Variability of naturally occurring fluoride in diverse community drinking-water sources, Tanna Island, Vanuatu

Large variations in fluoride concentrations exist in natural waters, many of which are the source of community drinking-water supplies. Determining fluoride concentrations in community drinking waters can be challenging in developing Pacific countries such as Vanuatu that have limited laboratory capacity. Knowledge of naturally elevated fluoride concentrations that cause irreversible, adverse health outcomes may allow communities the opportunity to treat and manage their drinking-water supplies. Community drinking-water samples (n1⁄4 69), sourced from groundwaters, roof catchment rainwaters, surface waters and springs, were sampled on Tanna Island, Vanuatu between 2017 and 2020. In an 18 km area of Western Tanna, a set of 30 groundwater-based drinking-water samples had a median fluoride concentration of 3.3 mg/L, with 20 samples >1.5 mg/L and seven samples >4.0 mg/L. These concentrations increase the risk of dental and skeletal fluorosis, respectively. Repeat resampling at five sites showed little variation over the sampling period. Rainwater-fed drinking-water supplies were lower overall and highly variable in fluoride concentrations (<0.05–4.0 mg/L, median of 0.53 mg/L), with variable inputs from volcanic emissions from Yasur volcano. We recommend a comprehensive oral health and bone health study for the whole island to determine adverse health effects of excess fluoride in this vulnerable population.


INTRODUCTION
The Pacific Small Island Developing States (PSIDS) are considered some of the most vulnerable nations on Earth. Pacific Islands often have challenging physical characteristics, undeveloped infrastructures and are exposed to increasing highimpact weather events (Barnett & Campbell ). They are listed amongst the nations with the highest disaster risk (United Nations University ) due to their exposure to hazards and lack of adaptive and coping capacity.
Water, sanitation and hygiene (WASH) frameworks initiated by the United Nations RES/18/1 (Human Rights Council ) allow PSIDS to move towards achieving 2030 Sustainable Development Goal 6 (SDG6) of safe, affordable, acceptable, available and accessible drinking water and sanitation for all (WHO ). Working towards this, it is necessary to consider all determinants of public health significance associated with drinking-water. The highest priority determinants are pathogenic microorganisms. However, chemical contaminants in drinking water, such as fluoride and arsenic, known to cause adverse health effects in global populations at concentrations in excess of drinking-water guideline values, also need to be considered when monitoring drinking-water supplies (Edmunds & Smedley ; Foster & Willetts ).
The primary source of human fluoride exposure, absorbed via the gastrointestinal tract, is considered to be drinking-water consumption (Fejerskov et al. ). Some fluoride intake is considered beneficial for dental health; however, this is dependent upon daily dose, age, diet and climate. Fluoride, when made bioavailable, easily bonds with calcium-rich structures of the human body such as teeth and bone (Fejerskov et al. ). The continuous ingestion of excess fluoride in drinking water from birth to age 8 years causes irreversible dental fluorosis in children with severe forms of dental fluorosis generating porous tooth enamel, which is more vulnerable to tooth decay (Fejerskov et al. ). concentrations in groundwater tend to be stable over time (Amini et al. ), meaning that residents could be exposed to sustained high concentrations. In contrast, fluoride concentrations in rainwater supplies may be highly variable, as they are dependent on the strength of volcanic activity and the direction and dispersion of gas and ash emissions (Stewart et al. ).
The primary aim of this study was to identify influences on fluoride concentrations in community drinking-water  Table S1). Samples were taken from sites most used by community members such as schools, communal village sites and sites outside of villages known to be communal collection points. Most households in a village shared the same collection point with reticulation in most villages non-existent. At each location, advice was sought from local community leaders about the most appropriate supplies to sample. Repeat sampling was possible only at some sites and was opportunistic when weather and road conditions were favourable. It was carried out at sites 6, 7, 9, 11, 12, 19 and 31. Samples were taken from groundwater sources at private and public bores (n ¼ 30), which included primary and secondary schools, the main town centre of Lenakel, the regional hospital and the recently built reticulated town supply bore at Isangel. Springwater samples (n ¼ 21) were taken across the whole of Tanna and included three beach springs, indicated as an alternative drinkingwater source during drought. Rainwaters collected off sheet metal roofs, at homes and schools, were sampled (n ¼ 13) when available, along with river water sites (n ¼ 5).
Two samples were taken from a chamber inside the masihur or masiwer tree, endemic to South West Tanna and an earlier method of water collection by villagers. Three commercial bottled waters were also tested for their fluoride content, along with a town supply tap water sample collected in Port Vila, all sourced from Efate's aquifer. Bottled water is used widely throughout Vanuatu and is often provided as part of emergency responses.
Water samples were collected in clean, pre-rinsed plastic bottles and stored in a fridge.
As no commercial internationally accredited laboratory facilities were available in Vanuatu, the samples were transported back to New Zealand. The New Zealand and Vanuatu government do not utilise sample transfer agreements. As this study did not involve human or animal participants, ethical approval was not required. This project was done in partnership with, and the support of, the Vanuatu DOWR.  Twenty-three drinking-water samples, across Tanna, exceeded Vanuatu's upper limit of 1.5 mg/L F (Table 1).

RESULTS AND DISCUSSION
Twenty-six of the 30 groundwater samples were located in West Tanna (Supplementary Material, Table S1), an area that includes the main business area of Lenakel, markets of Blakman Town, the regional hospital, multiple primary schools, kindergartens and a large area high school.   To date, there have been two studies performed on children to determine dental fluorosis for Tannese residents. Superscripts a and b denote statistical difference between these sets of data.