Estimation of submarine groundwater discharge and nutrient loading of Tolo Harbour

Abstract

Tolo Harbour is naturally eutrophic with frequent algal blooms in the past. Water quality of the Harbour has been improved in past 20 years but the phytoplankton density is still high compared to other waters in Hong Kong. This study aimed to quantify submarine groundwater discharge (SGD) to Tolo Harbour and to show the important role of SGD in terms of nutrient loading with ecological effect.

SGD was determined by one of the natural geochemical tracers, 226Ra. Mass balance of 226Ra was applied to quantify SGD and SGD was estimated to be 8.28 × 106 m3 d-1. A large portion of the SGD was recirculated seawater while the freshwater component of SGD, or submarine fresh groundwater discharge (SFGD), was only a small portion of that. SFGD was estimated to be 2.31 ×105 m3 d-1 using water balance. The flushing time of Tolo Harbour was estimated to be 77.9 d by using SFGD as one of the fresh water input components.

Different nutrients including NH3—N, NO2—N, NO3—N, PO43-—P, SiO2—Si and dissolved inorganic nitrogen (DIN) were applied to nutrient mass balance analyses. The mass balance is based on non-point sources which include aerosol deposition (dry deposition), rainfall (wet deposition), diffusion from sediment, river, SGD and tidal exchange. Removal of nutrients was determined once all the non-point sources were estimated. SGD is the major source which contributes more than 88 % for different nutrients. Other sources contribute less than 10 % individually. The total nutrient loads of DIN, PO43-–P and SiO2—Si are 8.33 ×105, 1.03 ×104 and 1.28 ×106 mol d-1 respectively. The removals of DIN, PO43-–P and SiO2—Si are 6.87 ×105, 5.10 × 103 and 1.28 ×106 mol d-1 respectively. Assume the removal of nutrients was only due to consumption of phytoplankton, primary productivity was estimated to be 1.16g C m-2 d-1.

Limitations of this study were about the determination of atmospheric depositions and diffusion from sediment. However, the contribution of nutrient loading from these sources was insignificant compared with SGD. Even if SGD is replaced by SFGD for the nutrient loading estimation, it is still the major contributor among all other non-point sources.

In spite of the limitations, the important role of SGD in terms of nutrient loading is evident. Groundwater quality should be monitored and controlled as it has significant ecological impact to the Harbour.