Geochemical evaluation of volcanic rocks and soils around Meru volcanic complex, northern Tanzania: Implications for fluorine source, mobility and contamination of groundwater systems

Abstract

Much of the water sources along the East Africa Rift System (EARS) are characterized by high fluoride. Geochemical evaluation of volcanic materials was undertaken to assess the possible source of high fluoride concentration, its mobility and eventual contamination of groundwater systems. In addition to rocks, this assessment involved evaluation of soils and water samples to elucidate the fluoride concentration in a rock-soil-water system. Results indicated that volcanic rocks are weakly to moderately fractionated with variable compositions that span from foidite to andesite. Petrographic examination indicated the presence of at least four minerals that are known to incorporate fluorine (F) in their lattice structures. In order of decreasing abundance, these minerals include: sphene or titanite, hornblende, apatite and biotite. These results indicated that fluorine occurs in all rock types in varying amount that ranges from 251 to 19,122 mg/kg (mean = 3921 mg/kg, n = 68) and that andesitic rocks contain the highest amount than the other rock types (mean = 7133 mg/kg, n = 13). This may suggest that F is largely incorporated during late crystallization process. Based on rocks and equivalent soils, fluorine was higher in rocks than equivalent soils at the ratio of 3:1. In general, distribution of F is largely controlled by parent material, the texture of the parent material and geomorphology. Furthermore, it is revealed that the amount of fluoride in well waters correlated well with fluorine in the rocks and/or soils proximal to the wells.