Parkinson, David John; (2000) Sources and controls on fluoride in groundwater of the Arusha Region, northern Tanzania. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Excessive fluoride in groundwater is a hazard in the Arusha Region of northern Tanzania. The WHO 1.5 mgl-1 guideline for fluoride is widely exceeded, concentrations are commonly above 5 mgl-1 and in places greater than 20 mgl-1. Yet the spatial variability of fluoride concentrations in groundwater is poorly understood. Relationships between fluoride concentration in groundwater and geology, hydrochemistry, hydrogeology and groundwater abstraction are not adequately known to support proposals for amelioration of the problem. Results of a study to define the spatial variability of fluoride concentration in groundwater, and resolve the factors controlling its occurrence within the Arusha Region of North Tanzania, are presented. Field relationships have been determined by sampling and analysis of water and rocks from four geologically distinct areas. This has lead to a hypothesis combining the mechanisms of fluoride release and controls on fluoride concentrations in groundwater. Hydrochemical evolution in the Arusha Region is dominated by the weathering of silicate minerals. The larger-scale spatial variability of groundwater fluoride concentrations across the study area can be related to the geological provenance of the water, with excessively high fluoride concentrations restricted to the Arusha Town volcanic area, except an isolated occurrence in the sedimentary area. Groundwater chemistry indicates fluorapatite as the likely fluoride source and fluorite solubility as the control on the fluoride concentration. The particularly low calcium concentrations in these predominantly Na+-HCO3 type waters allow the high fluoride concentrations to be attained. Petrography and electron probe microanalysis identified fluorine in a number of minerals, predominantly fluorapatite. Yet apatites account for only 32% of the total whole-rock F' content. The hypothesis of fluoride release from fluorapatite has been investigated by laboratory experimental work on the dissolution and exchange reactions of apatites. The experiments have been carried out in a variety of aqueous conditions, using both synthetic and natural apatites, in an attempt to specify the mechanism of fluoride release and to simulate the groundwater conditions in the field. Experiments show that F' released by dissolution is augmented by exchange with hydroxyl ions, and this is enhanced under the HCO3- -rich conditions of northern Tanzania groundwaters. Fluorite solubility is itself greater in the HCO3-rich environment. These results explain the large-scale geological control on the spatial variability of fluoride in groundwater of northern Tanzania. Hypotheses concerning hydrogeological control of the smaller-scale variations require further field measurements.

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