Master of Engineering Science
Civil and Environmental Engineering
Atoll island communities face socio-economic challenges together with environmental pressures imposed by climate change that are causing increased water quality degradation. Wastewater from domestic septic systems is released into the subsurface on Fongafale Islet, Tuvalu. It is unclear if this is contributing to high nitrogen (N) concentrations and eutrophication of the adjacent coastal lagoon. In this study, a variable-density groundwater flow and conservative contaminant transport model was developed in SEAWAT-2005 to simulate the salinity distribution and N transport through the Fongafale Islet aquifer. Model simulations evaluated the influence of tides, variable (interannual) recharge patterns, wastewater loading rates and wastewater source locations. The configuration of high and low permeability zones across the island strongly influenced the salinity distributions and N transport pathways. Tidal fluctuations were found to considerably increase freshwater-saltwater mixing and as a result reduce the zone of low salinity groundwater in the aquifer. In contrast, the impact of variable recharge was minor. The simulations indicated that maximum exit concentrations of N in the groundwater discharging to Fongafale Lagoon may be higher than concentrations previously measured in the lagoon, but subsurface travel times were found to be sufficiently long for N attenuation (denitrification) to be occurring in the subsurface. The findings contribute new conceptual understanding of processes influencing freshwater-saltwater dynamics and contaminant transport in an atoll island aquifer as needed to inform water quality and wastewater infrastructure programs.
Summary for Lay Audience
Many developing small-island communities such as those of atoll islands face environmental and socio-economic challenges due to rapid urbanization and climate change. One consequence of these vulnerabilities is reduced availability and quality of groundwater, which is often an important source of fresh water in atoll islands. Another consequence is the degradation of the surrounding coral reef ecosystem, which is important resource supporting coastal fisheries critical to the livelihoods of atoll island residents. This degradation is a result of human waste and fertilizers entering water resources and causing high input of contaminants such as nutrients into groundwater and adjacent coastal waters. High nutrient levels, particularly nitrogen (N), leads to a process known as eutrophication that causes the excess growth of algae and loss of biodiversity in the marine environment. The sources of N, its groundwater pathways, and concentrations discharging to coastal water of atoll islands remains largely unstudied. Poorly functioning septic systems on the atoll island of Fongafale, Tuvalu have been releasing wastewater into the subsurface and it is unclear if this is contributing N to the lagoon.
A computer model was developed based on Fongafale Islet, Tuvalu to simulate the movement and interaction below the surface between saltwater, freshwater from rainfall infiltrating into the ground, and wastewater and associated N injected into the ground from septic systems. Impacts of tides, rainfall, amount of wastewater entering the ground and septic systems locations were evaluated. It was found that tides increase mixing of freshwater and saltwater preventing the formation of a low salinity zone in the shallow aquifer and lowering N concentration discharging to the lagoon. Septic systems located further from the lagoon also led to low N concentrations in the groundwater entering the lagoon and the longest subsurface travel times. The N exit concentrations found by the model represent an upper value as subsurface reactions were not considered in this study. The findings in this study provide new conceptual understanding of atoll island groundwater processes and contaminant transport that will contribute to future atoll island computer modelling, field studies, and planning and management strategies for wastewater infrastructure and pollution control.
Gale, James D., "Evaluation of Fresh Groundwater Resources and Nitrogen Discharge to the Coastal Lagoon of an Atoll Island" (2022). Electronic Thesis and Dissertation Repository. 8568.