Evaluation of Fresh Groundwater Resources and Nitrogen Discharge to the Coastal Lagoon of an Atoll Island

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