SPATIAL MODELLING OF FLUORIDE CONTAMINATION IN GROUNDWATER SYSTEMS IN THE EAST AFRICAN RIFT VALLEY REGIONS OF NORTHERN TANZANIA

Geogenic occurrence of high concentrations of potentially toxic elements such as fluoride, arsenic and manganese in groundwater systems is one of the water quality problems impeding sustainable and safe water supply in many countries. Although fluoride release into groundwater systems is attributed to weathering and dissolution of fluoride bearing minerals such as titanite, amphibole, hornblende, and biotite., the variation in concentrations is controlled by many environmental factors. In this study, we investigated the influence of topography (landforms), soil types, exchange of groundwater between lithological units during lateral movements, recharge from surface water, tectonic processes, volcanic processes, and recharge from precipitation using spatial regression modelling Three models were developed including classical ordinary least squares regression, spatial lag, and spatial error models. Based on the calculated Akaike Info Criterion for each model, the classical ordinary least squares regression showed the largest value of 322 compared to 262 and 260 for spatial lag and spatial error model, respectively. Thus, the former model was used to study causal effects to the variation in fluoride concentrations in groundwater systems. A significant increase in fluoride concentrations with distance from Pleistocene pyroclastics and Miocene volcanic lavas (nephelinite, phonolite, alkali basalts) boundary was observed. For soil types, a significant decrease in fluoride concentrations with an increase in distance from Chernozems and Chromic Luvisols boundary was observed. Furthermore, a significant decrease in fluoride concentrations with an increase in distance from volcanic fault was revealed. Though not significant, fluoride concentrations increased with increasing distance from river channel. Based on these results, we hypothesize that variation of fluoride concentrations in groundwater in the East African Rift Valley regionsis aggravated by soil types, and natural dilution through groundwater exchange between adjacent hydrogeological layers. The occurrence of low fluoride concentrations near geological boundaries, provide evidence of potential sites for targeting safe sources for drinking water.