Regions:Chad/Guéra

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Guéra is a province in central Chad. Its provincial capital is Mongo. The province includes the town of Bitkine, where AOWIS is currently involved in local water-supply work and groundwater assessment.

Guéra lies across the transition from the Sahelian zone in the north to more humid Sudanian conditions in the south. Its mountainous crystalline basement, strongly seasonal rainfall, lack of permanent rivers, and highly variable groundwater productivity make water supply a major engineering challenge.

Geography and administration

The 2022 Schéma Provincial d'Aménagement du Territoire du Guéra gives an area of approximately 61,279 km². Following the 2021 administrative restructuring, the province consists of five departments:

• Guéra, capital Mongo;
• Abtouyour, capital Bitkine;
• Mangalmé, capital Mangalmé;
• Barh Signaka, capital Chinguil;
• Garada, capital Melfi.

The same planning document gives a projected population of 635,822 inhabitants in 2020, based on the 2009 national census and subsequent INSEED projections. The population is concentrated particularly in Guéra and Abtouyour.

Terrain and surface water

The landscape is dominated by plains interrupted by large granitic massifs, inselbergs, plateaus, and exposed bedrock.

The central Chadian massif reaches some of its highest elevations in Guéra. The Mont Guéra reaches approximately 1,613 m in the Bitkine area, while Mont Guédi east of Mongo reaches approximately 1,506 m.

There are no major permanent rivers across most of the province. Surface drainage is dominated by seasonal watercourses, locally known as barhs or ouadis, which flow during the rainy season and may retain water temporarily in depressions and ponds.

Seasonal streams draining the Guéra massif include the Bolong, Bang-Bang, Zilla, Zerzer and Melmele systems. Water from some northern catchments ultimately contributes toward Lake Fitri.

Surface-water availability therefore changes strongly between the rainy and dry seasons, increasing dependence on groundwater for permanent water supply.

Climate

Guéra spans several climatic zones. The north is predominantly Sahelian, the centre is Sahelo-Sudanian, and the southern part of the province approaches Sudanian conditions.

The main seasons are:

• rainy season: approximately May to September;
• dry season: approximately October to April.

July and August account for a large fraction of annual rainfall in hydrogeological studies of the central and northern part of the province.

The provincial planning document reports annual precipitation increasing broadly from around 800 mm in the north to 1,100 mm in the south. Hydrogeological studies covering more restricted parts of Guéra use lower ranges, including approximately 430–867 mm/year. These values refer to different geographic extents and datasets and should not be treated as a single uniform rainfall value for the province.

Mean annual temperature is around 28 °C, with high temperatures and long dry periods contributing to evaporation, drying of seasonal ponds, and strong seasonal pressure on water supplies.

Geology

Much of the Guéra massif consists of crystalline basement rock. Nouradine's regional hydrogeological work describes the study area as being dominated by granitoids and metamorphic rocks.

Around Bitkine, the basement includes:

• granite;
• biotite granite;
• diorite;
• granodiorite;
• dolerite.

The lithology strongly influences groundwater productivity. Biotite granites can develop permeable weathered and fractured horizons, while alteration of diorites, granodiorites and dolerites can produce larger amounts of clay, reducing infiltration and hydraulic conductivity.

Hydrogeology

Groundwater in Guéra does not occur as one large continuous regional aquifer. The principal groundwater resources are associated with:

• weathered crystalline basement;
• fractured and fissured bedrock;
• alluvial deposits;
• sands and sediments along seasonal drainage systems.

The basement aquifers are therefore heterogeneous and discontinuous. Two boreholes located relatively close to one another can encounter substantially different conditions.

A simplified basement-aquifer structure consists of:

1. superficial material and soil;
2. weathered basement, or alterites;
3. a fractured basement horizon;
4. progressively less fractured competent bedrock at greater depth.

The weathered horizon can provide substantial groundwater storage. Hydraulic transmissivity is often associated particularly with the upper fractured basement beneath it.

Nouradine's regional analysis found that important water inflows commonly occur at approximately 30–60 m depth. Productivity generally decreases where drilling continues far into relatively unfractured basement.

Groundwater occurrence and borehole productivity depend on the combined effects of:

• lithology;
• thickness and permeability of the weathered zone;
• development of the fractured horizon;
• geomorphology;
• recharge conditions;
• local drainage;
• position within the landscape.

Low-lying areas and locations favourable to infiltration and recharge can provide substantially better groundwater conditions than nearby elevated or poorly weathered basement.

Borehole productivity

Obtaining productive boreholes is a major difficulty in Guéra.

Data analysed for boreholes constructed between 2010 and 2016 show that, among 137 boreholes considered by the SEVES study:

• 47% were unsuccessful during drilling and were not equipped;
• 42% produced too little water for connection to an AEP water-supply network;
• only 11% provided an exploitable yield of approximately 4 m³/h for 8 hours per day.

The broader dataset analysed by Nouradine contains technical information from 798 boreholes, together with approximately 700 EM34 profiles and 592 electrical-resistivity tomography profiles.

The mean borehole yield in the analysed basement-aquifer dataset is approximately 1 m³/h. More productive boreholes are associated particularly with well-developed weathered and fractured horizons and favourable granite or biotite-granite formations.

The difficulty is therefore not simply drilling deep enough. Successful groundwater development requires identifying favourable combinations of geology, weathering, fractures, topography and recharge.

Groundwater recharge

Recharge is closely linked to the short rainy season.

Direct infiltration across exposed or poorly permeable uplands can be limited, while runoff concentrates water in lower terrain, seasonal drainage channels, alluvial deposits and favourable fractured zones.

The strongly seasonal rainfall regime means groundwater recharge can vary substantially between years. Quantifying long-term recharge remains an important uncertainty for sustainable groundwater development in Guéra.

Groundwater availability must therefore be evaluated not only in terms of instantaneous borehole yield, but also in terms of seasonal variation and sustainable long-term abstraction.

Groundwater quality

Regional groundwater studies generally describe Guéra groundwater as relatively weakly mineralised compared with strongly saline groundwater systems elsewhere in Chad.

Earlier regional studies cited by Nouradine reported dissolved mineral content generally below approximately 250 mg/L, with bicarbonate-dominated groundwater and relatively low chloride and sulfate concentrations.

More recent geochemical work by Nouradine analysed 211 groundwater samples, principally in and around the Mongo study area. Electrical conductivity ranged from approximately 83 to 1,183 µS/cm, with an overall mean of approximately 427 µS/cm.

Mineralisation increases in some deeper or fractured groundwater systems. Mean conductivity in the study was approximately:

Water source	Mean electrical conductivity
Shallow rural wells	~175 µS/cm
Rural boreholes in fractured basement	~445 µS/cm
Urban boreholes in fractured basement	~526 µS/cm
Wells intersecting both weathered and fractured horizons	~677 µS/cm

These figures describe the sampled areas around Mongo and should not be assumed to represent every groundwater body in Guéra.

Local groundwater chemistry can vary strongly because individual boreholes may intercept different weathered zones, fracture systems and groundwater residence times.

Local salinity

Saline or noticeably mineralised boreholes occur locally in the Guéra region, including around Bitkine.

AOWIS field observations around Bitkine indicate substantial local variation in water quality, with potable and saline groundwater occurring within the same general area. Some saline wells are unsuitable for drinking but remain in use for washing, construction, irrigation or other non-potable purposes.

This local variability is consistent with the discontinuous fractured-basement hydrogeology of the area. Salinity at one borehole does not establish that neighbouring groundwater will have the same composition.

Electrical conductivity, TDS and full chemical analysis should therefore be measured individually for each water point before treatment requirements or desalination systems are designed.

Bitkine and Abtouyour

Bitkine is the capital of the Abtouyour department and one of the main current areas of groundwater investigation in Guéra.

The 2023–2024 work carried out by SEVES, UMR METIS of Sorbonne Université and ANTEA concentrated specifically on the Bitkine area. Pierre Palomar reinterpreted 61 existing electrical-resistivity tomography surveys and associated borehole information to develop a local hydrogeological model and improve future borehole siting.

The work builds directly on Nouradine's regional research, which had concentrated particularly on the Mongo and Barlo areas.

The Bitkine study found that geology and weathering pattern are critical to borehole performance and that geophysical anomalies cannot be interpreted reliably as fractures without considering the geological and geomorphological context.

A dedicated page covers the local conditions and water-supply work:

Regions:Chad/Guéra/Bitkine

Water supply

Much of Guéra depends on boreholes, hand-pumped wells and small piped drinking-water systems.

The combination of low borehole yields, high drilling failure rates, seasonal demand and uncertain recharge makes reliable water supply difficult, particularly during the dry season.

The provincial development plan reported substantial deficits in both rural hand-pump coverage and piped AEP systems.

In the Bitkine area, SEVES and its Chadian partner ADES have been working with the Commune of Bitkine and the regional water authorities through the REAVIA programme.

Recent groundwater-development work reported by SEVES included:

• a new borehole at Bitkine producing approximately 6 m³/h;
• rehabilitation of a borehole at Koubo Adougoul producing approximately 4 m³/h;
• a new borehole at Koubo Adougoul producing approximately 7 m³/h.

These yields are high relative to many of the boreholes represented in the regional datasets.

Hydrogeological research

Recent groundwater research in Guéra has produced an unusually large combined hydrogeological and geophysical dataset for a crystalline-basement region.

Nouradine 2023

Haroun Nouradine's doctoral research combined:

• geological analysis;
• hydrogeology;
• electrical geophysics;
• groundwater chemistry;
• stable isotopes;
• piezometric monitoring;
• machine learning.

The underlying datasets include:

• 798 boreholes;
• 700 EM34 profiles;
• 592 electrical-resistivity tomography profiles;
• 211 geochemical and isotopic groundwater samples.

The work established a conceptual model of Guéra's crystalline-basement aquifers and identified factors controlling borehole productivity.

Full thesis:

Évaluation des ressources en eau dans les aquifères de socle dans la région du Guéra (Tchad), Haroun Nouradine, 2023

Palomar / SEVES 2024

Pierre Palomar's study, carried out within SEVES in collaboration with Sorbonne Université's UMR METIS and ANTEA, applies the regional hydrogeological model specifically to the Bitkine area.

It includes:

• a review of basement-aquifer hydrogeology;
• evaluation of geophysical prospecting methods;
• reinterpretation of existing ERT surveys;
• comparison with borehole results;
• a hydrogeological conceptual model for the Bitkine area;
• recommendations for future groundwater prospecting and borehole siting.

Full report:

Conceptualisation hydrogéologique et recommandations pour les prospections géophysiques et l'implantation des forages — Province du Guéra, Tchad, Pierre Palomar / SEVES, 2024

AOWIS work

Guéra is a current AOWIS field region, with initial work concentrated around Bitkine.

Current work includes documentation of existing water points, seasonal borehole yields, water quality, groundwater salinity, opportunities for cooperation between existing well operators, and options for additional storage, treatment and distribution infrastructure.

The combination of highly variable groundwater productivity and local differences in groundwater chemistry makes the region particularly relevant to integrated water-resource monitoring and planning.

External resources

• Nouradine 2023 doctoral thesis — AOWIS-hosted PDF
• Palomar / SEVES Guéra and Bitkine hydrogeology report — AOWIS-hosted PDF
• Nouradine et al. 2024 — Hydrogeology Journal
• SEVES — hydrogeology and borehole siting in Guéra
• Schéma Provincial d'Aménagement du Territoire du Guéra 2022–2032