The present paper provides evidence of the possible impact of shale-limestone-mudstone successions aquifers on groundwater chemistry by assessing the different hydrogeochemical processes. This was done by considering a sedimentary aquifer basin, namely the Hantebet sub basin (24.4 km2), Tekeze basin, northern Ethiopia. Groundwater is the main source of water supply in the sub basin extracted using hand dug wells, for domestic, irrigation and livestock uses. The sub basin is dominated by Paleozoic-Mesozoic sedimentary successions. Twenty groundwater samples were collected from hand dug wells using depth-integrated sampling techniques from both confined and unconfined aquifers. The major water bearing formations are gravely sand, weathered shale and weathered and fractured limestone, and intercalated weathered and fractured limestone and mudstone. The results indicate that groundwater is acidic to neutral, fresh, and hard to very hard. Ca2+, Na+, HCO-3 and SO2-4 are dominant ions compared to Mg2+, K+, and Cl- ions which show low to very low concentrations. Among eight hydrochemical facies identified, Ca-Na-HCO3 (40%), Ca-HCO3 (20%), Ca-Mg-Na-HCO3 (10%) and Ca-Na-HCO3-SO4 (10%) types dominate water chemistry. Dissolution of calcite and gypsum, and hydrolysis of feldspars, plagioclase, biotite and pyroxene are the major geochemical processes that control the chemistry of groundwater in the area. The intercalated shale beds are the source of sodium and chloride ions. Since, this study is based on groundwater from hand dug wells, the conclusions of this study should be further verified using groundwater from deep wells that are drilled in these successions.
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