Water circulation and impact on water quality in the southwest of Efate Island, Vanuatu
Introduction
Small Island Developing States (SIDS), such as Vanuatu, face multiple challenges in terms of environment and development including coral reef deterioration, damage to infrastructure, loss of coastal land and managing waste and growing population (Graham et al., 2020; Devlin et al., 2020). Vanuatu is considered one of the world's most vulnerable countries to natural hazards (Heintz et al., 2018) and inherently vulnerable to climate change impacts (Nurse et al., 2014). Vanuatu's is exposed to sea level rise impacts and other natural disasters and is vulnerable due in part to being comprised of many small remote islands with remote communities, its low level of economic development and the concentration of settlements and infrastructure in the coastal zone (Garschagen et al., 2016; Mackey and Ware, 2018; Boateng, 2010).
SIDS are also vulnerable to climate change impacts including sea level rise (Nurse et al., 2014) which can reduce the water quality from salinisation and increase coastal flooding (Thomas et al., 2020). These impacts are added to the multiple pressures facing SIDs and coastal systems. Vanuatu's population has increased by 66% (2000−2020) with 24.4% of the population now living in urban locations (United Nations, Department of Economic and Social Affairs, Population Division, 2019) with projections suggesting this could increase by 88% by 2050 (United Nations, Department of Economic and Social Affairs, Population Division, 2019). The population growth rate on Efate Island is almost double this national rate with the majority of the growth occurring in the peri-urban fringes of Port Vila municipality, Efate (Komugabe-Dixson et al., 2019; Zari et al., 2020). Port Vila, the nation's capital, has more than a quarter of Vanuatu's total population and the growth rate is estimated to be up to 8% per annum (Zari et al., 2020). Increasing population growth and tourism in Pacific SIDS (Gheuens et al., 2019) results in more marine pollution, wastewater discharge and land degradation and therefore an increase in the need for improved sanitation systems. The increase in coastal population growth and tourism in Pacific SIDS (Gheuens et al., 2019) results in increased demand for sanitation system from the additional marine pollution, wastewater discharge and land degradation. This increase in coastal population and the inadequate sewage infrastructure is one of the main drivers of marine pollution across the Pacific SIDS (McEvoy et al., 2017). This is the case of many south Pacific islands where wastewater is discharged directly into the ocean with little or no treatment (Graves et al., 2021) which may affect coral reefs and coastal ecosystems (United Nations, 2004). Done and Navin (1990) noted that the two main threats to the coral reefs of Vanuatu are siltation and eutrophication caused by domestic sewage discharged into reef waters. Tourist resorts in Vanuatu are concentrated along the coast, where limited or no wastewater treatment or management systems are in place (SOPAC, 2007). In Port Vila, the associated growth in population and development has resulted in wastewater being discharged into Port Vila Bay and Erakor Lagoon (Fig. 1) yet there are no effective wastewater management systems in place.
In Vanuatu, sewage contamination is also a serious health issue (Devlin et al., 2020) with many of the discharged wastes sourced from uncontrolled and unregulated piping and drainage (SOPAC, 2007; SPREP, 2016), And many businesses and houses having a direct-discharge sewage system (SOPAC, 2007). Sewage contamination is an additional stress on coral reef ecosystems, exacerbating climate change impacts and further highlighting the need to protect coastal water quality. Other pressures that may also be impacting on the water quality of the Bay include dredging and industrial runoff. In 2017, the channel on the southern end of the inner harbor in Vila Bay was dredged to 5.5 m depth with a channel 50 m wide, 200 m long (Hall, 2021).
To date, assessments of water quality and wastewater in Vanuatu coastal waters (Graham et al., 2020; Devlin et al., 2020; SPREP, 2016) with have shown that water quality issues are close to shore, though a lack of long-term data make it difficult to identify scale of change. Faecal contamination from sewage is often the reason for high bacterial levels (Hart et al., 2020). Total coliform, E. coli and Enterococci are the three types of bacteria tested as indicators of the presence of faecal material within Port Vila Harbor (Department of Geology, Mines & Water Resources, Government of Vanuatu, 2018; Taillardat et al., 2020). Such pathogens may pose health risk to people fishing, swimming and bathing in a water body. It was found that many businesses and houses in Port Vila Town still continue to empty human waste directly into the drainage system. Water quality issues became highly publicised when recreational activities including swimming, fishing, snorkelling were banned within Port Vila Bay in July 2018 due to high levels of bacteria with total coliform and E. coli (Department of Geology, Mines, and Water Resources, Government of Vanuatu, 2018). In 2021, the swimming ban was lifted in some parts of Vila harbor, though all swimming is banned during rainfall events. Erakor lagoon system is also highly used by locals and tourist for recreational activities There is also a further lack of water quality data in Erakor lagoon system (DGMWR, 2008) and therefore water pollution might go unnoticed, and might be unsafe for any recreational activities as found in Vila Harbor.
The effective management of reef-lagoon systems such as Erakor Lagoon requires a detailed understanding of the dominant circulation patterns that drive key processes such as sediment transport, biochemical cycling and spatial ecology (Lowe et al., 2010). The circulation can be driven by waves, tide, wind or buoyancy forcing (Lowe et al., 2010; Sous et al., 2017; Dumas et al., 2012). An understanding of circulation and the physical dispersal patterns of the wastewater are fundamental to managing water discharge and water quality in the system. However, knowledge of the flushing patterns and residence time within Erakor Lagoon and Port Vila Harbor is limited, and the factors influencing circulation patterns are unknown. Exposure to potential pollutants could be identified with a more informed understanding of the circulation and hydrodynamics of Erakor lagoon and Port Villa Bay. Furthermore, modelling of pollution transport would provide information on the likely impact of pollution on coastal coral reefs and human health.
The aim of this study, therefore, was to understand the fate of pollutants and their residence time within Port Vila Bay, where health impacts from wastewater discharge are well documented, and in Erakor Lagoon where there is little understanding of the impact of wastewater discharge on water quality. Given ongoing climate change impacts, we also modelled the potential impacts of project sea level rise under a range of scenarios. It was also anticipated that this study could generate information to support the development of water quality management action plans for the bay and lagoon which are currently lacking.
Section snippets
The study area
Vanuatu is an archipelago comprising 83 small islands in the South Pacific Ocean of mostly volcanic origin with 65 of them inhabited. There are about 1300 km between the most northern and southern islands and the total area is around 12,189 km2. Shefa is one of the six provinces of Vanuatu located in the centre of the country and includes the island of Efate which is volcanic and limestone/raised reef in origin. Erakor Lagoon system is located in the East side of Port Vila in Efate Island.
Methodology and approach
Modelling of pollution transport would provide information on the likely impact of pollution on coastal coral reefs and human health. Data on waves, tides and currents were collected by Griffith University to calibrate a numerical model for estimating pollution residence time inside Vila Bay and the Erakor Lagoon system and the dispersion of pollutants in the lagoons. The residence time characterizes the local transport rate and is calculated as the flushing time measured with passive tracers.
E. coli and faecal contamination
The health risks for water-based recreational activities of E. coli concentration levels are based on seven risk categories from “low” to “very unsafe” assessed against U.S. EPA Recreational Water Criteria Recommendation 1 (Aquagenx, 2021). US EPA guidelines are used across the US to assess water quality and potential impact. For the Erakor Lagoon System and Villa Bay, the results ranged from “low risk” to “very unsafe with predominance of high risk to very unsafe (Table 2).
Over the period of
Discussion
The aim of this study was to understand the fate of potential pollutants and flushing time of Port Vila Bay and in Erakor Lagoon systems, and the impact of wastewater discharge on water quality, inclusive of projected sea level rise and related climate variability. This in turn required a detailed understanding of the dominant circulation patterns that drive key processes and how circulation in the lagoons and bay is affected by waves, water levels, wind or buoyancy forcing.
Conclusion
This study provides new information on the residence time of pollutants in the Erakor Lagoon systems and Port Villa Bay. The results highlight the influence of wind, waves and water level in their patterns of circulation. This information can inform planning and management to reduce marine pollution of the lagoon and prevent future pollution associated with land-based activities and associated developments. Similar studies are needed throughout the SIDS. These results are a first step in the
CRediT authorship contribution statement
Conception and design of the study: Gaëlle Faivre.
Acquisition of data: Gaëlle Faivre, Erie Sami, Dr. Krishna Kumar Kotra, Jim Aimbie, Michael Maniel.
Analysis and/or interpretation of data: Gaëlle Faivre, Prof. Rodger Tomlinson, Prof. Hong Zhang, Dr. Guilherme Vieira da Silva, Prof. Brendan Mackey.
Drafting and editing the manuscript: Gaëlle Faivre, Prof. Rodger Tomlinson, Prof. Hong Zhang, Prof. Brendan Mackey.
Draft revisions: Gaëlle Faivre led draft revisions, with all authors contributing to
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful to the Department of Water Resources of Vanuatu, the Office of the Maritime Regulator, and the University of South Pacific for supporting data collection in this study. Data collection was enabled by the support and involvement of the local communities and customary owners of the lagoons and associated land. This research was undertaken with the approval of a research agreement between the Vanuatu National Cultural Council and Griffith University (May 17, 2018). The
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