Geochemical reactions and solute transport in coastal and deltaic groundwater flow systems

Abstract

Groundwater flow plays a significant role in geological and biogeochemical processes in coastal environments. Along with groundwater discharge, a large body of solutes, including nutrients, heavy metals, and other contaminants, are transported to open water bodies. This thesis investigates geochemical reactions and solute transport mediated by groundwater flow in coastal and deltaic environments. The iterative fitting method, numerical simulation, and analytical solutions were used to quantify the impact of wave motions in coastal zones. Field investigations, laboratory analyses of major ions, nutrients, heavy metals, the radium quartet and radon, and stable isotopes, and paleo-hydrogeological simulation were conducted in a coastal aquifer-aquitard system in the Pearl River Delta (PRD). Such coastal dynamic forces as waves and tides drive groundwater circulation and salt transport in intertidal zones, and thus field investigation considered the combined effects of waves and tides. Fluctuations in wave height were integrated with the tidal level using an iterative least-squares fitting method in which wave height was calculated by the measured sea level in the surf zone, with the fitted wave height then further verified by wind speed. An iterative algorithm separating the wave height from the mixed field data was employed to quantify the combined effects of waves and tides on the density-dependent beach groundwater flow. An analytical study was then conducted to investigate the wave-induced submarine groundwater discharge (SGD) in a subtidal aquifer, with the loading effect of the seawater weight overlying the seabed taken into account. The loading effect can decrease the head gradients between seawater and porewater. The analytical and simulated results may provide guidance for assessing the wave-induced SGD and solute transport in a wave-dominated seabed environment. Coastal aquifers are sensitive to the complicated evolution of marine transgressions and regressions in a river delta region. Paleo-hydrogeological reconstruction of the aquifer-aquitard system in the PRD was conducted for this thesis using hydrogeochemical data and a numerical model, which refined our understanding of the evolution of the groundwater system in Quaternary deltas and the present-day distribution of coastal groundwater reservoirs. Chemicals trapped in the deltaic aquifer system during the Holocene are transported to seawater owing to groundwater discharge, thereby contributing to future ocean eutrophication and acidification. Fluctuations in the groundwater level observed by permanent multilevel groundwater monitoring systems indicate that the paleo-sedimentary environment changed the groundwater flow system, with the local flow replaced by a regional flow. The radium quartet and radon were used to construct a radium reactive transport model employing the vertical migration of paleo saltwater in the aquifer-aquitard system to estimate the relationship between the recoil input and the decay of the sorbed parent nuclide. The model advances understanding of the salinization process in coastal aquifers, and has broad implications for comprehensive hydrogeochemical investigations in other deltaic aquifer systems with a similar hydrogeological setting.