Groundwater hydrogeological investigation in basement and sedimentary aquifers in El Obeid–Bara area, North Kordofan State; Sudan

Abstract

Abstract

The study area is situated in the North Kordofan State-western Sudan the

region is characterized by semi-arid and poor savannah climatic zone,

where acute shortages of water are experienced. The purpose of this study

is to assess the groundwater quantity and quality, through testing the

hydraulic properties of the aquifer and geological setting. Using remote

sensing techniques and geophysical electrical methods combined with the

hydrogeological methods to study the potentialities of the groundwater

resources.

The study area is covered by the basement complex (Precambrian age)

which is overlain by superficial deposits in the southern part, where the

northern part is covered by sedimentary rocks (Tertiary to Quaternary

age) under the overburden of superficial deposits.

The remote sensing studies were done to define some geological features

in the study area. Lineament-length density and frequency ranges from

0.02 to 0.3 km-1, and from 0.01 to 0.19; drainage-length density and

frequency are 0.006 to 0.1 km-1, and from 0.004 to 0.06 respectively.

Therefore, in the hard rocks terrains based on lineament studies the target

zones were indicated by the overlap of the high-intensity lineament

contours and the low-intensity drainage contours, and by intersection of a

stream channel, that was observed on major lineaments restricted at zones

(10, and 11) that provide better targets for groundwater prospecting.

The geophysical electrical resistivity and electromagnetic methods were

applied to define the anomalies, thickness and lateral lithological

variation of the sedimentary and basement aquifers. The results show that

these methods useful to locating boreholes sites. Geophysical logs

(electrical resistivity and self-potential) were applied in five wells located

at the sedimentary aquifer in order to provide better guideline for the

water bearing formation.

The present hydrogeological investigations have revealed two

inhomogeneous and anisotropic water-bearing formations: the

sedimentary basin consists of two aquifers called upper and lower, mainly

comprising of sand and a few amounts of gravels and clays. However, the

other aquifer weathered / fractured basement consist of clay and fragment

of rocks. The results of pumping test and grain size analysis revealed the

variation in hydraulic properties depending on geological formation.

Where, the average transmissivity ranges between 124 and 937 m2

/d,

hydraulic conductivity was estimated to be 1.8 and 19.5 m/d, and

storativity vary from 4.5x10-6 to 1.9x10-3, indicating higher hydraulic

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properties of the sedimentary aquifer and lower in the basement complex

one.

The present investigation has brought to light the fact that the area

promising in term of groundwater resources estimated to be 52.5x109

m

3

in the sedimentary basin, and 3.1x109

m

3

for basement complex. The

fluctuation in the static water level depending mainly on the annual

recharge and discharge rates range between 0.5 to 3.0 m. The volume of

annual recharge estimated to be 40% with compared to annual discharge.

On the other way, the total annual extraction values of groundwater

resources equal about 1.3 x109

m

3

, which is more than the annual

recovery rates.

In term of chemical composition of groundwater, the TDS values range

from 300 to 3060 ppm and EC values ranges between 415 and 4400

microohms/cm. Hydrochemical analysis using standard diagrams to

classified hydrochemical characteristics of both aquifers. Groundwater is

predominantly of sulphate- chloride alkaline-earth water (Ca-Mg-SO4-Cl

type) in the basement complex, while Ca-HCO3 to Na-Cl type of

hydrochemical facies presented in the sedimentary and it is apparently

suitable for drinking demands.