Abstract of the proposal

Although not as obvious as surface water, groundwater also discharges into the sea in the form of submarine groundwater discharge (SGD). While the impact of river water and sewage on the coastal sea water quality has been well recognized, the input of chemicals and nutrients to the sea in the form of SGD in Hong Kong has been largely ignored. We believe that it is essential to investigate the quantity and quality of SGD so that the geochemical budget in the coastal areas can be estimated more accurately. This study has two objectives. The first is to investigate the SGD in Tolo Harbour by collecting water samples and analyzing their radon and radium isotopes, which can serve as good natural groundwater tracers in igneous rock areas. The second is to investigate whether SGD is one of the controlling factors of algal blooms in the Harbour. Tolo Harbour is chosen to be the study area because: 1) the Harbour is a semi-closed bay and the groundwater catchment surrounding the bay is large so that relatively large amount of SGD is possible; 2) there are 6 marine water quality monitoring stations in the bay run by the Environmental Protection Department (EPD) in the past 20 years and 5 groundwater level monitoring wells near the Harbour owned by the Water Supply Department (WSD) with data over 30 years, and 3) in the past 20 years about half of the algal blooms in Hong Kong occurred in the Harbour according to the statistics from EPD.

This study will involve extensive field sampling, radon and radium isotope measurements, and long-term monitoring data interpretation by statistical analysis. Samples will be collected from seawater, groundwater and surface water in the study area and the radon (222Rn) and radium isotope (223Ra, 224Ra, 226Ra and 228Ra) contents in the samples will be measured. 222Rn and the long-lived 226Ra (half-life = 1600 yrs) and 228Ra (half-life = 5.75 yrs) can be used to quantify the amount of SGD. The short-lived 223Ra (half-life = 11.4 days) and 224Ra (half-life = 3.4 days) can be used to calculate the mixing ratio of the coastal waters. Groundwater chemistry in the monitoring wells will also be investigated so that the amount of chemical and nutrient discharge into Tolo Harbour in the form of SGD can be estimated. The marine quality and algal bloom data from 1986 to 2004 in the Harbour will be collected from EPD and analyzed. The water level measurements taken in the same period by WSD will be used to study the groundwater flux to the Harbour. The potential relations among the flux of SGD, marine water quality, and eutrophication in the study area will be investigated.

This project will present the first attempt to study SGD by measuring the radon and radium isotopes in coastal environments in Hong Kong with important environmental implications such as to delineate the relationship between eutrophication and groundwater discharge in the study area. By quantifying SGD, it may be possible to calculate the amount of materials and nutrients transport from land to the sea. Besides knowing more about the geochemical budget in coastal water in the Harbour, this study will shed some light on water pollution remediation in the study area. On the other hand, it has long been suggested that algal blooms in Hong Kong is the direct result of discharge of untreated sewage to the rivers and the near coastal areas. By studying the 20-year continuous marine water quality data and the corresponding groundwater level data, it may be possible to investigate the relationship between groundwater discharge and eutrophication in the Harbour, Hong Kong. Based on the initial findings from this project, we will apply for major external funding such as RGC grant.

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