Hydro-geophysical Investigations of the . Upper Aquifer Abyei area, South Kordofan State, Sudan

Abstract

The Geological and Geophysical Electrical method in combination with Hydrogeological methods were applied to study the availability and potentialities of the groundwater resources and quality in the upper part of Um Ruwaba Fonnation in Abyei Locality- South Western Sudan. The geological succession in the study area consists of Basement complex overlain by Nubian Cretaceous Sandstone Formation, Um Ruwaba Fonnation, and Superficial deposits. Baggara basin composed of Nubian sandstone formation overlain by Um Ruwaba series considered as main aquifers in the study area. Umm Ruwaba Series is composed of a variety of sediments, coarse and fne, well sorted and poorly sorted, penneable and impenneable. The depth to water table for upper aquifer varies from 09 m to 45 m. and from 36 m to 127m for the lower aquifer; the transmissivity generally varies between 6.42 X l0'3 m2/s and 3.39 X l0'3 ml/sj the average Hydraulic conductivity lies between 3.4 X l0'4 m/s and 9.61 X l0'4 m/s and the average porosity between 28% and 39%. ' The hydraulic gradient in this particular part is ranging from 0.002 to 0.003. The low hydraulic gradient indicates relative homogeneity and isotropy of good porosity and permeability of the aquifer. The close spacing of the groundwater flow contours in the northeast, west and east of the study area indicates relative anisotropic aquifer. The Wide spacing contours at the middle part of the northeast side are particularly due to the thick saturated sedimentary section and the high transmissivity of Um Ruwaba fonnation. The final directional component merge to the SW of the study area, the resultant flow moves out of the study area in the direction to the Bahr El Arab. The surface and near surface layer reflect wide range of resistivity values from 6 to 200 Qm. A few anomalous values are shown to fall between 540- 2240 Qm (VES 17, 18 &l9). The lower range generally describes clays & sandy clay sediments. This unit extends to an average depth of 40 m. The second unit in the column is indicated by a range of 18 — 280 Qm, high resistivity values observed on the top of the layer especially under VES (8), (9), (10) and (11) with the resistivity values 546, 688, 898 and 468 Qm respectively reflecting dry gravelly and/or coarse sandy formation. The lower resistivity values zones in the second unit is interpreted as Water bearing horizon as it lies generally below the static water level in the region. The aquifer also shows low resistivity values at the bottom of VES (5), which probably refer to sandy clay fonnation. The water bearing horizon shows depth of 30-140 m below ground surface. The third layer is characterized by consistent resistivity values ranging between 5- 20 Qm this unit is most probably clays, which dominates underneath. This unit is known here as the upper clays of the Baggara basin. The chemical composition of water indicates Na-Ca-Mg-HCO3 water type which is suitable for drinking, irrigation and industrial purposes.