Effect of hydrogeological structure on geogenic fluoride contamination of groundwater in granitic rock belt in Tanzania

Abstract

• Effect of geophysical structure on fluoride contamination have been researched. • Results suggest weathering promote fluoride leaching. • Intake from the fractured zone reveals fluoride depend on the fault direction. • Intake from the weathered zone increases fluoride with well yield potential. In the East African Rift Valley, geogenic fluoride contamination of groundwater is particularly noticeable as a drinking water problem. In the Tabora region, in the central part of Tanzania, which consists of two basins, Lake Tanganyika Basin (LTB) and an Internal Drainage Basin (IDB), within a semi-arid region, fluoride-contaminated groundwater that exceeds the World Health Organization standards is pumped from many wells in the fractured and weathered zones of the granitic layer underlying these basins. Fluoride contamination of groundwater is heterogeneously distributed in the granitic layer, posing an obstacle to groundwater development. In this study, a relationship between the fluoride concentration and the hydrogeological structure was investigated using geophysical survey data to achieve efficient groundwater development in geogenic contaminated areas. Based on the results, the following two mechanisms have been determined regarding fluoride contamination in the Tabora region. 1) With respect to the weathered zones in both the LTB and IDB, the greater the extent of weathering, the more readily fluoride leaches into groundwater. This suggests that fluoride leaching due to the chemical weathering of minerals containing fluoride increases with the weathering of rocks in the field. 2) With respect to the fractured zones in the LTB, a large well yield potential and a deeper water strike depth to the water vein result in lower fluoride concentrations. The suggested flow mechanism for the LTB is that water recharge in a groundwater depression area with a low fluoride concentration flow to substantial depth and over long distances through fault systems, which form preferential flow paths. This implies that the fractured zone along the fault system in the LTB is a high-priority reservoir for the development of safe water resources in the Tabora region. The hydrogeological mechanisms discussed in this study provide a basis for establishing estimation methods for the prediction of safe groundwater resources before a borehole is drilled. However, more reliable data needs to be collected to clarify the effects of hydrogeology on groundwater fluoride contamination for the fractured zone in the IDB.