Geoelectrical and Hydrogeological Characteristics of the Groundwater Aquifers in the Gezira Area, Central Sudan

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Mohammed, Elzein Ahmed Elzein
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Geoelectrical measurements in conjuction with boreholes data have been used to develop a conceptual hydrogeological model for the exploration of groundwater in the Gezira area, Sudan. This is the land between two Niles (The Blue and the White Niles), occupying an area of about 32000 km2, which is bounded by lat. 13° 30\\\\\\\' and 15° 30\\\\\\\'N. The surface geology of the Gezira area is largely dominated by the sediments of the Gezira Formation (late Quaternary) which rest unconformably on the Cretaceous Nubian Sandstone Formation. This in turn lies with a significant unconformity on the crystalline non-pervious Precambrian Basement Complex rocks. All these sequence and rock units are covered in some places of the Gezira area by the recent Superficial deposits. The Nubian Sandstone Formation is composed of sandstones, mudstones and conglomerates. It has a thickness of more than 240m. The Gezira Formation is made up of unconsolidated sediments (sands, gravels, silts and clays). It has a thickness which varies from 0.1m to 120m. The Gezira and Nubian Formations represent the main water- bearing formations in the Gezira area, while the Superficial deposits and the Basement Complex rocks are less important as groundwater aquifers. The saturated thickness of the Gezira aquifer varies from a few meters to more than 50m, while the Nubian aquifer has a saturated thickness which varies from a few meters to more than 100m. The groundwater quality ranges from fresh water to extremely poor and saline water which is unfit for human and animal consumption or agricultural use. The salinity increases to a maximum in the trough which lies in the center of the Gezira and decreases towards the Blue and White Niles. These Niles represent the major source of the groundwater recharge in the area. A direct recharge is also reported during this study from the irrigation canals of the Gezira and Managil schemes. The groundwater levels vary between 4 and 12m (below the land surface ) near the rivers, but reach up to 45 m in the central part of the Gezira. A total number of 300 vertical electrical soundings (VES) are used in the present study. The VES data were correlated with 730 well logs data (hydrogeological, lithological, hydrochemical and borehole geophysical logs). The data are analysed and interpreted to assess and evaluate the main groundwater aquifers of the Gezira area by determining their hydrostratigraphic units and their geoelectrical characteristics. The resistivity data were collected with help of an ABEM (SAS 300, 1000 & 4000) Terrameters. The data inversion was done by using automatic one-dimension inversion software (RESIX version 3.08 Interpex limited, 1994). The results which are presented in the form of a geoelectrical and geological sections as well as electrical conductivity, salinity and hydrochemical maps have shown a close agreement with general hydrogeological condition of the region. The geoelectric characteristics of the different groundwater aquifers, their water quality and lithological units are identified. Moreover the interface between fresh and saline groundwater in the region has been well delineated. These results are used to construct the conceptual hydrogeological model. The proposed model can identify the different hydrostratigraphic units of the Gezira and the Nubian Formations, their resistivity ranges, their lateral and vertical extent and their groundwater potential as well as their water quality (salinity). The stratigraphy of the hydrogeological model reveals from top to bottom the following units: a vadose zone which is characterized by resistivity values (2 to 500]7;.m), fresh Gezira aquifer (15 to 250]7;.m), saline Gezira aquifer (5 to 40]7;.m), aquitard-1 (5 to 50]7;.m), fresh Nubian aquifer (15 to 300]7;.m), saline Nubian aquifer (5 to 30]7;.m), aquitard-2 (5 to 40]7;.m), lower Nubian aquifer (25 to 150]7;.m), mudstone (10 to 40]7;.m), and the bedrock which is characterized by resistivity values (150 to 1000]7;.m). However, these hydrostratigraphic units below the vadose zone are not always present throughout the study, area and one or two units might be missing. The hydrostratigraphic units are displayed in the form of two and three dimension hydrostratigraphic models as well as hydrostratigrphic cross sections. Finally based on these finding, maps of potential sites for drilling in the fresh Nubian and Gezira Formation aquifers are constructed. It is observed that the area of the fresh aquifer geometry is closely related to the location of the proto- Blue Nile paleochannel and therefore indicates a direct recharge from the Blue Nile through the coarse sediments of these paleochannels. Borehole yields are generally related to the aquifer thickness, and the high yields are always associated with the thick aquifers along the fresh water zone. The geophysical well logs have also well identified the static water level, the Gezira and the Nubian Formations and their groundwater quality (salinity). The Nubian Formation is generally characterized by relatively high resistivity values compared with the Gezira Formation, and the fresh water in both aquifers has relatively high resistivity than the saline water zone. The resistivity of the vadose zone varies from high to low due to its lithological heterogeneity. The geoelectrical findings indicate that the resistivity is controlled by the water quality (salinity) and the lithology rather than the porosity. The geoelectrical characteristics adopted in this model could be applied to evaluate water prospects of new target areas in the region and elsewhere. The promising sites for groundwater exploration, giving more production and better quality, throughout the Gezira area can easily be identified
Geoelectrical, Hydrogeological Characteristics , Groundwater Aquifers , Central Sudan