Spatial uncertainties in fluoride levels and health risks in endemic fluorotic regions of northern Tanzania

Spatial uncertainty caused by large-scale variation in fluoride (F⁻) occurrence remains a setback for water supply authorities in the F⁻ belts of the world. It is estimated that approximately 80 million people in the East African Rift Valley (EARV) regions and volcanic areas exhibit a wide variety of fluorosis symptoms due to drinking water with F‾ concentrations higher than 1.5 mg/L (WHO guideline limit). In this study, we combined geostatistical techniques, spatial statistical methods, and geographical information systems (GIS) to (i) map the probable places with F‾< 0.5 mg/L and F‾> 1.5, 4.0 and 10.0 mg/L using probability kriging (PK) method, (ii) estimate the probable total population at high or low F⁻ risk levels using univariate local Moran's I statistic, and (iii) map the spatial distribution of population at high and low F⁻ risk levels in Manyara, Arusha and Kilimanjaro regions using GIS. It was predicted that places along the major and minor EARV mountain ranges and around the flanks of major stratovolcanoes were dominated by groundwater sources with extremely low F‾(<<0.5 mg/L). In contrast, places within EARV graben were dominated by groundwater sources with F‾> 1.5 mg/L. About 1 million people (~20% of the total population) living around Mt. Kilimanjaro in Rombo, Moshi, and Mwanga districts are at high dental caries risk. Furthermore, it was estimated that about 2 million people (~41% of the total population) in Siha, Hai, Arusha City, Hanang’, Arusha, Simanjiro, and Meru districts are at high risk of dental, skeletal, and crippling fluorosis. Fluorosis, especially dental and crippling fluorosis, is an increasing disease burden at the community level due to prolonged consumption of F⁻ contaminated water within EARV graben. The major findings of the present study are very crucial for authority to minimize the uncertainty caused by high spatial variability in geogenic F⁻ occurrence.