Anthropogenic impacts on water quality of the lagoonal coast of Fongafale Islet, Funafuti Atoll, Tuvalu
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Water pollution, evident by negative values of redox potential in waters, occurs at the lagoonal coast located near the densely populated area of Fongafale Islet on Funafuti Atoll, Tuvalu, Central Pacific. Sediment microbial quinone analysis revealed that these coastal sediments exhibit 2.7–10.4 times more microbial biomass, significantly different microbial community structure and low microbial diversity, when compared to an undisturbed natural coastal sediment. Thus, the pollution is chronic. By considering the total land use/coverage on the islet, the situation of septic tank installations, temporal changes in water redox potential and Escherichia coli numbers in the coastal waters and the spatial distribution of acid volatile sulfide in the sediments, we conclude that domestic wastewater is the primary source of pollution. This pollution is proposed to occur via the following mechanism: during ebb tides, domestic wastewater leaking from bottomless septic tanks and pit toilets run off into the lagoonal coast. Tide changes control the pollution load of domestic wastewater.
KeywordsWater pollution Escherichia coli Redox potential Domestic wastewater Septic tank Pacific Island
Anthropogenic pollution in reef-flat seawater is of great concern for coastal conservation. This is because reef island sediments are produced by calcifying organisms, such as coral, coralline algae, molluscs and large benthic foraminifera, that live in the adjacent reefs. In Central Pacific atolls (e.g., Tuvalu, Kiribati, Marshall Islands), shells of large benthic foraminifera are the primary components of sand-sized sediments (Collen and Garton 2004; Yamano et al. 2005). Thus, corals and foraminifera are two major sand producers. Coral reefs on the ocean side act as a natural breakwater and provide bioclastic materials. If a coral reef is healthy without receiving adverse impacts such as rising acidity of seawater, it has an upward growth potential of as much as 400 mm/100 years, which matches the median predicted value of sea-level rise. Thus, a healthy coral reef has the potential to keep up with rising sea level (Kayanne et al. 2005).
Recent studies have suggested that reef islands and adjacent coral reefs located near densely populated areas are being affected by wastewater discharge and waste disposal (Abraham et al. 2004; Richmond et al. 2002; Vieux et al. 2004). The main islands of atoll nations are densely populated (e.g., 8,300 people/km2 on Fongafale, Tuvalu; 2,558 people/km2 on South Tarawa, Kiribati and 11,724 people/km2 on Majuro, Marshall Islands) (Secretariat of the Pacific Community 2005, 2007; Economic Policy, Planning and Statistics Office 2007) owing to limited habitable areas. Concentrations of nutrients were high in reef-flat seawater near densely populated islands, resulting in both direct and indirect negative effects on foraminifera through habitat changes and/or the collapse of algal symbiosis (Osawa et al. 2010). Such reduced water quality on coral reefs caused changes in benthic foraminiferal communities (Hallock et al. 2003; Uthicke and Nobes 2008; Carilli and Walsh 2012). Large benthic foraminifera were rare or absent in the ocean reef flat of Majuro Atoll (Fujita et al. 2009), in lagoons and ocean reef flats of the south Tarawa Atoll (Ebrahim 2000) and in the vicinity of wastewater outfalls on Enewetak Atoll (Hirshfield et al. 1968). The decrease in sediment supply has the potential to contribute to increased coastal erosion (Collen and Garton 2004); however, the mechanisms causing such high nutrient concentrations are as yet unknown.
Reef islands and their populations are considered vulnerable to a range of climatic changes including sea-level rise and similar extreme occurrences (Mimura et al. 2007). The most anticipated physical impacts of sea-level rise on reef islands are shoreline erosion, inundation, flooding, salinity intrusion and reduced resilience of the coastal ecosystem (Khan et al. 2002; Leatherman 1997; Mimura 1999; Yamano et al. 2007). If the atoll nations disappear, there will be no islands left and nothing to inhabit (Connell 2004).
Considering the above studies, a degradation of coral reefs and a decline in large benthic foraminifera, caused by anthropogenic impacts, will accelerate the onset of serious problems that may be caused by future sea-level rise. Therefore, studies are urgently needed to develop and implement countermeasures in order to protect these areas against coastal water pollution. In this paper, we investigate the current water quality of the densely populated lagoonal coasts in Fongafale Islet, Central Pacific and the occurrence of water pollution. We then compare them with less populated natural coast in the islet. The primary pollution sources and pollution mechanism are identified. Through this investigation, we demonstrate the need for effective water quality control measures for coastal conservation.
Materials and methods
Water quality measurements
A water quality sonde (Model 6600V2, YSI/Nanotech, Kawasaki, Japan) was installed at ~20 cm from the reef-flat sediment and at 40–60 cm water depth at sites 1, 2-2 and 3, on 5, 3 and 4 April 2010, respectively. Water temperature, electrical conductivity (EC), salinity, dissolved oxygen (DO), pH and redox potential (Eh) were observed routinely at intervals of 10 min for around 1 day on the same days. Further observation was conducted at site 2-2 from 6 to 10 August 2010 at the same intervals for 4 days, in order to investigate the behavior of domestic wastewater runoff.
Water samples (10 mL) were diluted 10-fold with sterile distilled water and were subjected to most probable number analysis using a commercial test kit (Colilert 18/QuantiTray™, IDEXX Laboratories, Tokyo, Japan) (Fricker et al. 1997). The samples were incubated at 37 °C for 18 h, in accordance with the manufacturer’s instructions.
Microbial quinone is an essential component in the electron transport chain of microorganisms (Hiraishi et al. 1989). Quinones are divided into two groups: respiratory quinones and photosynthetic quinones. Respiratory quinones, ubiquinone (Q) and menaquinone (MK), exist in bacteria that use respiration to gain energy. In general, ubiquinone is used for aerobic or anoxic respiration and menaquinone for aerobic or anaerobic respiration (Jones 1988). Photosynthetic quinones, plastoquinone (PQ) and vitamin K1 (VK1), are present in photosynthetic microorganisms such as microalgae and cyanobacteria (Collins and Jones 1981; Jones 1988). Each microorganism has only one predominant quinone associated with that species, which is stable even when environmental conditions change. The content of quinone corresponds to the amount of biomass of the microorganisms (Hiraishi et al. 1989). Therefore, quinones have been used as a biomarker to quantitatively analyze a microbial community structure in aqueous environments, such as tidal flats or seabed sediments (Hasanudin et al. 2004, 2005).
It is known that quinone species are assigned to phylogenetic taxa on the basis of the available chemotaxonomic information (Hiraishi et al. 1989). Q-8, Q-9 and Q-10 are assigned to the beta, gamma and alpha subclasses of Proteobacteria, respectively (Yokota et al. 1992). MK-6, MK-7 and MK-8 are assigned to taxonomic groups including the Flavobacterium-Cytophaga group (Nakagawa and Yamasato 1993) and gram-positive bacteria with low G + C contents (Collins and Jones 1981). In addition, MK-7 occurs in sulfate-reducing bacteria such as Desulfotomaculum and Desulfococcus species (Collins and Widdel 1986).
To evaluate microbial community structure, 250 mL surface sediments, up to ~10 cm depth, were sampled at sites 1, 2-2 and 3 on 10 August 2010. Samples were stored at −20 °C. Microbial quinone in the sediments was assayed according to a procedure reported previously (Hasanudin et al. 2004, 2005). Lipids, including quinone, were extracted from the sediment sample with a chloroform–methanol mixture (2:1, v/v) that was re-extracted with hexane. The crude quinone extract in hexane was concentrated using a solid-phase extraction cartridge (Sep-Pak® Plus Silica, Nihon Waters, Tokyo, Japan) and was separated into menaquinone and ubiquinone with 2 and 10 % diethylether–hexane, respectively. Ubiquinone and menaquinone were analyzed using a high-performance liquid chromatography (SCL-10A VP, Shimadzu, Kyoto, Japan) with a photodiode array detector (SPD-M10A, Shimadzu, Kyoto, Japan). Quinone species were identified by their spectrum and the equivalent number of isoprene units (Hiraishi et al. 1989).
Acid volatile sulfides
Sediment samples were collected at up to ~10 cm depth from surface layer of all sites on 20 and 21 January 2011, and the concentration of acid volatile sulfides (AVS) in the sediments was determined in triplicate using an AVS detector tube (210H and 210L, Gastec, Ayase, Japan) following the manufacturer’s instructions.
Results and discussion
Water pollution status
DO and pH ranged from 4.5 to 7.2 and from 8.1 to 8.3 at site 1, respectively. Site 2-2 and site 3 in particular displayed more variation. DO and pH decreased during the night and increased during the day. These variations are likely in response to respiration and photosynthesis by photosynthetic microorganisms.
Surprisingly, negative Eh values were found at sites 2-2 and 3, whilst site 1 showed positive values during the entire observational period. Site 2-2 displayed quite a different trend to that of site 3. The minimum Eh value of −61 mV appeared at midnight at site 2-2, although the trend of variation in Eh was quite similar to those in DO and pH at site 3. From the results, there is a possibility that wastewater flows into the coastal area at site 2-2.
Sediment microbial community structure
Content of photosynthetic quinones, plastoquinone (PQ) and vitamin K1 (VK1), in coastal sediments at each site
At site 1, the most predominant quinone species was ubiquinone with eight isoprene units (Q-8), followed by menaquinone with six isoprene units (MK-6) and MK-8. The order of occurrence of the units at sites 2-1, 2-2, 2-3 and 2-4 was Q-8 > Q-9 or Q-10 or MK-7 > Q-9 or MK-7 or MK-8 and that at site 3 was Q-8 > Q-10 > MK-7. MK-7 has been detected as the second or third major quinone species at these sites near the populated area, indicating the presence of sulfate-reducing bacteria. This is also indicated by the occurrence of negative Eh values in the coastal waters at site 2-2 and the probable presence of organic matter and nutrients in the coastal areas at sites 2-1, 2-2, 2-3 and 2-4, and site 3.
Water pollution mechanism
Water pollution sources
Considering the land use/coverage on Fongafale Islet (Yamano et al. 2007), it is unlikely that non-point source pollution and/or industrial wastewater were the primary sources of pollution. Fongafale Islet has 639 households (Secretariat of the Pacific Community 2005). Although there is no centralized treatment system such as a wastewater treatment plant, 424 households have buried septic tanks that receive domestic wastewaters including human waste. Specifications require the septic tank to have two compartments: one for settling and one for anaerobic treatment. In addition, 163 households have pit toilets with a pour flush (Secretariat of the Pacific Community 2005; Lal et al. 2006). Thus, 92 % of households have access to improved sanitary facilities. However, studies have shown that septic tank systems (Borchardt et al. 2003; DeWalle and Schaff 1980; Scandura and Sobsey 1997; Viraghaven and Warnock 1976) and pit toilets (Dzwairo et al. 2006; Montgomery and Elimelech 2007; Pedley and Howard 1997) are a source of groundwater contamination. Thus, the disposal of human waste using these facilities is a key issue for groundwater quality and public health protection. The Public Works Department of the Tuvalu government was surveyed about the design and integrity of the septic tanks on the islet. Surprisingly, it was determined that the bottoms of the septic tanks were not sealed—so called ‘bottomless’. Construction specifications proposed by Australia require these tanks to be sealed; however, these tanks were constructed with a disregard for these specifications. Thus, considering also the fact that the Holocene sand aquifer with high permeability extends from the surface to the depth of ~ 20 m on Fongafale Islet (Ohde et al. 2002), the potential sources of pollution of the lagoon side coast are bottomless septic tanks and pit toilets.
Wastewater runoff mechanism
Nakada et al. (2012) reported ground water dynamics in the lagoonal coast using electrical resistivity. Saline water extended landward from the coastal area during flood tides, and brackish water receded coastward from the inland area during ebb tides. This indicates that if there are leaks from bottomless septic tanks and pit toilets, they subsequently flow into the coastal lagoon. The Eh value should then respond and fecal indicator bacteria, E. coli, would be detected, because the wastewater includes human waste.
Surficial sediments at sites 2-1, 2-2, 2-3 and 2-4 were grey sand with a hydrogen sulfide odour. AVS concentrations ranged from 0.024 to 0.133 mg/g. This corresponds to the sediment quinone analysis that detected MK-7, which occurs in sulfate-reducing bacteria. Digging in the sandy beach between the households and the coast revealed similar grey sand. However, no grey sand was found at the other sites and AVS concentrations were less than the detection limit (0.002 mg/g). Therefore, sulfate reduction occurs in sediments from sites 2-1, 2-2, 2-3 and 2-4. This further lends support to the hypothesis that domestic wastewater runoff migrates to the coast through the groundwater.
There is a strong possibility that the coastal water pollution in the lagoon due to poorly constructed sanitary facilities is connected to the decrease in sand supply as observed in other atolls (Ebrahim 2000; Fujita et al. 2009; Hirshfield et al. 1968), because the coastal environments are chronically damaged. In other words, the results from this study demonstrate an urgent need for the development and implementation of effective water quality control strategies. To consider such strategies, we should pay attention to both hard and soft infrastructures. The former in order to improve the purification capability of existing sanitary facilities for wastewater treatment. Improved sanitary facilities should be suitable for the geophysical and social surroundings specific to atolls. The latter in order to establish a policy for the water quality improvement and to develop local capacity building. We have reported the current status and mechanism of the lagoonal water pollution to the Tuvalu government. Government officials have deep concerns about the serious situation and their Tuvaluan counterparts are working on a proposal for a project based on our results to improve remediation of water pollution. Our scientific results are being utilized by working together. On the other hand, we have trained them in skills for water quality assays so they can get by on their own. We very much hope that our work finally connects with their policy decisions, and that this will become a good example of working practice because many atolls are facing a similar situation due to either installation of similar sanitary facilities or no treatment of wastewater.
Coastal water pollution of atolls due to human impacts has long been recognized (e.g., Johannes et al. 1979; Kimmerer and Walsh 1981). This paper has demonstrated water pollution mechanisms in lagoonal coasts for the first time by surveying near the densely populated area of Fongafale Islet on Funafuti Atoll, Tuvalu. Water pollution is a chronic problem, and domestic wastewater is cited as the primary pollution source. This occurs even though 92 % of households have access to improved sanitary facilities such as septic tanks and pit toilets. However, this study determined that these so called ‘improved sanitary facilities’ were not built as per the design specifications or they are not suitable for the geophysical characteristics. Although the septic tanks should be sealed at the bottom, many of the tanks within the study area were not sealed. Thus, during ebb tides, domestic wastewater leaking from bottomless septic tanks and pit toilets runs off into coastal waters. Tide changes control the pollution load of domestic wastewater.
The authors would like to thank Mr. Yoichi Ide (Oceanic Planning Corporation, Japan) for the AVS measurement and Dr. Murray Ford (The University of Auckland, New Zealand) for English language review and informative comments on the early version of this manuscript. This research was supported by JST/JICA SATREPS (0808918), Ibaraki University ICAS Research Project, JSPS KAKENHI (24560658), and JGC-S Scholarship Foundation Grant for Young Researchers.
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