Geochemical evolution and processes regulating groundwater chemistry in the upper Omo-Gibe River basin, Ethiopia: An integrated method of ionic ratios, inverse geochemical modeling, and chemometric analysis

Study area: Upper Omo-Gibe River Basin Study focus: The study aims to investigate the major geochemical processes and understand the sources of chemical constituents using various methods, including the conventional hydrochemical approaches, chemometric analysis, and inverse geochemical modeling. New hydrogeological insights: The results indicated that silicate weathering and ion exchange are the key factors controlling the groundwater chemistry. Scatter plots of Ca+Mg versus SO4+HCO3, total cations versus Na+K, and total cations against Ca+Mg confirm that silicate weathering is the dominant geochemical process in the area. The analysis of chloro-alkaline indices shows that normal ion exchange processes primarily influence the levels of major cations in the groundwater. Principal component analysis identified four major components, which are attributed to geogenic rock weathering and anthropogenic agricultural contamination. Hierarchical cluster analysis revealed three primary water groups: Cluster 1 includes Ca-HCO3 and Ca-Mg-HCO3 in recharge zones. Cluster 2, in transitional zones, consists of mixed water types such as Ca-Na-HCO3, Ca-Na-Mg-HCO3, and Na-Ca-HCO3. Cluster 3 contains Na-HCO3 water type in discharge zones associated with regional and deep flow systems. Inverse geochemical modeling indicated that the processes responsible for the geochemical evolution of groundwater along the flow path include the normal ion exchange processes, dissolution of primary silicate minerals, precipitation of secondary silicate minerals, including kaolinite, Ca-montmorillonite, along with calcite and quartz.