Abstract

The hydrochemical evolution and flow pattern of groundwater is analyzed in a complex closed rift basin on the basis of multivariate hierarchical statistical cluster analysis. The analysis was made on the basis of water samples collected from over 120 wells and limited surface waters. The result revealed dissimilar hydrochemical phases in the rift and highlands representing different groundwater flow regimes. Five groundwater zones with different hydrochemical facies were identified. Both the conventional hydrochemical and multivariate statistical analyses indicate that the central sector of the rift represents a complex groundwater system influenced by thermal waters and alkaline lakes. It also demonstrates that the close similarity of lakes and groundwater with high fluoride, sodium, Total Dissolved Solids (TDS) and pH. The waters associated with acidic volcanic rocks are characterized by a positive alkalinity residual of calcite. When they concentrate due to the effect of climate, the precipitation of calcite causes a decrease in the chemical activity of calcium and increase in alkalinity. These waters become NaHCO₃-dominated with distinctly different from highland systems characterized by Ca-MgHCO₃ type waters with low TDS. Alkaline waters are intimately linked to acidic volcanics while fresh waters are associated with dominantly basic volcanics. Waters with similar hydrochemical facies as highland waters are evident in local fractures of the rift floor. The zonation of hydrochemical facies on the basis of hierarchical cluster analysis indicates increase in ionic concentration and pH along the groundwater flow path.

Key words: Ethiopian rift, groundwater evolution, hierarchical cluster analysis, hydrochemistry, volcanic rocks.