Hydrchemical evaluation of groundwater in Khartoum state, Sudan 5. Results and discussion The pH of groundwater in upper and lower aquifers, varies within small ranges (Table 1), elaborates a fresh and a slight trend of alkaline chemical reaction within the groundwater environment [6]. The electric conductivity (EC) varies from 281 to 4070 mS/cm in the eastern area (East of River and Blue Nile), from 263 to 3180 µS/cm in the Central area (Between Blue and White Nile) and from 116 to 4690 µS/cm in Omdurman area (West of River and White Nile) see Table 1, & Figure 1. These measurements indicate that there are probably fresh water (<500 µS/cm), marginal water (500-1500 µS/cm) and brackish water types (1500-5000 µS/cm) in the area. Alkalinity varies within considerable ranges in the eastern, central and Omdurman area elaborates alkaline water type (Table 1). This indicates the existence of calcretes (CaCO3) and silicate weathering processes in the upper Gezira and Omdurman formation. Hardness concentrations, with respect to [7] classifications, indicate that Omdurman area Characterized by moderately soft (50-100 mg/l) to hard (200-300 mg/l), whereas, the eastern and central areas range from moderately soft to very hard (>300 mg/l) see table 1. Mean total dissolved solids (TDS) at eastern, central and Omdurman area are 625, 491 and 751 mg/l respectively (Table 1). The hydrochemical map shows high salinity zones in the eastern area (Hattab-Idd Kalamon zone), central area (Waghara-Isealy zone) and Omdurman area (AlGulei-Tureis zone) see Figure 2. A tendency of decreasing concentration with respect to aquifer depth (Figure 3) marks a brackish water type (TDS >1000 mg/l) in the upper aquifer and moderately fresh water type (TDS < 1000 mg/l)) in the lower aquifer (Table 1). These salinity ambiguities are sanctioned to the dissolution of silicate minerals, evaporite deposits, low hydraulic gradient (< 0.001), increasing distance from the recharge sources and high evaporation and evapotranspiration rates in this semi-arid area.  Figure 2. Areal distribution of TDS  Figure 3. TDS and sampling depth distribution Sodium is comparatively low and varies within small ranges (Table 1). This is ascribed to adsorption on mineral surfaces of high cation exchange capacities such as clays. Hence, relatively high concentration appears as irregular troughs generally trending northwest-southeast concordant with the regional flow direction (Figure 4). Significant increasing of sodium along the regional flow direction, suggests the silicate weathering and/or dissolution of soil salts (Na2SO4) that characterize the upper part of Gezira and Omdurman formations.  Figure 4. Areal distribution of sodium |