Assessment of Nitrogen Inputs from Groundwater to a Coastal Atoll Lagoon

ABSTRACT: Small island development states (SIDS) are subject to compounding anthropogenic and climate change stresses including growing populations and sea level rise. The inhabitants of low-lying coral atoll islands are especially vulnerable and facing numerous challenges including increasing pollution of coastal lagoons and subsequent deterioration of fisheries. Due to the high connectivity between the ocean and land on coral atolls, it is challenging to evaluate the pathways delivering land-derived pollutants to coastal lagoons. Groundwater may be a potentially important pathway but its relative contribution to coastal nutrient loads remains unclear. This study focuses on Funafuti Atoll, Tuvalu where excessive nitrogen inputs to Fongafale lagoon have caused eutrophication and associated macro-algal blooms. Poorly functioning septic systems and pig manure are thought to be the main contributors to the excessive nitrogen loads to the lagoon. Studies have shown that the groundwater is highly polluted and therefore discharge of nutrient-rich groundwater to the lagoon may be an important delivery pathway. The study objective was to examine the potential role of groundwater in delivering nutrients to the lagoon. This was addressed by variable density groundwater and nutrient transport (conservative and reactive) modelling. A vertical cross-section of the atoll was simulated and spatial and temporal variability of groundwater discharge to the lagoon driven by seasonal and tidal fluctuations was considered. Subsurface transport of nitrogen was simulated with consideration of nutrient sources, pathways, residence times and potential for natural attenuation to occur prior to its discharge to the lagoon. The study findings will be used specifically to inform future field investigations and management decisions on Funafuti Atoll related to groundwater nutrient inputs to the lagoon. Moreover, the findings will increase knowledge of nutrient groundwater contamination on atoll islands and its potential contribution to coastal eutrophication.