Abstract.A multidisciplinary research effort, including geological, hydrogeological, hydro-chemical, geophysical and hydrological investigations, was aimed at locating a source of safe groundwater for a district of northern Tanzania, within the western branch of the East Africa Rift Valley, where water shortage is common and much of the surface water carries unacceptable levels of dissolved fluoride. The 440 km 2 study area lies in the northern part of Arumeru district and is dominated by Mt. Meru (4565 m a.s.l.). The local climate is semi-arid, with distinct wet and dry seasons. Four hydrogeological complexes were identified, occurring within different volcanic formations, either alone or superimposed upon one another. The groundwater flow system was interpreted from the spatial distribution of the springs, combined with a lithology-and geometry-based reconstruction of the aquifers. The dominant pattern consists of a multi-directional flow from the higher elevations in the south towards the lower areas in the north, but this is complicated by structures such as grabens, faults, lava domes and tholoids. After the identification of the major fluoride source, an interference pattern between groundwater and high fluoride surface water was drawn. Finally, vertical electrical soundings were performed to define the location of aquifers in regions where release of fluoride was prevented. The methodological approach for the prospecting of safe water in a semi-arid, fluoride polluted region was validated by the drilling of a 60 m deep well capable of supplying at least 3.8 l/s of low fluoride, drinkable water.

Abstract. The objective of this study is to analyze the geochemical conditions associated with the presence of fluoride (F − ) in the groundwater of an area of Northern Tanzania. The studied aquifers are composed of volcanic rocks such as phonolitic and nephelinitic lavas, basalts, lahars of various ages and mantling ash. Sedimentary rocks consisting of fine-grained alluvial and lacustrine deposits occur as well. Samples collected from springs, borehole and surface water, during two monitoring surveys, were analyzed for the various physico-chemical and isotopic parameters. The geochemical composition of water is typically sodium bicarbonate. High values of F − (up to 68 mg l −1 ) were recorded. The highest values of fluoride agreed with the highest values of pH, sodium and bicarbonate. Dissolution of major ions, exchange processes and precipitation of Ca 2+ from supersaturated solutions joined with the local permeability and hydraulic gradients, control the fluoride mobilization and the contamination of the area.

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