Assessment of Major Sources Controlling Groundwater Chemistry in Kombolcha Plain, Eastern Amhara Region, Ethiopia

The study area, Kombolcha town, forms an important industrial town situated in the Eastern Amhara region, Ethiopia. The geology of the area is mainly composed of basalts, rhyolitic ignimbrites, and Quaternary sediments. Hydrogeochemistry and the source of ions in the groundwater of the study area are poorly understood. Therefore, the current study aims to assess the factors and the different hydrochemical processes significantly controlling groundwater quality, source, and chemistry. For this purpose, a total of eighteen groundwater samples were collected using 250 ml sampling bottles at selected points in the dry season (May 2017) and wet season (November 2017). Gibbs diagram, correlation analysis, scatter plots of ionic molar ratio relations, saturation index values (estimated using PHREEQC Interactive 2.8) were used to decipher the hydrogeochemical process. Gibbs diagram shows that the rock-water interaction process is the predominant, Na+/Cl- and Ca2+/Mg2+ molar ratio value of all groundwater samples in both seasons reveals that the groundwater chemistry of the area is controlled by silicate minerals weathering. The strong correlation of Ca2+ with Mg2+ in the dry season, and Ca2+ with HCO3- and Na+ with HCO3- in the wet season could also be an indication of silicate weathering and ion exchange processes. The impact of anthropogenic practices on groundwater chemistry is also seen from the strong correlation of Ca2+ with Cl-, NO3-, PO43- and F-, NO2- with K+, Mg2+, and PO43- , PO43- with F- , and NO3- with Na+, Cl-, HCO3- . The negative values of chloro-alkaline indices in both seasons indicate base-exchange reaction where an indirect exchange of Ca2+ and Mg2+ of the water with Na+ and K+ of the host rock occurs. Saturation indices results for the wet season show that the groundwater is under-saturated with respect to calcite, aragonite, dolomite, gypsum, and anhydrite. In the dry season, however, some of the waters are oversaturated with respect to calcite and aragonite. To sum up, the groundwater quality of the study area is controlled by geological processes and anthropogenic effects.