Geological controls on brine discharge in Itumbula Salt Dam within the Rukwa Rift in Momba District, Tanzania

Abstract

The Itumbula salt dam of the Rukwa Rift Basin is a depression formed through extraction of spring-derived salt crystals. Brine yield by springs which is the primary cause of significant amounts of salt in the dam required further geological investigations to understand yield controls. In this study, detailed field geological investigations in the salt dam and its surroundings were conducted to ascertain brine discharge controls. These included documentation of lithology and surface manifestations of brine deposition. Geophysical methods (i.e. magnetic and electric surveys) for studying geologic structures associated with brine deposits, and laboratory analysis of cations and anions (e.g. chlorides, bicarbonates or sulphates) essential to characterize composition of waters were also performed. The information on the springs discharge rate was retrieved from the previous studies. The magnetic profile revealed a very low magnetic anomaly across the salt dam, trending NW to SE direction, which is interpreted to be the main structure that controls fluid movements in the dam. Electric resistivity survey results delineated a low resistivity body in the central part of the dam interpreted as porous formation with saline water. Hydro-chemistry of the hot spring brines indicated high levels of sodium and chloride ions contents than magnesium, calcium, potassium, sulphate, and carbonate and bicarbonate ions, interpreted to be mature water with minimal water mixing. The structurally controlled brines of approximately 2.5 kg/s are discharged in the study area.

Keywords: Geologic Structures, Brine, Salt Production, Momba, Rukwa Basin.