Updating of Groundwater Conditions of Ennahud Basin, North Kordofan, Sudan

Abstract

Abstract The work mainly aims to characterize the hydrogeology of Ennahud basin and delineate favorable sites for groundwater development. Conventional quantitative hydrogeological methods have been used to provide local estimates of hydraulic properties and average the overall hydrogeologic conditions. Then a conceptual model is used with the aid of visual modflow software to evaluate groundwater budget, calculating inflow, outflow and fluctuation in hydraulic head.

The main geologic units are; Basement Complex, Nubian Sandstone, (Cretaceous), Labda Laterite deposits and Quaternary Superficial deposits. The Cretaceous Sandstone is a single group composed of intercalations of sandstone, siltstone, mudstone and conglomerates.

The geophysical survey provided results pertaining to basin geometry, geological structure and stratigraphic sequences of the subsurface. The results delineated a large faulted basin structure (grabin) of 150km long and 100km wide. The deepest part of the grabin has a maximum depth of 1200m. The groundwater occurrence and availability is good within the horizon between 125 to 400m.

There are three hydrogeological divisions in the area; impervious Basement rocks, semi pervious unit of sandy mudstone and pervious Sandstone. Geologically Ennahud is considered as an outlier, structurally controlled basin and it was created during the tectonics processes of African Rift. In the study area, groundwater occurs in the Sandstone aquifer under unconfined condition, with slight variation in hydraulic properties. The saturated thickness varies from 30m to 300m. The depth to water level varies from 56m to 144m. There is a large fluctuation in the groundwater level between dry and wet season. Infiltration from rainwater constitutes the source of recharge to the sandstone aquifer in the area (149Mm3), while a considerable portion of recharge percolates from the surrounding faulted Basement.

The well yield in Ennahud basin ranges between 6.5m3/h and 120m3/h while the specific capacity varies between 0.36m2/h/m and 60m2/h/m. The total annual abstraction is about 6.2 million cubic meters. The average transmissibility (T), hydraulic conductivity (K) and storage coefficient (S) are 383.4m2/d, 8.2m/d and 4.5x10-3 respectively. The groundwater quality of the Sandstone aquifer is generally good for drinking and household purposes, excluding few areas. The total dissolved solids (TDS) range between 85 ppm and 2366ppm.

The finite difference method is selected for the numerical solution of the partial differential equation as a method for groundwater modeling and the Visual MODFLOW version 3.1 (software) for numerical computation. The maximum computed annual pumpage at year 2015 is 45 million cubic meters while the total annual recharge to the aquifer is computed as 2897 million cubic meters. The study indicated that all basin area is good for future development, but the most favorable are; the areas which extend from El Khuwei south in the north to Abu Gulb north in the south.