Identification of hydrogeochemical processes in groundwater of Dawa River basin, southern Ethiopia

Dawa River basin in southern Ethiopia is covered by volcanic, basement, and sedimentary rocks. Locating good quality groundwater is a challenge in most parts of the basin. Statistical analysis and graphical plots of 94 hydrochemical data of groundwater were used as a main tool to acquire an insight into the major processes that control groundwater chemistry. In the volcanic terrain groundwater is dilute (mean total dissolved solids (TDS): 152 mg/l), while salinity is the highest in the sedimentary terrain (mean TDS: 1750 mg/l). NO3- varies from below the detection limit to 433 mg/l NO3-. In 26% of the water samples, nitrate concentration is above the human-affected value, 5 mg/l NO3-. In 6 % of the samples, NO3- concentration is above the limit recommended in drinking water, 50 mg/l NO3-, by WHO. Concentration range of the other major ions is also high and hydrochemical water types are diverse, suggesting the effect of various hydrogeochemical processes on the water chemistry. Chemical data analysis revealed that in the volcanic and most parts of the basement terrains silicate hydrolysis is the dominant process. Gypsum dissolution is the main process in the sedimentary terrain. Dissolution of gypsum is also important at few locations along dry riverbeds in the semiarid area where the effect of evaporation on the water chemistry is considerable. Loading of factors with K+ and SO42-, K+ and NO3-, and NO3- and correlation of SO42- with Cl-, along with the observed high nitrate concentration, indicate the effect of surface contamination sources on the water quality.