The Chadian company of Water Supply and Electric Energy STEE supplies groundwater
to NDjaména and environs from the aquifer of the Quaternary formation.
This is the only aquifer in the region from which private agencies and individuals
also exploit groundwater for socioeconomic development. Already, in the Chari
Baguirmi area, the aquifer has experienced a great piezometric depression (Kouka
depression) located in the Northeastern part of NDjaména (Town
Council of NDjamena, 2004). In addition to abstraction by boreholes,
evaporation of water from the shallow water table of this aquifer is said to
have contributed to the lowering of the about 10 m water level in the Kouka
depression. Additionally, Djoret (2000) has shown that
there is an indication of rapid recharge from meteoric source, a deduction based
on concentration of some chemical species in the water implying that recharge
to the aquifer is seasonal. Given the historic climatic conditions and the present
changes in climate worldwide even the speculations that there is a determinable
rate at which groundwater can be withdrawn indefinitely from an aquifer or a
groundwater system in a defined area without causing undesirable effects (Lohman,
1972; Fetter, 1994) has been one of the most discussed
and a controversial concept in groundwater hydrology. Unfortunately, in Ndjaména
area, despite the heavy dependence on groundwater for various uses, very little
is known about this aquifer from which water is abstracted. The aim of this
study therefore, is to estimate the hydraulic characteristics of the aquifer
using available scanty data in order to highlight the groundwater potential
so that abstraction can be controlled.
LOCATION AND CHARACTERISTICS OF THE STUDY AREA
NDjaména town and environs are situated midway between the Northern and Southern Chad Republic at the extreme West of the country. They are located within latitude 12° 02 N-12° 08 and 15°° 04 E-15° 08 E (Fig. 1). The terrain is nearly flat but shows a progressive slope Southwards towards Lake Chad with an altitude varying between 293 and 298 m above mean sea level. The elevation gradually rises as one moves to the Northern and Eastern parts away from Lake Chad. These low slopes have been observed to be opposite gravitational flow towards River Chari. This study area forms an important part of the Chad Basin.
The study area experiences a tropical climate characterized by two seasons,
wet and dry. The wet season lasts from June to September with a maximum rain
in August and the mean annual rainfall is about 500 mm. Temperature varies between
20 and 25°C during this season. The dry season lasts from October to May
and is characterized by dry winds coming from the East and high temperatures.
|| Location of the study area
Sedimentary rocks form the Tertiary and Quaternary terrains in NDjaména
(Gerard, 1958). The sediments accumulated from detrital
materials of variable grain size carried by rivers and streams coming from massif
of Adamaoua (granite) in the South, Tibesti (eruptive rocks and granite) in
the North and Guéra-Mongo-Abou Deia (granite) in the centre. The sediments
themselves have been eroded, transported and deposited to produce alluvium and
primary sandstone of Borkou-Tibesti-Ennedi and Ouaddaï. Eolian sedimentation
also contributed to deposition wherein winds have changed fluvial sandy formations
to a sandy landscape. The thickness of sediments is variable, being shallow
in the vicinity of massifs and increases progressively to about 400 m away from
it. Bardeau (1956) represented a succession of sedimentation
observed above the granitic basement in the East of the region and Northeast
of NDjaména as a series dominated by clay of about 10 m thick in
contact with the basement, a series dominated by sand of about 50 m, a series
dominated by clay of 150 to 200 m and a series dominated by sand of 50 to 60
m. Sediments from the Continental Terminal overlie the Basement Complex and
underlie the Pliocene formation and the Quaternary formations.
MATERIALS AND METHODS
Field work for the project took place in NDjaména and environs
from 12th November, 2008 to 27th May, 2009. During this period geological, hydrogeological
and hydrometeorological data were acquired. Analyses of these data were carried
out at Ahmadu Bello University, Zaria, Nigeria. Pumping test data from 26 boreholes
(Table 1) located in the study area were used. Each well was
pumped at the given discharge rate three times with each pumping session lasting
1 h. Both the second and the third pumping sessions commenced after the well
was allowed to recover from the previous stress. But the data for the third
hour were the only ones used in this work. The limitation of the data is linked
to the duration of the test and the lack of observation boreholes. The analyses
were based on (Jacob, 1960) modified equation for free
For a fully penetrating well screened up to the static water level transmissivity T, is expressed as:
where, K is the hydraulic conductivity (m/d), h1 and h2
are elevations of the water table in observation wells 1 and 2 (Fig.
2) above the datum (m), (h1+h2)/2 is the saturated
thickness of the aquifer (m).
From Fig. 2, s1+h1 = s2+h2 and so:
From (Jacob, 1960) initial formula of the plot of drawdown
versus log t:
where, Q is discharge rate, t1 and t2 are times corresponding to times at which drawdown measurements were made in an observation well.
Substituting T of Eq. 1 and s2-s1 of Eq. 2 in Eq. 3 we obtain:
So plotting h2 against log t will produce a slope, which over one log cycle of t (that is, log t2/t1 equals log 10), will yield:
Sample plots based of some pumping test data (Table 2) are presented (Fig. 3).
During the pumping tests changes in elevation of the water table were recorded in the pumping well as water was being pumped from the screened portion of the aquifer.
From Eq. 1, (h1 + h2)/2 is the portion through which discharge (Q) takes place. But now in this case, discharge was through the screened portion only. Therefore, transmissivity was estimated as the product of the hydraulic conductivity and the screen length.
Storage coefficient is the volume of water that the unit volume of aquifer
releases from storage under a unit decline in hydraulic head. This term applies
essentially to confined aquifers. The fact that this quantity normally varies
directly with aquifer thickness enables the rule-of-thumb relationship (Lohman,
where, S is storage coefficient and b is the thickness of the aquifer.
For the confined aquifer, water released from storage is controlled by secondary
effects of compaction of the aquifer (aquifer compressibility, a) caused by
increasing effective stress and expansion of the water (compressibility of water,
β) caused by decreasing pressure. Normal range in value of storage of confined
aquifer is 0.00005 < S < 0.005, implying that large pressure changes over
extensive areas will be required to produce substantial water yields.
|| Information on boreholes in NDjamena area
But the Quaternary aquifer in NDjaména area is unconfined. Therefore water releases from it amounts to dewatering the aquifer. Normal range in values for such an aquifer is 0.01 < S < 0.30 implying that substantial volumes of water can be obtained with small changes in head over relatively small areas. For the unconfined aquifer, storage is termed specific yield. The ratio of unit drawdown induced by pumping the Quaternary aquifer to the length of the screen in each borehole was considered for the determination of aquifer saturated thickness in the present calculation. Comparing the least values of storage for both confined and unconfined aquifer Eq. 7 was modified as:
where, S is specific yield and b is (length of screen in borehole/drawdown in the borehole)
The aquifer constants were thus estimated and the results are presented in Table 3.
||Schematic section showing radial variation in water level
around a well in an unconfined aquifer
||Time-Water level above stratum curves for boreholes FN°
2, FN° 25 and FN° 53
RESULTS AND DISCUSSION
Table 3 presents the results of analyses. The least value
of transmissivity ( 5.8x10¯1 m day-2) was recorded in borehole
FN° 22 and the highest (2.69x10¯4 m day-2), was recorded
in borehole FN° 41. Very low values of transmissivity (below 1000 m day-2)
were recorded in boreholes numbers FN° 14, FN° 12, FN° 25, FN°
51, FN° 34, FN° 6 and FN° 22 while very high values (above 18, 000
m-2 day) were obtained for boreholes numbers FN° 5, FN° 42,
FN° 41, FN° 4, FN° 1 and FN° 30. But the remainder of the boreholes
have 2, 592.34 m day-2 as the mean value of aquifer transmissivity.
||Pumping test data in the 3rd h for borehole FN°.2, FN°.25
The range of values compares with those of (Town Council
of NDjamena, (2004) elsewhere in the basin, accommodates those of
Cotei (1967) - 3.2 x10-3 m sec-1-6.6x10-3
m sec-1-using six boreholes within NDjaména township
and Schneider and Wolf, 1992-3.0x10-4 m sec-1 < T <
7.0x10-3 m sec-1 and 1.7x10-3 m sec-1
< T < 2.5x10-2 m sec-1 for Northern and Southern
NDjaména, respectively. The mean hydrodynamic parameters of Chari
Baguirmi, according to Artis and Garin (2007), BRGM
(1987) and Schneider (1967) are 3.0x10-4
m sec-1 < T < 7.0 x10-3 m sec-1 and 2.8x10-8
m sec-1 < T < 2.0x10-2 m sec-1,
respectively and compare with the present results.
Using Gheorghe (1978) standards (Table
4) to interpret transmissivity only borehole FN° 22 records a moderate
potential while the rest boreholes show high potential for the aquifer. By Krasny
(1993) standards (Table 5) the aquifer generally has high
to very high transmissivity capacity that provides withdrawals of regional importance
because even in borehole FN° 22 intermediate capacity from which local water
supplies can be withdrawn is indicated. Results from the rest of the boreholes
indicate that well yield can be adequate for industrial, irrigation and municipal
purposes as these can provide withdrawals of great regional importance.
The hydraulic conductivity varies from 0.12 m day-1 to 5,460.48
m/d which on Bouwer standards (Table 6) an aquifer varying
in composition from sand-and-gravel mixes to gravels. These results show that
the aquifer in the study area is heterogeneous. The results are in reasonable
agreement with the exception of borehole FN° 5, FN °1, FN°42, FN°4,
FN° 30 and FN° 41 for which figures for the porous medium tends to be
essentially for gravels. Indeed Schneider and Wolf (1992)
values for hydraulic conductivity are 1.4x0-5 m sec-1 <K<4.7x10-4
m sec-1 and 1.3x10-4 m sec-1 for Northern and
Southern parts of NDjaména, respectively.
||Hydraulic conductivity, transmissivity and specific yield
of the aquifer in NDaména area
Specific yield varies from 0.006 to 0.052. However, Cotei
(1967) and Schneider and Wolf (1992) values are 4.0x10-4<S<
10-3 and 3.0x10-5<S<5.0x10-2, respectively
tend to indicate that the lower value in each range may imply that the aquifer
is semi confined.
|| Standards for hydraulic conductivity (Bouwer,
Our values fall generally within the normal range for unconfined aquifers,
but the aquifer storage is low. This may not be difficult to understand given
the climatic environment and the general phreatic nature of aquifer in the area.
The evaluation of hydraulic properties of the Quaternary aquifer of NDjaména and environs indicate that the aquifer has good yield potentials and that groundwater development potential of the area is very high. The likely problem that could lead to inadequate supply of water will be related to its availability rather than the hydraulic properties of the aquifer. Accordingly, abstraction should go on with caution in the face of prevailing climate change that may adversely affect recharge. It may be necessary to set a pumping rate for each borehole based on standard step drawdown pumping test in order to guarantee sustainable withdrawal. The estimation of hydraulic conductivity was based on a formula that is not quite familiar or commonly used, but which derivation is logical. Further confirmation of it is necessary using data obtained from standard pumping tests with observation borehole(s). This will subsequently confirm the validity of results obtained for both transmissivity and specific yield using this procedure or enable a comparison between this and the standard procedure.
We wish to thank Dr. Doumnang Jean-Claud, Head of Geology Department of the University of NDjamena, for his encouragement during the field work, Mr. Doudarial Moussa, Coordinator of European Fund for Development, the General Manager of the Ministry of Water and Environment, all the staff of the Ministry of Water and Environment for their support and assistance in providing the necessary data for the work.