Hydro-geochemistry and Related Processes Controlling the Composition of Thermal Waters in the Lake Natron Basin, Northern Tanzania

Abstract

Lake Natron Basin (LNB) forms part of the eastern branch of the East African Rift System (EARS) in Tanzania. The basin is endowed with thermal springs flowing towards the lake from the western, northwestern, southwestern and eastern parts. The western, northwestern and south-western thermal springs emanate from fractures in the basaltic rocks while in the east, they originate from the western flank of the Gelai volcano. There are limited studies on these springs, thus their geochemical differences and distributions of physicochemical parameters are not well understood. A hydrogeochemical study was conducted to interpret available geochemical data from the LNB springs, including cold and thermal water to allow their geochemical characterization. Results have shown that LNB water is dominated by sodium (Na+), chloride (Cl-) and bicarbonate (HCO3-) ions. Three hydro-chemical facies have been identified forming Na-Cl, Na-HCO3 and Ca-HCO3 water types. This study has also revealed that thermal water in the north-western part of the basin is highly mineralised and gradually becomes diluted toward south-west and south due to groundwater incursion. Water-CO2-rock interaction affects the overall chemistry of thermal water leading to HCO3- water, particularly in the east of the basin. While few springs from the western side of the basin indicated maturity of the thermal waters, other springs indicated mixed Cl-HCO3/peripheral water; while the cold waters are HCO3 type. It is recommended to take precautions when planning geothermal projects to avoid corrosion and scaling of production facilities. Similarly, mixing or dilution of thermal water with shallow groundwater can affect the temperature and the composition of Cl-.