A geochemical model for the abnormal fluoride concentrations in waters in parts of northern Tanzania

Abstract

Natural waters in parts of northern Tanzania are characterized by exceptionally high fluoride contents in some rivers (12-26 mg/l), springs (15-63 mg/l) and alkaline ponds and lakes (60-690 mg/l). In lowland rivers most of the fluorides are derived from weathering of F-rich nephelinitic and carbonatitic rocks and soil whereas, in the rivers draining Mt. Meru crater, gaseous emanations through mineral springs may also contribute to the concentrations. The flushing of F-rich sodium bicarbonate ("magadi") from the surface and top soil at the onset of the rainy season affects the F concentrations in shallow ground water and in surface water bodies. The steady flux of fluorides in ground water and overland flow and interflow containing dissolved "magadi" into internal alkaline ponds and lakes increases F (as well as CI) contents as a result of continuous evaporative enrichment over a long period. Very low Ca and Mg contents render the abnormally high F contents possible. The excessive F concentrations in drinking water cause endemic dental and skeletal fluorosis in people and in animals. The fluorous "magadi" used as an additive in food preparation is also considered to be a source of deleterious amounts of fluorides. The geomedical aspects of fluorine are discussed in the light of findings in the literature.