Major and Trace Elements Geochemistry of Natural Waters at Mt. Meru Volcano, Tanzania

Abstract

The study area of the present study is on the eastern side of Mt. Meru volcano and lies at an elevation of 1230 to 4565 meters above sea level. It is in the volcanic province of northern Tanzania, in the Gregory Rift of the East Africa Rift System. In the study area, processes controlling the composition of major and trace elements, and mobility of various elements in thermal and non-thermal waters are not well understood. This study was intended to investigate the geochemistry of major and trace elements in the ground-, surface- and lake waters at Mt. Meru volcano. It focuses on the origin of the waters and processes affecting water composition, mobility of major and trace elements, and characterization of end-member water sources. The study involved sampling and analysis of spring-, stream-, river-, lake-, and groundwaters. PHREEQC was used for the calculation of aqueous speciation and mineral saturation indices. Results suggest that the studied waters are of meteoric origin as indicated by δ2H-H2O and δ18O-H2O, with Na-HCO3 composition. The chemistry of the studied waters is mainly controlled by the progressive dissolution of rocks at relatively low temperatures as indicated by the correlation between major constituents (Na, K, Mg, SO4, and Cl). Two possible end-member water compositions, namely non-reacted rainwater and reacted water have been identified in this study. The composition of most of the studied waters fall between these two end members. The saline lakes are formed by evaporation of waters sourced in the higher altitudes of Mt. Meru volcano, and the high concentration of dissolved constituents is attributed to evaporative concentration. The composition of studied waters is further attributed to dissolution of carbonates, mixing with biogenic and atmospheric CO2, and input of deep volatiles as suggested based on δ13C-CO2. The element mobility relative to Na is generally low in the studied waters, suggesting that many elements are incorporated into weathering and hydrothermal alteration minerals. Geothermometry based on multiple mineral equilibria suggests a possible low-temperature geothermal system on the eastern flank of Mt. Meru with a subsurface temperature of less than 100 °C possibly resulting from the shallow circulation of meteoric water.