Geochemical processes and solute transport in coastal groundwater mixing zone

Abstract

Submarine groundwater discharge (SGD) is a major component of the hydrological cycle and also an important pathway for the groundwater borne material transporting from land to the sea. Physical and geochemical processes in coastal groundwater mixing zone (CGMZ) substantially influence the chemical composition of groundwater in coastal aquifers. Geochemical processes in CGMZ have been largely ignored despite their significance in estimating SGD-derived material discharge to the sea. This thesis investigated physical and geochemical processes including (1) mixing between fresh groundwater and seawater, (2) adsorption/desorption of major ions and radionuclides, (3) dissolution and precipitation of minerals, (4) production and removal of nutrients, and (5) redox reactions of metals in CGMZ under the effect of tidal fluctuation and seasonal hydrologic variation. The study on SGD estimation indicates that SGD driven by tidal fluctuation occurs mainly in the near-surface area of coastal aquifers where the porewater exchange between saline groundwater in CGMZ and nearshore seawater is dynamic. Therefore, the near-surface area is recommended to be the best location for the groundwater endmember sampling in estimating tide-driven SGD and associated material fluxes. In near-surface area of CGMZ, nutrients (NH4+ and PO43-) are enriched due to the remineralization of organic matters along with the infiltration of nearshore seawater. Meanwhile, total alkalinity and other elements in groundwater such as Fe2+, Mn2+, and short-lived isotopes are diluted owing to the mixing of nearshore seawater depleted in these elements. Biogeochemical processes such as anammox and denitrification are responsible for the significant N loss in the aquifer. Through this removal mechanism, a large amount of terrestrial groundwater borne nutrients is attenuated in CGMZ. In particular, cation exchange and mineral dissolution/precipitation are the main processes of major cations such as Na+, K+, Mg2+, and Ca2+. The CGMZ is a potential location for dolomite production. The sulfate reduction is responsible for the depletion of SO42- and is also the main reason of the enrichment of the total alkalinity. The quick variation of Fe2+ in the transition zone indicates that the fast transformation between Fe2+ and Fe oxides. The variations of Mn and Sr are not as significant as Fe. Due to the rising ionic strength in CGMZ, desorption from sediment surface coating is the main enrichment process of radium isotopes in CGMZ. Tidal fluctuation influences the physical and biogeochemical processes through altering the regional land-sea hydraulic gradient, and changing groundwater redox conditions and chemical compositions through mixing with nearshore seawater. The seasonal hydrologic variation affects the physical and biogeochemical processes through varying groundwater residence time in the aquifer, chemical compositions in mixing endmembers of CGMZ, and redox conditions in the aquifer. The pH, dissolved oxygen, and temperature are found to be of importance to the production of nutrients, cation exchange, recycling of Fe and Mn. The salinity is demonstrated to be the main controlling factor for desorption processes of radium isotopes in CGMZ. Findings of this study will benefit the understanding of the physical and biogeochemical processes and their responding mechanism to environment change in CGMZ.