wells.

The pH, salinity, and redox conditions vary greatly in the freshwater-saltwater mixing zone in coastal aquifers. Some chemicals sensitive to the geochemical environment changes such as radium and nutrients may be added or removed before they enter the sea. Consequently, using groundwater chemicals from the wells may lead to erroneous SGD estimation because the well chemicals can be very different from those of SGD leaving right at the aquifer-seawater interface. The understanding of the dynamics of tracer and nutrients, their addition and removal processes in coastal aquifers, and the implications of these processes on SGD estimation is very limited.

We propose to investigate the dynamic hydrogeochemical processes of flow, nutrients, radon and radium in the freshwater-saltwater mixing zones of Tolo Harbour by installing and then monitoring multi-level sampling piezometers and the implications of these processes on SGD estimation. Numerical models will be used to simulate the transport of nutrients in the mixing zone and predict the nutrients leaving at the aquifer-seawater interface. We will then estimate the amount of nutrients transported via SGD to Tolo Harbour using the nutrients at the aquifer-seawater interface predicted by the numerical models. The work will be conducted by an interdisciplinary team using multiple approaches, including multi-level monitoring piezometers, field hydrogeological studies, radioisotope tracers, and numerical modelling. Successful completion of this study will provide a deeper understanding of the hydrogeochemical processes of geochemical tracers and nutrients in the subterranean estuary and insights and methodology to better estimate SGD.