Characterization of volcanic deposits along the slopes of Mount Meru, Northern Tanzania : insights into the potential sources and release of fluoride

A water quality problem exists in populated areas along the flanks of Mt. Meru in northern Tanzania, with excessively high fluoride (F-) concentrations exceeding the WHO drinking water standards (1.5 mg/L). Little is known about the potential sources of F- among the various rocks types forming the Meru aquifers. Nineteen samples (Debris avalanche deposits (DAD n=4), lava flows n=6, brecciated lava n=4, pumice n=2, scoria n=1, ash n=1, carbonitic n=1) representative of the materials covering the slopes of Mt. Meru were characterized for their mineralogical, chemical, and total and water-soluble F- compositions. Mt. Meru is mainly composed of alkaline volcanic rocks of basaltic to phonolitic composition. The total F- analysis indicated that F- occurs in all rock types with a mean value varying per rock type from 0.6 to 3.2 g/kg. The DAD in the east and northwest of Mt. Meru contained the highest amount of F- (mean 3.1±0.17 g/kg), whilst the lava flow samples had the lowest mean value (0.6±0.25 g/kg). Water rock-interaction experiments further revealed the highest release of F- in the analyzed DAD samples, possibly associated with their weathering status that progressively converted the primary minerals into secondary clay-bearing minerals assemblage, and favoring F release into the interacting water. Unlike DAD, pumice and ash have a moderately high level of total F- (1.76±0.04 g/kg) yet; release a minimal amount of it through leaching. Petrographic observations showed that the analyzed volcanic rocks consist of volcanic glass and rare F- -bearing accessory minerals (amphibole, titanite, biotite, and apatite), among others. Using electron microprobe analysis, the F- concentrations were found to be as high as 3- 6.5 g/kg in the glassy groundmass and up to 4 g/kg, 5 g/kg, and 45 g/kg in accessory phases of titanite, amphibole, and fluorapatite, respectively. Comparing the abundance and the composition of the glassy groundmass with the mineral phases, the former harbors most of the total F- content. The findings of leaching experiments are congruent with past water quality which show that, low F is found in water from lava and tephra-dominated areas at higher altitudes and Mt. Meru west, respectively. This new information could guide future explorations for safer locations to place wells for water consumption. It could also be of interest for other East African Rift sectors and similar volcanic settings