Hydrochemical reactions and origin of offshore relatively fresh pore water from core samples in Hong Kong

- The hydrochemistry of the submarine groundwater in Hong Kong is first studied.
- Pore water with low Cl− concentration shows the presence of offshore brackish waters.
- The stable isotopes of water suggest the terrestrial origin of the pore water.
- Corrections for evaporation during transportation and storage are introduced.
- Leaching and decomposition reactions recognised locally can occur in other reserves.

SummaryThe existence of relatively fresh pore water offshore has been well recognised over the globe but studies on the chemistry of the pore water from offshore geological formations are extremely limited. This study aims to characterize the hydrochemistry of the submarine groundwater body in Hong Kong. It looks into the major ion concentrations and the stable isotopic compositions of pore water extracted from core samples from an offshore 42.30-m vibrocore in the southwestern Hong Kong waters. A minimum Cl− level of about one-third of that in typical seawater was noted in the terrestrial sediments, suggesting the presence of offshore relatively fresh water. Unexpectedly high NH4+ levels are attributed to organic matter decomposition in the terrestrial sediments. The leaching of shells due to exposure of marine sediments at sea-level low stands raises the Mg2+ and Ca2+ concentrations. Base Exchange Indices show weak cation exchange reactions in which Na+ and K+ are released while Mg2+ and Ca2+ are adsorbed. Isotopic compositions of pore water reveal that the low-salinity water is probably the relic water sequestered in fluvial systems during relative sea-level low stands. Cores properly stored in a freezer for a long time has been used to study the pore water chemistry. For the first time, this study introduces an approach to correct the measured data by considering the possible evaporation effect during the transportation and storage of the samples. Corrections for evaporation were applied to the major ion concentrations and the stable isotopic compositions of pore water measured. It is found that the corrections determined by the Cl− mass balance approach are more reliable. The corrected measurements give more reasonable observations and hence allow sensible conclusions on the hydrochemical reactions and the origin of pore water.