Hydrogeological and Hydrochemical Studies of the Area Around the Two Banks of the Blue Nile River between Longitudes 32° 33 & 33° 29' E and Latitudes 15 00 & 15° 34' N (South of Khartoum Province - North of Gezira States).

Abstract

This study covers Hydrogeological and Hydrochemical investigations in the area around the two banks of the Blue Nile river. It is situated between longitudes 32° 33' and 33°29' E and latitudes 15° 00' and 15°34' N, South of Khartoum province North of Gezira state. The rock sequences of the study area comprise the Precambrian Basement Complex rocks. The Cretaceous Nubian Sandstone Formation, the Quaternary Gezira Formation and the Recent Superficial deposits. The Basement Complex rocks are the oldest rocks, which are covered by the other formations and occur at variable depths in the study area. The Nubian sandstone Formation rests unconformable on the Basement Complex rocks. It is situated under the Gezira formation in the west and under the superficial deposits in the area east of the river. The formation is composed of sandstones, mudstones and conglomerates. The Gezira Formation overlies the Nubian sandstone formation in the area west of the river and is absent east of it. The formation consists of unconsolidated sands, gravels, silts and clays. The thickness varies from a few meters to a maximum of about 95 m. The Recent Superficial deposits include clays, sandy clays and wind blown sands. It covers most of the ground surface of the study area with a maximum thickness of 15 m. The Gezira and the Nubian sandstone formations represent the major water bearing formations in the study area, while the superficial deposits are less important as groundwater aquifer. Most of the deep boreholes drilled in the middle and the southern parts of the left bank produce water from these two aquifers, which are hydrogeologically interconnected and contain large quantities of water. Groundwater occurs mainly within the sand and the gravelly sand of the Gezira formation. The Gezira aquifer is unconfined and the depth to the saturated zone of this formation is ranging from 12m to 40m, while the saturated thickness of the aquifer varies from 6m to about 57m.

Nubian sandstone aquifer is considered to be the most important aquifer in the study area. The sandstone, the pebbly sandstone and the conglomerates constitute the main aquiferous zone of the Nubian sandstone formation. In some localities more than one aquifer horizon exists within this formation. The groundwater of the Nubian formation aquifer occurs under semi-artesian conditions. The depth to top of the saturated horizon of the Nubian sandstone formation varies from 23 m to 164m. The aquifer has variable thickness, ranging from 23 m to about 118 m. The groundwater flow of the aquifers in the southern and the middle areas is directed towards the Northwest direction. In the Northern area, east of the Blue Nile, the local groundwater flow is observed from west to east away from the right bank. However on the left bank, the local flow of groundwater is observed from east to west and from northeast to southwest, which indicate the influent nature of the Blue Nile. The hydraulic parameters for the studied aquifers are estimated mainly from the pumping test data. For the Gezira aquifer, the average transmissivity values are 281 m2/day. While the hydraulic conductivity of the aquifer is relatively low; it's ranging from 0.5 to 48 m/day. The specific capacity of the Gezira aquifer ranges from 19 to 190 m2/day. The well loss in wells tapping the Gezira aquifer varies from 66 % to 51 % of the total drawdown; this indicates that these wells were badly designed. The average transmissivity value of the Nubian Sandstone aquifer is 300 m2/day, and the hydraulic conductivity varies from 1 to 10 m/day. The step-drawdown test interpretation show that the well loss of wells tapping the Nubian sandstone aquifer varies from 13% to 74 % of the total drawdown. The specific capacity varies from 30 to 357 m2/day, with an average value of about 150 m2/day. The groundwater reserves of the Nubian aquifer up to a bout 300 meters below the surface are estimated to be 21 X 109 cubic meters. Where the total groundwater reserves of the Gezira aquifer up to about 90m below the surface (west of the river) is estimated to be about 8 X109 cubic meters. The annual groundwater recharge from the Nile to the Nubian aquifer on the left bank of the river about 3.1 X 106 m3/year, while on the right bank it is about 3.7X103 m3/year. While the total recharge from Blue Nile to the Gezira aquifer is estimated to be 3.3 X 106 m3/year. Therefore, the total recharge to the Nubian and Gezira aquifers on the left bank is computed to be 6.4 X106 m3/year. Since the recharge of groundwater is mainly derived from the Blue Nile river, so the composition of the river water affects the chemistry of the groundwater. The discharge represented by the annual pumped groundwater estimated for 100 boreholes is approximately 6.5 million m3. The total dissolved solids of the groundwater of the Gezira aquifer ranges from a minimum of 180 ppm to a maximum of 1495 ppm. Whereas the salinity (TDS) of the Nubian sandstone aquifer east of the river varies from 170 ppm to 1280ppm, while the .values of TDS in "the west bank tract varies from 180 ppm to 568 ppm . Generally, the groundwater of the study area in the two aquifers display low salinity content (less than 500ppm TDS). An exception is the high levels of salinity (TDS concentration more than 1000 ppm) that are recorded at Soba Agriculture Scheme area in the northwestern periphery of the study area. The dominant concentration of alkalinity as CaCO3.in the Gezira aquifer ranges from 73 to 688 ppm, with average values of 380 ppm, while the pH values of the aquifer range between 7.4 and 8.8: In the Nubian sandstone aquifer the concentration of alkalinity varies from 80 to 406 ppm, with an average of 243 ppm The pH values of the Nubian aquifer ranges from 7.0 to 8.9. The hydrochemical composition of the Gezira and the Nubian sandstone aquifers indicates that the sodium bicarbonate water represents the most predominant water types. The groundwater of Nubian and Gezira aquifers is suitable for irrigation of most types of plants, except in the high salinity areas, where special management of salinity control is needed and certain kinds of plants with good salt tolerance have to be adopted.