Hydrochemical characterization of the main aquifers in Khartoum, the capital city of Sudan

The present study assesses groundwater resources of the densely populated area of the Khartoum State, Sudan by integrating conventional hydrochemistry and statistical analysis. Groundwater is exploited from two aquifers: Gezira and Nubian. The Gezira aquifer, which occupies the upper part, is separated by impermeable unit from the lower Nubian aquifer. Both aquifers are part of thick sedimentary accumulations formed under fluviolacustrine environment during Tertiary and Cretaceous, respectively. The groundwater flows in the two aquifers from the White and Blue Niles (both rivers bound the area from west and east, respectively) into the center of the study area suggesting recharge from the rivers. This river-induced recharge is demonstrated by the hydrochemical evolution of the groundwater along the flow direction. Groundwater evolves in the Nubian aquifer from Ca²⁺–Mg²⁺–HCO₃ ⁻ type in the vicinity of the Niles to Na⁺–K⁺–Cl⁻–SO₄ ²⁻ type in the central part of the area indicating precipitation of carbonates and dissolution of clay minerals and evaporites reported in the well logs. A third type of groundwater Na⁺–K⁺–HCO₃ ⁻ is formed in the mid zones between the Niles and the central part showing the ionic exchange and minerals dissolution/precipitation with the flow direction. In the Gezira aquifer, the water only changes from Ca²⁺–Mg²⁺–HCO₃ ⁻ type to Na⁺–K⁺–HCO₃ ⁻ type indicating a lesser degree of mineral dissolution. The factor analysis reveals three factors controlling the chemistry of groundwater in the study area: mineral dissolution and weathering, carbonate system, and anthropogenic effect. Factor 1 (mineral dissolution and weathering) includes high values of TDS, EC, Na⁺, K⁺, CI⁻, HCO₃ ⁻, and SO₄ ²⁻ in respective order; Factor 2 (carbonate system) includes Ca²⁺, Mg²⁺, CO₃ ²⁻, and pH; Factor 3 (anthropogenic effect) includes NO₃ ⁻ and F⁻. It has been demonstrated that the integration of conventional hydrochemical analysis with factor analysis represents a powerful mean to understand the factors controlling groundwater chemistry.