Reconstructed chloride concentration profiles below the seabed in Hong Kong (China) and their implications for offshore groundwater resources

Jiao, Jiu Jimmy; Shi, Lei; Kuang, Xingxing; Lee, Chun Ming; Yim, W. W.-S.; Yang, Shouye

Reconstructed chloride concentration profiles below the seabed in Hong Kong (China) and their implications for offshore groundwater resources | Profils de concentration en chlorure reconstruits sous le fond de mer à Hong Kong (Chine) et leurs implications sur les ressources en eaux souterraines en domaine marin Perfiles de concentración del cloruro reconstruidos debajo del lecho marino en Hong Kong (China) y sus implicancias en los recursos de agua subterránea mar adentro 香港海底沉积物中氯离子浓度剖面的重建以及它们对近海地下水资源的意义 Perfis reconstruídos de concentração de cloretos sob o leito do mar em Hong Kong (China) e suas implicações para os recursos hídricos subterrâneos offshore

2015

Offshore hydrogeology has been much less studied compared to onshore hydrogeology. The marine Quaternary system in Hong Kong (China) consists of interlayers of aquitards and aquifers and was part of the Pearl River Delta when the sea level was low before the Holocene. Core samples from six offshore boreholes were collected to measure the chloride concentration in the system by adding deionized water. A method was proposed to convert the sediment chloride into that of the original pore water. A one-dimensional sedimentation-transport model was developed to simulate the historical conservative transport of the reconstructed pore-water chloride. The model integrates present knowledge of stratigraphy and the historical evolution of the geological system. The chloride concentration profiles show that the chloride decreases from an average of 13,800 mg/L in the first marine unit to an average of 5,620 mg/L in the first aquifer. At the bottom of one borehole, the concentration is only 1,420 mg/L. The numerical model shows that the vertical chloride distribution is due to diffusion-controlled downward migration of seawater. The second marine unit obstructs the downward migration, indicating its low permeability and good aquitard integrity. The relatively fresh or brackish water in deep aquifers protected by the aquitard has the potential to be used as drinking water following some treatment, or at least as raw water with much cheaper desalinization compared with using seawater. The methodology and findings in this study are instructional for other coastal areas with similar geology and history in the South China Sea.

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