TC Lake Chad Basin: Sustainable Water Management

Report of the project:

Fig. 1: Watershed of the Lake Chad

Background:
The Lake Chad Basin, situated in the eastern part of the Sahel region at the southern edge of the Sahara desert, is one of the largest sedimentary groundwater basins in Africa. Although the Lake Chad Basin (Figure 1) extends over an area of about 2,381,000 km², the area of the active hydrographic basin covers about 967,000 km². As a result of a long lasting drought period that ravaged the region from 1973 to 1984, the surface of the lake shrank from about 25,000 km² to less than 2,000 km². However, the size of the lake has remained more or less constant since 2000.

Annual rainfall varies between 1,500 mm in the south of the basin to less than 100 mm in the north. The potential evapotranspiration exceeds 2,000 mm per year at the centre of the basin. The most important influent rivers are the Chari and its tributary the Logone, the Komadougou-Yobe, the El Beid and the Yedseram.

The population of the conventional basin is estimated at about 20 million distributed as 2,550,000 in Cameroon, 193,000 in Niger, 11,376,000 in Nigeria, 5,049,000 in Chad and about 634,000 in Central African Republic. The majority of these people practice agriculture, nomadic and semi-nomadic animal husbandry and fisheries. High population growth rate (35 million in 2020) and development of irrigated agriculture exacerbate water scarcity leading to, among others, reduction of fish availability and increase of food insecurity, migrations, social conflicts and poverty.

Fishing village at Lake Chad

The Lake Chad Basin Commission (LCBC) is responsible for monitoring the use of water and other natural resources within the Lake Chad Basin. The LCBC, which was created in 1964 with headquarters in N'Djamena, has six country members: Chad, Central African Republic, Cameroon, Nigeria, Niger, and, since April 2007, Libya. However, the LCBC lacks basic information as well as management strategies to fulfill its tasks in a sustainable way. Despite these shortcomings, it is the sole institution with potential to deal with transboundary issues, as it is politically accepted by the riparian states. The project intends to strengthen the LCBC so that it is able to coordinate the exchange of groundwater data between the member states, integrate them in a management system and elaborate water resources strategies of sustainable character.

Geology:
The basin was originated during the Cretaceous (more than 100 million years ago) and is underlain by a basement complex (Figure 2). It is composed of a series of sediment deposits. These sediments comprise four aquifers: the Upper Quaternary, the Lower Pliocene, the Continental Terminal and the Continental Hamadien. Since the main recharge to the aquifers is from surface water bodies, it is very sensitive to changes in the surface runoff regimes.

Fig. 2: Hydrogeological cross section

The groundwater quality of the Quaternary phreatic aquifer is suitable for domestic consumption of the local population and livestock. The Lower Pliocene Aquifer, found at depths of about 250 m with average thickness of 60 m, is an artesian aquifer intensively used in the Nigerian part of the basin. Although the reserve of the Lower Pliocene Aquifer is unknown, the exploitation of this aquifer is estimated at about 3 million cubic meters per year.

Fig. 3: Sampled locations

The Continental Terminal Aquifer is essentially an alternation of sandstone and clay with some 250 m in thickness. Lastly, the Continental Hamadien Aquifer is an important aquifer in the West African sub-region but very little information is available on this aquifer in the conventional basin.

Field work:
Under the frame of the project, field investigations and groundwater chemical analyses were performed. Aim of these investigations is to enhance and complete the knowledge of the hydrogeological behavior of the Quaternary aquifer to the north of the Chari-Baguirmi Department. The study area comprises most of the Chadian part of the aquifer. Sampling campaigns took place in three different periods: November 2008 to April 2009, November 2009 to March 2010, and November 2010 to February 2011. During the first phase of the project, a total of 441 water samples distributed throughout the Chadian part of the basin (Figure 3) were taken from surface water (19) and groundwater (422) to investigate their chemical composition and water quality. Groundwater comprised mainly the upper quaternary aquifer (417), but 5 samples were taken from deep boreholes that get water from the lower Pliocene. The chemical analyses have been performed in the BGR laboratories and isotopic analyses in the LIAG laboratories in Germany. The results were used to investigate aquifer zonation and to define areas of groundwater recharge.

Results:
Groundwater dynamics
Figure 4 shows the groundwater contour line map for 2008-2010. Groundwater mounds appear along the Chari River, the Massénya swamps and the surroundings of the Lake, which according to the chemical analyses have a weak mineralization. These two effects combined lead to the conclusion that recharge to the Quaternary aquifer is mainly produced by surface water bodies.

Fig. 4: Groundwater contour line map

The picture also shows groundwater domes in the Harr region and to the north of the lake, as well as a discharge to the northeast of the study region towards the low lands through the Bahr el Ghazal. This presently dry river acts as a discharge of the Lake Chad whenever the lake’s level is higher than 285 m above mean sea level.

Further, the map shows a depression to the southeast of the Lake with its deeper point at 235 m above mean sea level. Because groundwater quality in the area presents enhanced salt content, it can be concluded that the depression is caused by evaporation from groundwater, which certainly surpasses whatever recharge from precipitation takes place in the area. Evaporation occurs probably through the palaeo-delta sands of the Chari River.

Fig. 5: Groundwater type

Groundwater type
Groundwater in the Lake Chad region displays regionally different chemical compositions as shown in Figure 5. Groundwater of Ca-Mg-HCO₃-type (light color) appears in the proximities of the Chari and Logone rivers in the southern part of the study area as the result of direct recharge either from surface water or from precipitation. Further weakly mineralized water is present in the north of Lake Chad, surely as a result of direct recharge from precipitation because no surface water is present in this area.

Fluoride
Groundwater shows fluoride contents above the upper WHO limit of 1.5 mg/l in 12 of the 441 samples (Figure 6), especially along the Bahr el Ghazal, to the south of the Lake Chad, in the vicinity of Lake Fitri, and in the Mayo-Kebbi region. Elevated fluoride concentrations between 0.5 mg/l and 1.5 mg/l are present along the Logone River and along the Chari River in the north of parallel 11° north.

Fig. 6: Fluoride concentrations

The reason for the high fluoride concentrations are various, according to the locations as follows:

The zones in the south of the Lake and east and west of the Bahr el Ghazal are characterized by alkaline water (pH above 7) and enhanced concentrations of bicarbonate, potassium, and sodium as well as magnesium and lithium. Thus, the presence of mica would be the cause for the high fluoride content in this region.
There are post-tectonic granite intrusions in Moyto, in the proximities of the Lake Fitri, and in the Mayo Kebbi region. The groundwater flow through weathered granite could be the explanation of high fluoride in these areas.
The Logone River is known to be flowing along a structural feature (fault zone). Thus, upwelling groundwater from the basement into the shallow aquifers is probably the cause of high fluoride here.

Fig. 7: Nitrate concentrations

Nitrate
A total of 59 samples (or 13%) present levels of nitrate above the 50 mg/l allowed by the WHO norms (Figure 7). The sources of nitrate are different depending on the main human activities in the region. In the north of 12° latitude north, pollution is due to livestock watering directly from the borehole leading to accumulation of animal faeces at the wellhead. In the south, agriculture is the main activity, especially cultivation of rice and cotton. The high pollution here might be an effect of the excessive use of nitrogenised chlorinated fertilizers because a good correlation is achieved between nitrate and chloride (Figure 8).

Fig. 9: Suitability for irrigation

Irrigation Suitability
A map of suitability of groundwater for irrigation (Figure 9) was produced taking into consideration three different parameters (Wilcox, Sodium Adsorption Radio SAR, and Magnesium Hazard MH). In the map of Figure 9, the blue dots indicate water points where groundwater is suitable for irrigation according to all the three indices considered. The light red dots correspond to locations where groundwater is classified as unsuitable by one of the methods. The red dots show those places classified as unsuitable by at least two of the methods. Finally, the dark red dots are locations where groundwater has been classified as unsuitable for irrigation by the three methods considered.

Groundwater is unsuitable for irrigation in the east and west of the Bahr el Ghazal, in the south of the Lake, and in the vicinities of the Lake Fitri.

Isotopic analyses
A profile of oxygen-18 and deuterium contents was drawn using the isotope data from the project and those published by Djoret (2000) for the Chari-Baguirmi area.

Fig. 10: Profile of isotope values

The profile starts at the Lake Chad and crosses the Chari-Baguirmi region up to the Lake Fitri (Figure 10). Both lakes are characterized by the presence of heavy isotopes. The decline of the isotope values with increase of distance to the lakes is an indication of recharge or leakage from these water bodies into the aquifer. A simple calculation based on mass conservation results in a mix of 20% to 30% of groundwater and 80% to 70% of Lake Chad water at Guirbe, some 15 km from the lake. At Salga, some 52 km from the lake shore, the proportion changes to 80% of groundwater to 20% of lake water. Finally at Naala, some 85 km from the lake shore, only 10% of the mix corresponds to lake water. The sudden increase in deuterium values at Massaguet shows probably the effect of a surface water source, probable a palaeo-channel of the River Chari, as proposed by Djoret (2000).

Conclusions:
It can be concluded that groundwater in the area is generally suitable for human consumption. However, there are problem regions where water is inappropriate for consumption, as concluded below:

Nitrate pollution is punctual and due to livestock watering directly from the borehole in the north of 12° latitude north. In the south of the study area, the high nitrate pollution is regional and is caused by an excessive application of nitrogenised fertilizers in the rice production.
High salt concentration in the east and west of the Bahr el Ghazal, in the SE of the Lake Chad and in the proximities of the Lake Fitri suggests that these areas as unsuitable for irrigation with groundwater.
The high fluoride concentrations might have different genesis as follows:
- The zones in the south of the Lake and along the Bahr el Ghazal are characterized by neutral to slightly alkaline groundwater (pH above 7) and enhanced concentrations of bicarbonate, potassium, and sodium as well as magnesium and lithium. Thus, the presence of mica could be the cause for the high fluoride content in this region.
- Post-tectonic granite intrusions are encountered at Moyto, in the Mayo-Kebbi region, and in the proximities of the Lake Fitri. Here, groundwater flow through weathered granite would lead to high fluoride concentrations.
- The Logone River is known to be flowing along a structural feature (fault zone). Thus, upwelling groundwater from the basement into the shallow aquifer is probably the cause of high fluoride in this area.

Recommendations:
It is highly recommended that groundwater protection measures be implemented to safeguard the resource against pollution caused by human activities.

In regions with high nitrate concentrations, hand dug wells should be abandoned in benefit of drilled, well constructed and protected boreholes. If this technique is not possible, then a better management of hand dug wells is recommended.

Further, due to high salt concentration, groundwater should not be used for irrigation purposes in the east and west of the Bahr el Ghazal, in the south of the Lake Chad and in the proximities of the Lake Fitri.

Literature: