Hydrogeology and hydrogeochemistry of an alkaline volcanic area: the NE Mt. Meru slope (East African Rift – Northern Tanzania)

摘要

The objective of this study is to analyze the geochemical conditionsassociated with the presence of fluoride (F−) in the groundwater of an areaof Northern Tanzania. The studied aquifers are composed of volcanic rockssuch as phonolitic and nephelinitic lavas, basalts, lahars of various agesand mantling ash. Sedimentary rocks consisting of fine-grained alluvial andlacustrine deposits occur as well. Samples collected from springs, boreholeand surface water, during two monitoring surveys, were analyzed for thevarious physico-chemical and isotopic parameters. The geochemicalcomposition 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 ofpH, sodium and bicarbonate. Dissolution of major ions, exchange processesand precipitation of Ca2+ from super-saturated solutions joined withthe local permeability and hydraulic gradients, control the fluoridemobilization and the contamination of the area.