The hydrochemical evolution and water balance of the emakat lake in the northern crater highland of Tanzania

Abstract

This study aimed to ascertain the hydrochemical evolution and water balance of Emakat Lake, of the Empakaai Crater. Water and rock samples were collected from the lake and springs on the inner and outer crater rims, and at the foot of the Empakaai Crater. The results showed that the lake is a highly alkaline (pH > 10) and saline (electrical conductivity (EC) = 28,860 - 29,460 μs/cm) with the concentration of total dissolved solids (TDS) ranging from14,432 to 14723 mg/L. Springs exhibited lower pH (6.85 - 8.69), EC (562 - 1584 μs/cm) and TDS (276 - 1016 mg/L). The dominant ions in Emakat Lake were Na+ and CO3 2-+HCO3 - which occupy about 80% and 85% of the cation and anion phases with ion distribution of Na+ > K+ > Ca2+ > Mg2+ and (CO3 2- + HCO3 - ) > Cl- >SO4

2- > F- > NO3- > PO4 3- . Piper, chloro-alkaline indices, Chadha, and Gibbs plots revealed that Na-K-HCO3 water type dominated Emakat Lake, and a majority of springs exhibited mixing characteristic water type. Base ion-exchange dominated the hydrochemical evolution of both lake and springs, influenced by evaporation and water-rock interaction for the lake and springs respectively. The water balance of Emakat Lake was highly influenced by groundwater flow which accounted for 49% of the inflow and 56% of the outflow. This suggest that Emakat Lake plays a major role in the hydrological system in the area alongside the springs which are the sources of the major rivers of Engaruka and Engaresero.