Fluoride: Geochemical and ecological significance in East African waters and sediments

Abstract

The geochemistry of fluoride in East African lakes and rivers was examined to elucidate processes of fluoride acquisition, concentration, removal, and diagenesis in inland waters in a region relatively little influenced by man. The range of fluoride concentrations (0.02- 1,627 mg liter-l) is the greatest found anywhere. A strong correlation was found between the concentration of fluoride in the predominant crystalline rocks of each particular drain- age basin and the F:CI ratio in surface waters. Because fluoride-rich volcanic rocks are common in East Africa very high fluoride concentrations, the products of chemical weathering, are often observed. Fluoride removal from surface waters is not significant; concentration by evaporation proceeds until saturation with respect to villiaumite is reached. Fluoride : chloride ratios for the interstitial waters of a 7-m core from Small Momela Lake, Tanzania, indicate that in sedimentary environments fluoride may be removed from pore waters as fluorite and possibly fluorapatite. Geographical evidence suggests that the high fluoride concentrations may influence the distribution of man, livestock, zooplankton, phytoplankton, and higher aquatic plants.