Regions:Chad/Guéra/Bitkine

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Bitkine is a town in the Guéra province of central Chad and the capital of the Abtouyour department. It lies immediately north of the Mont Guéra massif, in an area where strongly seasonal runoff from the mountains meets the flatter plains around the town.

Groundwater is the principal dependable water resource, but obtaining productive and consistently potable boreholes is difficult. The local aquifers occur in weathered and fractured crystalline basement and can vary substantially over short distances. Boreholes in the same general area may therefore differ greatly in yield and water quality.

Bitkine has become an important local study area for groundwater prospecting in Guéra. A 2023–2024 hydrogeophysical investigation by SEVES, Sorbonne Université's UMR METIS and ANTEA specifically analysed existing boreholes and electrical-resistivity surveys around the town to improve the siting of future boreholes.

Physical setting

Bitkine lies on the northern side of the Mont Guéra massif. The massif rises abruptly south of the town and forms a major local catchment for seasonal rainfall.

The terrain between the mountain front and Bitkine is crossed by seasonal drainage systems. During the rainy season, runoff from the massif is concentrated into valleys, wadis and branching channels before spreading across the lower terrain north of the mountains.

This relationship between steep uplands, seasonal drainage and low-lying piedmont areas is important for groundwater recharge. Rainfall falling on exposed or relatively impermeable mountain slopes may run off rapidly, while infiltration can become concentrated where this water reaches flatter terrain, weathered rock, sandy deposits and fractured basement.

Geology

The Bitkine area forms part of the crystalline basement of the Guéra massif.

The local geology includes:

• granite;
• biotite granite;
• granodiorite;
• diorite;
• dolerite;
• locally other intrusive and metamorphic rocks.

The wider Guéra basement is dominated by granitoids rather than one homogeneous granite body.

Rock type influences the development of the aquifer. Weathering of granite tends to produce relatively sandy material, while weathering of more mafic rocks such as diorite can produce more clay-rich material. Thick clay-rich alteration can inhibit infiltration, whereas permeable weathered granite together with an underlying fractured horizon can provide favourable groundwater conditions.

The area around Mont Doli, southwest of Bitkine, contains both granitic and dioritic/granodioritic formations and should therefore not be treated as geologically uniform.

Groundwater system

Groundwater around Bitkine occurs mainly in discontinuous hard-rock aquifers.

A simplified local profile consists of:

1. soil, alluvium or superficial deposits;
2. weathered crystalline basement;
3. a fractured or fissured basement horizon;
4. progressively less fractured competent bedrock.

The weathered horizon can provide groundwater storage, while the fractured horizon beneath it can provide the main pathways through which groundwater moves toward a borehole.

Unweathered crystalline rock has very low primary porosity. Productive groundwater therefore depends on the local development and hydraulic connection of weathered material and fractures.

This makes the aquifer highly heterogeneous. A productive borehole does not imply that another borehole a few hundred metres away will encounter the same water-bearing structure.

Hydrogeophysical investigation

Pierre Palomar's study reinterpreted 61 electrical-resistivity tomography (ERT) sites around Bitkine and compared the geophysical profiles with available drilling information.

The study classified hydrogeophysical conditions into several conceptual models.

A and A* represent sites with relatively well-developed weathered and fractured horizons. These were the most favourable classes for obtaining productive boreholes.

B and B* represent conditions where alteration is poorly developed or where production depends more strongly on an isolated fracture in otherwise competent basement.

C represents strong lateral discontinuities such as fractures, dykes, veins or changes in lithology, where the interpretation and drilling result can be less predictable.

In the analysed dataset, A* sites performed particularly well. Approximately 31% of A* sites produced at least 5 m³/h. For A sites, approximately 14% produced at least 5 m³/h and approximately 36% produced at least 3 m³/h.

The results nevertheless show that geophysical prospecting cannot guarantee a successful borehole. ERT must be interpreted together with geology, topography, weathering, drainage and drilling information.

South of Bitkine

One of the most important results of the Bitkine-specific study is the geographic distribution of the hydrogeophysical models.

North of Bitkine, many investigated sites belong to the B* class, in which groundwater production depends largely on deeper fractures.

South of Bitkine, toward the Mont Guéra massif, A and A* sites are more common. Palomar therefore identifies the southern sector as a preferred area for drinking-water prospecting.

This does not mean that any point south of Bitkine will produce water. It indicates that the geological and weathering conditions associated with productive basement aquifers occur more frequently there in the available dataset.

The relationship with the massif is also hydrologically significant. Surface runoff from Mont Guéra is channelled toward lower terrain north of the mountains. Low areas, seasonal drainage corridors and their margins can therefore receive concentrated recharge.

A potentially favourable setting consists of:

• a catchment draining the massif;
• a mountain-front or piedmont position;
• permeable superficial or weathered material;
• a sufficiently developed weathered basement horizon;
• an underlying fractured horizon connected to recharge;
• geophysical characteristics corresponding to an A or A* type structure.

The active centre of a wadi is not necessarily the preferred location for a drinking-water borehole. While it may receive strong recharge, it is also more exposed to flooding and rapid infiltration of microbiological and chemical contamination from surface runoff. A favourable fractured aquifer intercepted from the side of a drainage corridor may provide a better compromise between recharge and sanitary protection.

Groundwater recharge

Recharge is strongly seasonal.

During the rainy season, runoff becomes visible along a dense network of temporary channels draining the Mont Guéra massif. These channels can concentrate water from a catchment considerably larger than the immediate area around a borehole.

The most important recharge may therefore occur during relatively short rainfall events in:

• valleys;
• wadis and talwegs;
• piedmont depressions;
• alluvial deposits;
• permeable weathered zones;
• fractures hydraulically connected to these features.

Quantification of recharge remains a major uncertainty. SEVES notes that the sustainability of the groundwater resource around Bitkine cannot yet be determined confidently because long-term piezometric measurements and systematic records of abstraction volumes are lacking.

Borehole productivity

Groundwater development around Bitkine is difficult even after hydrogeophysical prospecting.

The wider Guéra dataset analysed by Palomar shows that among 137 historical boreholes:

• 47% were unsuccessful during drilling and were not equipped;
• 42% had yields considered too low for connection to a piped drinking-water network;
• only 11% provided approximately 4 m³/h for 8 hours per day.

The Bitkine study therefore concludes that a town of this size cannot normally be supplied from one exceptionally productive borehole. A reliable supply is more likely to require a well field consisting of several moderate-yield boreholes.

Under the REAVIA programme, SEVES and ADES reported a newly developed borehole at Bitkine producing approximately 6 m³/h. Several unsuccessful drillings were also encountered during the same groundwater-development work.

Water quality and salinity

Water quality varies locally around Bitkine.

AOWIS field observations include both potable and noticeably saline groundwater sources within the same general area. Saline water is in some cases unsuitable for drinking while remaining usable for washing, construction or other non-potable purposes.

This spatial variability is compatible with the fractured-basement aquifer system. Nearby boreholes may intersect different fracture networks with different recharge, residence times and geochemical histories.

Regional geochemical work by Nouradine was concentrated primarily around Mongo rather than Bitkine. It therefore provides a useful conceptual framework but should not be used as direct evidence for the chemistry of individual Bitkine boreholes.

The Guéra data show that deeper fractured groundwater is generally more mineralised than shallow groundwater. Prolonged water-rock interaction with silicate minerals can increase dissolved sodium, calcium, magnesium, potassium and bicarbonate. Older or poorly renewed fracture water may therefore become more mineralised than groundwater receiving frequent recent recharge.

This provides one possible explanation for local differences around Bitkine, but the origin of the saline water has not yet been established by adequate local chemical analysis.

In particular, water described locally as salty should not automatically be assumed to be sodium-chloride water. Determining the actual hydrochemical type requires analysis of the major dissolved ions.

Useful measurements include:

Measurement	Purpose
Electrical conductivity (EC)	Rapid indication of total ionic mineralisation
Temperature	Required for comparable conductivity measurements
pH	General water chemistry
TDS	Approximate total dissolved mineral content
Na, K	Major alkali cations
Ca, Mg	Hardness and hydrochemical classification
Cl	Identification of chloride-rich salinity
SO4	Sulfate contribution
HCO3 / alkalinity	Bicarbonate-dominated basement groundwater
NO3	Important indicator of surface contamination
F	Drinking-water quality
Fe, Mn	Common groundwater quality problems
Microbiological analysis	Required to establish drinking-water safety

Electrical conductivity should preferably be measured directly in µS/cm rather than relying only on the calculated ppm value displayed by inexpensive TDS meters.

Seasonal behaviour

Local groundwater availability changes substantially through the year.

AOWIS field observations indicate that some potable boreholes can maintain considerably higher yield after the rainy season and decline during the late dry season. Other local wells provide only moderate yields during the March–June period.

This behaviour makes a single short pumping test insufficient for understanding the long-term capacity of a source.

For important water points, monitoring should include:

• pumping rate;
• pumping duration;
• static and pumping water level;
• electrical conductivity;
• temperature;
• seasonal changes in yield.

For saline or mildly saline boreholes, EC should also be recorded during prolonged pumping. In a fractured aquifer, water quality can change during pumping as the relative contribution of different fractures changes. A declining EC may indicate displacement of stagnant or more mineralised water by a fresher source, while an increasing EC may indicate recruitment of a more mineralised fracture. Such behaviour must be demonstrated by measurement rather than assumed.

Existing water services

Since 2022, SEVES and its Chadian partner ADES have been supporting the water services of Bitkine and surrounding smaller centres.

SEVES reports that existing services frequently experience interruptions or are unable to meet demand, particularly during the dry season, largely because insufficient groundwater has been successfully mobilised.

The REAVIA programme combines groundwater prospecting and drilling with support for the organisation and long-term operation of the water services. Work is conducted with the regional delegation of the Ministry responsible for water and with the Commune of Bitkine.

AOWIS field work

AOWIS work around Bitkine is currently focused on building a more detailed local picture of the available groundwater resources.

Local work includes:

• identifying existing boreholes and wells;
• recording GPS position;
• recording dry- and wet-season yield;
• distinguishing potable and saline sources;
• measuring electrical conductivity and other water-quality parameters;
• documenting borehole depth where available;
• recording ownership and willingness to participate in a shared water-supply system;
• investigating additional groundwater sources;
• evaluating storage, pumping, treatment and distribution options.

Several private well owners have agreed to cooperate in the local water-supply work. Field observations already show that useful groundwater resources are distributed across multiple relatively modest sources rather than one known large aquifer.

The immediate objective is therefore not only to find new water, but also to understand how the existing sources are hydraulically and chemically related and how they could be used together sustainably.

Mont Doli investigation area

AOWIS has identified a preliminary groundwater investigation area east of Mont Doli at approximately:

11.948855° N, 18.187213° E

This location is a prospecting hypothesis only. No successful borehole at this point is currently known to AOWIS.

The site was selected from topography and seasonal satellite imagery. It lies north of the massif in terrain crossed by seasonal drainage from the mountains.

Dry-season imagery shows the mountain-front morphology and drainage depressions. Rainy-season imagery makes the active drainage system considerably clearer and shows runoff being concentrated from the high ground toward larger channels farther north.

The broader setting is consistent with several characteristics considered favourable for groundwater prospecting:

• proximity to the Mont Guéra catchment;
• concentrated seasonal runoff;
• relatively low mountain-front terrain;
• potential recharge through drainage and piedmont deposits;
• location in the southern Bitkine sector where A and A* hydrogeophysical models are more common.

These observations do not establish the presence, depth, yield or quality of groundwater at the candidate point.

Before drilling, the site and the adjacent drainage corridor should be investigated geophysically.

A useful survey would include ERT profiles crossing:

1. the principal seasonal drainage route;
2. its flanks;
3. the original candidate point.

This would allow comparison of the weathered and fractured basement beneath each setting.

For a potable-water borehole, a productive target on the flank of the principal drainage corridor may be preferable to a location directly within the active channel. This could retain hydraulic connection to the recharge area while reducing exposure to direct floodwater, erosion and surface contamination.

Any final drilling location should be selected from the combined evidence of:

• geology;
• topography;
• drainage;
• ERT;
• nearby borehole results;
• nearby groundwater chemistry;
• sanitary risks.

Research priorities

The main remaining uncertainties around Bitkine concern both quantity and quality.

A useful local groundwater programme should establish a consistent dataset for each important water point containing:

Parameter	Unit / information
Position	latitude / longitude
Borehole depth	m
Static water level	m
Pumping water level	m
Pumping rate	m³/h
Seasonal yield	m³/h
Electrical conductivity	µS/cm
Temperature	°C
pH	—
Potability / use	drinking, washing, irrigation, construction, etc.
Major-ion chemistry	mg/L
Microbiology	laboratory result

Repeated measurements are particularly important. A single yield or EC value does not describe the long-term behaviour of a fractured basement borehole.

Once sufficient observations are available, the combination of groundwater chemistry, borehole performance, ERT interpretation and terrain analysis should allow the local water sources to be grouped into different hydrogeological systems.

Hydrogeological research

The principal recent technical sources for the Bitkine groundwater system are the regional doctoral work of Haroun Nouradine and the later Bitkine-focused study by Pierre Palomar.

Nouradine 2023

Nouradine developed a regional conceptual model for the crystalline-basement aquifers of Guéra using geological, hydrogeological, geophysical, geochemical and isotopic information.

Although the detailed geochemical investigation concentrated primarily around Mongo, the work provides the principal regional framework for understanding groundwater occurrence, recharge, productivity and mineralisation.

Full thesis:

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

Palomar / SEVES 2024

Palomar's work specifically extends this approach to the Bitkine area.

The study:

• reinterprets 61 existing ERT sites;
• compares the geophysics with drilling results;
• develops a local hydrogeophysical classification;
• identifies characteristics associated with productive boreholes;
• demonstrates a geographic difference between the northern and southern Bitkine sectors;
• gives practical recommendations for future groundwater prospecting.

Full report:

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

External resources

• Guéra regional overview
• Nouradine 2023 doctoral thesis — AOWIS-hosted PDF
• Palomar / SEVES Bitkine hydrogeology report — AOWIS-hosted PDF
• Nouradine et al. 2024 — Hydrogeology Journal
• SEVES — groundwater prospecting in Guéra
• SEVES — REAVIA groundwater development at Bitkine and Koubo Adougoul