Potential offshore groundwater reserves in Hong Kong by characterising submarine aquifer-aquitard systems

Abstract

Offshore low-salinity groundwater has been reported to be a global phenomenon, but related hydrogeological research has not been carried out in Hong Kong despite of its potential as another valuable water resource. An integrated geophysical and hydrochemical approach has been adopted to investigate sand and gravel deposits beneath the seafloor, which are likely to constitute regional submarine aquifers and contain brackish or even fresh groundwater. Marine seismic reflections profiles across waters southwest to Lantau Island and along Tolo Harbour were analysed to depict the distribution of sand and gravel deposits. The recognition of extensive sand deposits in the study areas is compelling, with an average thickness of about 11 m. This results in an estimated total offshore low-salinity water volume of 3850-6850 mcm in Hong Kong. This amount of water is comparable to the annual water import to the city. If these sand deposits are recharged through onshore outcrops, then the offshore groundwater reserves may be a sustainable resource. Moreover, these sand deposits can also be used for artificial aquifer recharge and recovery. Secondly, more direct observations on the sedimentological, geophysical and hydrochemical properties of the sediments below the seabed were obtained from laboratory measurements on an offshore vibrocore southwest to Lantau Island. Based on the sedimentological and geophysical results, a four-fold sequence closely associated with the relative sea-level history is noted. This agrees with the hypothesis that the offshore groundwater reserves are the relics of flow systems formed during interglacial periods. In particular, pore water with chloride concentration one-third of that in seawater is found in the lowermost oxidized sediments, indicating the presence of relatively fresh water in the terrestrial units. Integrating these results gives the distribution and the nature of the submarine aquifer and aquitard materials. This study also provides preliminary information for feasibility study on exploiting the relatively fresh groundwater as an alternative to seawater in desalination, which has been proposed as one of the future water supplies in Hong Kong. Thanks to its lower salinity, the desalination cost can be lowered with the use of the relatively fresh offshore groundwater.