The Gedaref basin in eastern Sudan is believed to be a great tectonic depression filled mainly with continental deposits and Tertiary basalt lava flows. The main aquifers are the Gedaref sandstone aquifer system and the basalts. 70 wells drilled in the Gedaref basin were sampled and analysed for the major cations, major anions, total dissolved solids, alkalinity and hardness. The data were treated with simple statistics, represented with Piper trilinear plots and were the subject of cluster analysis. The interpretations of the applied methods confirm each other and the groundwater was classified into two groups. The water in the sandstone aquifer was dominated by NaHCO3while the water in the basalts was partially dominated by alkaline earth.