Volcanic crater lakes are unique important natural resources with the capacity to provide drinking water and several other ecosystem services to humans (Nankabirwa et al., 2019; Rubaihayo et al., 2008). However, this ability is threatened by the occurrence of elevated levels of mineral ions of geogenic and anthropogenic origins (Nankabirwa et al., 2019; Tumwebaze et al., 2019). For instance, over 37 % of the inhabitants in the Ndali-Kasenda area obtain their water for domestic use from crater lakes (Rubaihayo et al., 2008) whose origin of potential contaminants is ambiguous. Fluorine is the 13^th most abundant mineral in the earth crust and occurs in water resources in varying proportions depending on the plethora of fluoride-bearing minerals in the underlying rocks (Ali et al., 2019; Kashyap et al., 2020; Thakur et al., 2013). Fluoride in water sources emanates from anthropogenic and geogenic sources (Ali et al., 2019; Dongzagla et al., 2019; Kashyap et al., 2020; Thakur et al., 2013; Tiwari et al., 2020). The application of pesticides and fertilizers containing organic fluorine is the predominant anthropogenic source of fluoride in surface waters. However, groundwater in the study area is predominantly impacted by geogenic sources of soluble fluoride-containing minerals (Ijumulana et al., 2020; Kimambo et al., 2019; Smedley et al., 2002). More than 150 fluoride-rich minerals occur in the upper layers of the earth’s lithosphere, with the most abundant sources being clay and silicate minerals (Chandrajith et al., 2020; Kimambo et al., 2019; Thakur et al., 2013; Tiwari et al., 2020). In arid and semi-arid regions of the world, high rates of evaporation and low precipitation exacerbate fluoride enrichment (Chandrajith et al., 2020; Tiwari et al., 2020) in groundwater-fed surface water sources. Thus, the occurrence of fluoride at elevated levels in water for drinking is a serious problem in arid and semiarid regions of the world, affecting an estimated 200 million people worldwide (Kashyap et al., 2020; Tiwari et al., 2020; Wanke et al., 2017). Nevertheless, humans are mainly exposed to fluoride contamination by ingestion of water with elevated fluoride levels (Ali et al., 2018; Kimambo et al., 2019). Depending on its concentration, fluoride may have beneficial or harmful effects on human health. Exposure to low fluoride concentrations (0.5 – 1.5 mg/L) helps prevent tooth decay by inhibiting lactic acid production by bacteria on the tooth surface. However, ingestion of excess fluoride above 1.5 mg/L has detrimental health effects with varying mechanisms of toxicity. In the human gut, fluoride ions combine with protons from the gastrointestinal mucosa to form hydrofluoric acid, which is then absorbed leading to abdominal pains, diarrhea, nausea, gastrointestinal irritation, and vomiting (Ali et al., 2018; Edmunds & Smedley, 2005; Toolabi et al., 2021). Chronic ingestion of fluoride ladened water (1.5 mg/L – 4.0 mg/L) leads to the development of dental fluorosis, the permanent disruption of enamel production in children. Dental fluorosis is characterized by discolored, blackened, mottled, or chalky white/brown teeth(Edmunds & Smedley, 2005; Ijumulana et al., 2020; Toolabi et al., 2021). Ingestion of higher fluoride concentration (>4.0 mg/L) may result in skeletal fluorosis, which is a malformation of bone structure (Chandrajith et al., 2020; Onipe et al., 2020). Other reported health effects of excess fluoride ingestion include the retardation of physical and intellectual development in children, spontaneous abortion, neurological damage, kidney failure, and other forms of morbidities(Ali et al., 2019; Toolabi et al., 2021). At present, an estimated 80 million people are suffering from dental fluorosis and other fluoride-related morbidities in East Africa (Ijumulana et al., 2020; Kimambo et al., 2019). Due to the health hazards of excess fluoride in drinking water, delineation of its release mechanism and its genesis in various water resources has received considerable attention from researchers in recent years (Ali et al., 2016; Kashyap et al., 2020; Kimambo et al., 2019). On that note, many researchers have studied sullying of water sources with fluoride (Egor & Birungi, 2020; Emenike et al., 2018; Ijumulana et al., 2020; Kashyap et al., 2020; Kimambo et al., 2019; Thakur et al., 2013; Tiwari et al., 2020). For example, Singh et al. (2013) used GIS and geochemical model WATEQ4F to map fluoride contamination in Allahabad District in India. Similarly, geochemical modeling of fluoride contamination was done to decipher the fluoride release mechanism in the hard rocks of Madhya Pradesh in India by Thakur et al. (2013). Their study employed saturation indices computation and found spatial fluoride variation ranging from 0.6 mg/L – 4.74 mg/L.

More recently, Tiwari et al. (2020) evaluated fluoride contamination in groundwater in a semi-arid region of India with fluoride concentration in the range 0.48 mg/L – 3.64 mg/L while Egor & Birungi (2020) found fluoride in the range 0.2 mg/L- 3.0 mg/L in the Sukulu hills in Eastern Uganda with an upper limit calculated using a modified Galagan equation found to be 0.4 mg/L. Relatedly, studies involving multivariate statistics and non-carcinogenic human health risks assessment using the US EPA (2011) through ingestion and dermal contact pathways were employed in South Western Nigeria (Emenike et al., 2018), Tanzania (Ijumulana et al., 2020), Sri Lanka (Chandrajith et al., 2020), Bangladesh (Bodrud-Doza et al., 2020), Iran (Toolabi et al., 2021), India (Mohanta et al., 2020), and North China (Chen et al., 2017). Their results showed the greater vulnerability of infants and children to high fluoride levels in the studied water resources. Although some researchers have done work on the crater lakes, hydrochemical and fluoride contamination in them is still at a nascent stage in the literature. Such a multidisciplinary approach for understanding the occurrence, spatial distribution of fluoride, and its associated health risks have not yet been carried out in the Ndali- Kasenda crater lakes located in the Albertine Graben which provide water for domestic use for over 37% of the inhabitants located in the area. This study reports for the first time a multidisciplinary approach employing hydrogeochemical, multivariate statistics, and Human Health Risk Assessment (HHRA) model to interpret information on spatial variation of fluoride and its associated non-carcinogenic health risks via ingestion. Hence the main objectives of this present study were to:

1. i) determine hydrochemical characteristics of Ndali-Kasenda crater lakes serving as domestic water sources
2. ii) apply GIS, correlation, and multivariate statistical techniques to map spatial fluoride distribution and evaluate sources responsible for high fluoride in the crater lakes of Ndali- Kasenda cluster
3. iii) assess non-carcinogenic health risks due to exposure to fluoride within the crater lake system, using the HHRA model via ingestion pathway.

This study is envisaged to provide hydrogeochemical information for the development of efficient and effective management strategies and remedial measures for the provision of safe drinking water in rural communities in the vicinity of the Ndali- Kasenda area.