The Transboundary Aquifer Productivity Map of the Lower Chari-Logone River Basin

Project: Sustainable Management of Groundwater Resources in the Lake Chad Basin

Report N° 16

Berlin, November 2020

On behalf of:

Author: Kolja Bosch and Michaela Rückl

Commissioned by: Federal Ministry for Economic Cooperation and Development (Bundesministerium für wirtschaftliche Zusammenarbeit und Entwicklung, BMZ)

Project: Sustainable Management of Groundwater Resources in the Lake Chad Basin

BMZ-No.: 2018.2225.3

BGR-No.: 05-2409

Elvis link:

BGR-Archive No.:

Date: November 2020

Content

Summary ...... 1 1. Introduction ...... 2 2. Study area ...... 3 3. Lithological map ...... 4 3.1 Analogue maps ...... 4 3.2 Digital processing ...... 5 3.3 Lithological units ...... 9 3.4 Attribution of the lithological units in the harmonized lithological map ...... 13 4. Aquifer productivity mapping ...... 16 4.1 Methodology ...... 16 4.2 Data sources and quality ...... 18 4.3 Analysis ...... 20 4.3.1 Classification results following the approaches of Krásný and Struckmeier & Margat ..... 20 4.3.2 Subdivision of alluvial deposits NE of Bama Ridge into two aquifer productivity domains 23 5. Conclusions ...... 25 References ...... 26 Appendix ...... 28

List of Figures

Figure 1: Location of the study area within the Lake Chad Basin ...... 3 Figure 2: Analogue lithological maps of the study area as basis for a harmonized lithological map ...... 4 Figure 3: Example of self-connected polygon and manual correction of the topological error ...... 5 Figure 4: Manual correction of remaining gaps in the merged dataset ...... 6 Figure 5: Manual addition of ancient ergs mapped by Péronne & Dumort (1968) ...... 7 Figure 6: Changes along the transition zone of the Cameroonian maps ...... 8 Figure 7: Chari deltas and beach ridge deposits (“Cordons sableux”) (Pias 1967) ...... 12 Figure 8: Harmonized lithological map of the lower Chari-Logone River Basin ...... 15 Figure 9: Aquifer classification after Struckmeier & Margat (1995), modified after Bäumle (2011)...... 17 Figure 10: Temporal distribution of pumping test data ...... 18 Figure 11: Cumulative relative frequency of specific capacity (Sc) and yield (Q) values...... 21 Figure 12: Conceptual hydrogeological cross section ...... 24 Figure 13: Correlation between Index Y value and water table depth (WTD)...... 24 Figure 14: Simulation results for the testing of different water table depth (WTD) contours as outlines for an area where > 30% of the boreholes indicate low aquifer productivities...... 24

List of Tables

Table 1: Attribution of harmonized lithological units ...... 13 Table 2: Classification of the hydraulic heterogeneity of a lithological unit...... 16 Table 3: Aquifer productivity classification based on transmissivity, specific capacity and yield magnitude, adapted from Krásný (1993) and Struckmeier and Margat (1995)...... 17 Table 4: Parameters and results of productivity classification after Krásný (1993) and Struckmeier & Margat (1995)...... 22

List of Abbreviations

BGR Federal Institute for Geosciences and Natural Resources

GIZ German Corporation for International Cooperation GmbH

IHME International Hydrogeological Map of Europe

LCBC Lake Chad Basin Commission

WTD Water table depth (depth to groundwater table)

Summary

The transboundary region of the lower Chari-Logone River Basin is located in the south of the Lake Chad Basin and extends over the south-west of Chad, the north of Cameroon and a small portion of eastern Nigeria. The study area is characterized by the Yaéré and Naga floodplains and the Logone and Chari Rivers, representing the main feeders which supply the Lake Chad. Due to the enhanced water availability, the lower Chari-Logone River Basin represents an ecologically and economically important portion of the Lake Chad Basin. In the context of changing environmental parameters and dynamics in water demand due to population growth, urbanization, industrialisation and agricultural irrigation, the investigation of groundwater resources in this area is of major interest.

An aquifer productivity map of the transboundary region was developed based on available lithological maps and borehole data. The lithologies of the uppermost aquifer as well as cover layers were identified and attributed following the scheme of the International Hydrogeological Map of Europe (IHME) outlined in Duscher et al. (2015). The borehole data of over 1500 boreholes were analysed and classified following the classification method after Krásný (1993). The transmissivity index Y for each borehole was calculated based on available measurements of specific capacity or yield. The range and distribution of the index Y values determined the productivity class of the lithological units. The classification method after Krásný (1993) was translated into the aquifer productivity categories of the Standard Legend for Hydrogeological Maps (SLHyM) by Struckmeier & Margat (1995) and presented in an aquifer productivity map.

For the Nigerian and Cameroonian parts of the study area, the results are of limited significance due to the lack of borehole information. Here, the productivity classes were inferred based on lithological analogies.

Keywords: aquifer productivity map, lithological map, Chari-Logone River Basin, Lake Chad Basin, hydrogeology

1. Introduction

The Federal Institute for Geosciences and Natural Resources (BGR), within the programme “Sustainable Water Management of the Lake Chad Basin”, supports the Lake Chad Basin Commission (LCBC) for strengthening regional groundwater management in the Sahel zone. The programme is financed by the German Federal Ministry for Economic Cooperation and Development (BMZ) and consists of the BGR module “Sustainable Management of Groundwater Resources in the Lake Chad Basin” and the GIZ (Gesellschaft für Internationale Zusammenarbeit GmbH) module “Applied water resources management in the Lake Chad Basin”.

The current project phase is scheduled until June 2022 and the BGR module focuses amongst others on the implementation of technical solutions for a sustainable groundwater management in the Lake Chad Basin and the elaboration of local hydrogeological maps. The present report documents the data availability, the data treatment and the data analysis for the aquifer productivity map of the lower Chari- Logone River Basin. In the previous project phase aquifer productivity maps were realized for the Salamat and Komadugu Yobé regions (Rückl 2018 a, b).

Aquifer productivity maps are a common tool to determine regions, where wells for water supply can be drilled. On a regional scale, these maps can help to get an overview of potentially vulnerable areas, areas suitable for managed aquifer recharge or regions, where extensive pumping for an increased water demand may be possible. In terms of water resources availability and related ecological and economical importance the lower Chari-Logone River Basin has an outstanding role in the Lake Chad Basin. The study area is characterized by the Chari and the Logone Rivers representing the main feeders of surface water for the Lake Chad and the Yaéré and Naga floodplains in Cameroon and Chad, respectively. These hydrological features are closely linked to the available groundwater resources in the region with the floodplains acting as major groundwater recharge zones. Unmanaged withdrawals of groundwater therefore represent a risk for both surface water and groundwater resources. Thus, the present aquifer productivity map serves as decision making tool for water management in the context of changing environmental parameters and dynamics in water demand due to population growth, urbanization, industrialisation and agricultural irrigation.

The elaboration of the aquifer productivity map consisted of three major steps described in detail in the present report:

1) Elaboration of a harmonized transboundary lithological map based on the existing lithological maps of the countries Chad, Cameroon and Nigeria. Based on this map, fissured and porous media were differentiated, 2) Determination of aquifer productivity proxies (specific capacity or yield) from data bases and literature (point information derived from more than 1500 boreholes), 3) Assignment of productivity classes to the different lithologies following the approaches by Krásný (1993) and Struckmeier & Margat (1995).

2. Study area

The study area of the lower Chari-Logone River Basin is located between 10-13° N latitude and 14- 16° E longitude (Figure 1). Further details on climate, hydrology and hydrogeology can be found in the previous technical report n°10 “Groundwater - Surface Water Interaction in the Lower Logone Floodplain” of the LCBC-BGR project (Vassolo et al., 2016).

The geology of the study area is characterized by the transition of the Precambrian basement to the sedimentary fill of the Lake Chad Basin, while Precambrian basement and Cenozoic intrusions are only exposed in Cameroon, in the SW of the study area (Mandara Mountains). The sedimentary fill of the basin comprises the Continental Terminal, the Pliocene and the Quaternary. The former two are not exposed and covered by quaternary sediments. The fluvio-lacustrine and aeolian sedimentary sequence is of considerable thickness of around 250-365 m in the south (Yagoua) and in the middle (Waza) of the study area. In the north, next to the Lake Chad, its thickness reaches up to 700 m (Detay, 2000).

The Continental Terminal deposited during the upper Eocene/Oligocene to Miocene (Schneider & Wolff, 1992). It is a predominantly detrital series with sandy layers. In some parts, where cementation of the sands occurs, sandstones can be observed. Intercalations with clay often become predominant in the lower part of this series. The general thickness of the Continental Terminal ranges between 200 m and 400 m (Detay, 2000).

The lower Pliocene is characterized by predominantly sandy sediments with interbedded clays and a thickness varying between 50 m and 80 m. The upper Pliocene is constituted of clays with few interbedded sands. Its thickness varies between 100 m and 200 m (Detay, 2000).

Quaternary sediments are composed of alternating sandy and clayey layers related to the trans- and regression cycles of the Lake Chad. The thickness of the quaternary sequence generally varies between 30 m and 70 m with an increasing trend from south to north (30 m at Yagoua, 50 m at Zinah, 60 m at Logone Birni) (Schneider & Wolff, 1992; Biscaldi, 1970).

Figure 1: Location of the study area within the Lake Chad Basin.

3. Lithological map

3.1 Analogue maps Four different maps consisting of 8 map sheets were used to create a harmonized lithological map of the study area. Their spatial extent and reference is shown in Figure 2. The four maps vary in their topic (hydrogeology, geology and hydrology), scale and degree of detail. Nevertheless, they all contain lithological information with varying spatial resolution corresponding to the different scales of the respective maps (1: 200 000, 1: 250 000 and 1:500 000).

Figure 2: Analogue lithological maps of the study area as basis for a harmonized lithological map.

3.2 Digital processing Digitization of the maps for the study area was contracted to the company DGIS Service in Radeberg, Germany. As geographic projections are rarely documented on old analogue maps, it was agreed that all maps were to be georeferenced to the reference system of WGS 84 and provided as .tif-files. Although the original reference system was unknown, the result of the georeferencing was adequate and no further spatial adjustment was needed.

File geodatabases were created in ArcGIS, containing all displayed elements of the maps in different feature classes. To be able to import the created datasets to the Open Source Software QGIS, every feature class was delivered in shapefile format as well.

As QGIS sets different topological rules to polygon layers than ArcGIS, further corrections in the original datasets of the maps were necessary to eliminate the topological errors of self-connected polygons (Figure 3). Only if the datasets of the individual maps are free of self-connected polygons the QGIS tools of the first two steps of the merging process are working properly:

Figure 3: Example of self-connected polygon (left) and manual correction of the topological error (right).

Difference tool: Removal of overlapping parts of the maps

The map of (Biscaldi, 1970a) was used with its whole extent as its scale is the largest. Overlapping parts of the three other maps were removed by the difference tool. The same process was applied to the map of Péronne et Dumort (1968) to eliminate the overlapping parts with the hydrological map of Nigeria (Department of Hydrogeology and Hydrology, 1992).

Merge vector layers: Combine individual datasets to one layer

After creating an identic structure (same column number and column head) of the attribute tables for each map, the datasets were merged to create a dataset containing the polygons and respective attributes of all original maps.

Manual corrections: Filling of arising gaps

For the manual correction process the different lithological descriptions which are available on the digitized maps have been translated into 17 IDs shown in Figures 4, 5 and 6. The respective lithology of every relevant ID is given in the specific process descriptions here below.

The transition zone of the maps of Biscaldi (1970a) and Torrent/ Schneider (1966) is characterized by the recent alluvial sediments of the Chari and Logone rivers. Therefore, in case of small remaining gaps between the maps, the polygons of the “recent alluvial sediments” (ID 2) were enlarged to fit seamlessly to the ones of the adjacent map. Satellite images were used to further validate the location of the riverbed. The same process was done for the transition zone between the maps of Biscaldi (1970a) and

5 of the Department of Hydrogeology and Hydrology (1992). Here, the “recent alluvial sediments” (ID 2) belong to the river El Beid.

The gap with the biggest extent is located in the NW of the study area, in the area of the former Lake Chad (Figure 4). This gap results from different extents of Lake Chad during the mapping periods of the different geological maps. When the Chadian and Cameroonian maps were created, the Lake Chad had still its large extent of pre-drought times (severe drought years in the 1970s to 1980s). The Nigerian map (Department of Hydrogeology and Hydrology, 1992) in contrast displays lithological units up to the current lakeshore. A detailed differentiation of lithological units as in Nigeria cannot be provided for the respective areas in Chad and Cameroon. Nevertheless, the gap was filled based on the comparison with satellite images and a flood frequency map to separate the marshland of Lake Chad from the older sedimentary deposits further south.

Two further big gaps were identified on the Nigerian eastern boundary (A and B in Figure 4). Units described in the map of the Department of Hydrogeology and Hydrology (1992) were used to fill the gaps based on visible structures on satellite images.

Figure 4: Manual correction of remaining gaps in the merged dataset.

Manual corrections: Adding structures

A comparison of the maps of Cameroon revealed structures of an ancient erg (ID 16) only mapped by Péronne & Dumort (1968) but not by Biscaldi (1970a). The information was manually added by adjusting the respective polygons (Figure 5).

Figure 5: Manual addition of ancient ergs mapped by Péronne & Dumort (1968).

Manual correction: Thematic adjustment at the map sheet borders of Biscaldi (1970) and Péronne & Dumort (1968)

Several corrections concerning the geometry and the lithological description were done for the transition of the Cameroonian maps of Biscaldi (1970a) and Péronne & Dumort (1968). On the one hand, the geometry of several polygons had to be manually modified based on satellite images in order guarantee a seamless transition without sharp edges (Figure 6, map A). On the other hand, it occurred that lithological descriptions of matching polygons were not identical (Figure 6, map B). At the map sheet borders the polygons of the two lithologies “river alluvium” (ID 2) and “recent clayey series” (ID 6) described by Biscaldi (1970a) match with the polygons of the more generalized lithological description “river alluvium” (ID 2) mapped by Péronne & Dumort (1968). In order to establish a continuous lithological description the matching polygons at the map sheet borders were merged and the more generalized lithological description “river alluvium” (ID 2) was assigned. This generalization was only applied at the map sheet borders. For all other parts of the lithological map the differentiation between “river alluvium” (ID 2) and “recent clayey series” (ID 6) was retained.

Figure 6: Changes along the transition zone of the Cameroonian maps: smoothing of abrupt transitions (A) and changing of lithological IDs in case of non-matching lithological information (B).

3.3 Lithological units Based on the original map units and on further literature 12 outcropping lithological units were identified and shown in the harmonized lithological map (Figure 8):

Crystalline basement [b] – Precambrian and Cenozoic –

Basement rocks are mainly exposed in the Mandara Mountains in the Cameroonian part of the study area, between Kaele and Mora. They are constituted of the volcano-sedimentary group of Maroua comprising migmatites, anatexic granites and plutonic calco-alcaline, sub-alcaline and alcaline intrusions. The Precambrian formations are in discordance with Cenozoic syenite and granite intrusions and their associated volcanic rocks which are exposed at Gréa, Balda, Waza and Golda/Zouelva (Dumort & Péronne 1966; Péronne et Dumort 1968; Ngounou Ngatcha, 1993).

Alluvial deposits [a1-a2] NE of Bama Ridge – Pleistocene “ancient reworked series” –

The alluvial deposits [a1-a2] summarize sandy sediments originating from the Pleistocene reworking of Continental Terminal series. Pias (1970a) describes these alluvial sediments as a mainly sandy and fluviatile “older reworked series” (“série ancienne remaniée”). Reworking took place during humid periods by submersion during the succeeding Lake Chad high stands and by abrasion by streams of the Quaternary. In the area between Bongor, Dourbali, Bokoro and Miltou the deposition of this series is associated with the 1st Chari Delta (see Figure 7). Lake Chad water level during that period ranged between 380-400 m a.s.l. and can be related to the 1st transgression cycles of the Lake Chad. Micro- dune structures and erg remnants (5-10 m high) in Nigeria and Cameroon document aeolian reworking during dry phases (Pias, 1967; Pias 1970a). Biscaldi (1970a) describes this series as “older alluvium” (“alluvions anciennes plus ou moins remaniées”) [a1] and “recent sands” (“sables récentes”) [a2]. According to this study both series are difficult to distinguish and therefore were mapped as [a1-a2]. Where lacustrine deposits [l2] occur, the “older alluvium” [a1] and the “recent sands” [a2] are separated by lacustrine deposits [l2]. In Cameroon, the thickness of the alluvial sediments [a1-a2] is generally less than 10 m but locally sums up to 20 m (Biscaldi, 1970a). In the region between Peté and Goudoum- Goudoum, Morin (2000) describes those fluviatile sediments as fine sands, clayey sands, and clays of around 14-27 m thickness, superposing lacustrine deposits. In Nigeria, this series is described as sands, loams and silts (Department of Hydrogeology and Hydrology, 1992).

Alluvial deposits [a1-a2] SW of Bama Ridge – Pleistocene “ancient reworked series” –

Alluvial deposits [a1-a2] southwest of Bama Ridge were less affected by alluvial reworking during Lake Chad transgressions because Bama Ridge represents the former coastline of Lake Chad (3rd transgression of Lake Chad). Structures of Pleistocene aeolian reworking of the Continental Terminal series are preserved in form of flattened dunes south of Yagoua, belonging to the Erg de Doukoula. The Erg extended over large areas of the Chadian basin which nowadays are covered by younger sediments. East of Bama Ridge the reworked erg sediments prolong under the younger lacustrine sediments [l2,l3]. The Erg consists of homogeneous fine sands ranging in thickness between one to tens of meters (0.6 m to tens of meters at Doukoula, 29 m to 60 m next to Bougaye). Interrupted by several alluvial channels, the sandy plane extends further south to the “Bec-de-Canard” of the Cameroon national border (Morin, 2000).

Lacustrine deposits [l2] – Pleistocene “ancient clayey series” –

Superposing the alluvial deposits of [a1] there is the clayey series of lacustrine deposits [l2], sedimented during a humid phase (dating 33 300 years B.P.) (Biscaldi, 1970b). Deposition of this series is associated with the 2nd transgression of Lake Chad (Pias, 1970a). At some places, intercalations with “recent sands” [a2] can be observed. Lacustrine deposits [l2] have an average thickness of about 10 m and are characterized by post-genetic calcium carbonate enclosures (Biscaldi, 1970b). Pias (1970) describes thicknesses of 5 m to 6 m along the Chari and Logone Rivers and thicknesses of 3 m to 4 m around Bongor and even less in the Logone floodplain (north of Bongor) where the series is covered by younger lacustrine deposits [l3] or alluvial deposits [a2,a3]. The sedimentation of lacustrine deposits [l2] is characterized by a coarse grained phase in the beginning (gravel, sands) but generally exhibits a clayey- sandy texture (Pias, 1970a).

Ancient dunes [ds] – Pleistocene –

In Cameroon, ancient dunes oriented in the direction of SW-NE document aeolian reworking of alluvial deposits [a1-a2] during an arid period (Biscaldi 1970b). Most of the dunes are related to the Erg de Kalfou (20.000 - 15.000 B.P.), whose main dunes are formed in the area between Moulvoudaï, Doukoula and Yagoua (Figure 8). It consists of aeolian deposits of 10-20 m thickness which are built of very homogenous fine sands, close to silts. Further north, between the Bama Ridge and Waza, ancient dunes consist of more or less sorted sands (Morin, 2000).

Lacustrine deposits [l3] – Holocene “recent clayey series” –

The deposition of the clayey lacustrine deposits [l3] results from the 3rd transgression of Lake Chad up to an altitude of 310 to 320 m a.s.l (5400-12000 years BP) (Biscaldi,1970b and Pias, 1970a). These deposits are in majority characterized by clays which settled by decantation. In the vicinity of shorelines and river mouths sedimentation occurred under the influence of weak currents and therefore becomes clayey to sandy. Carbonate enclosures can be observed but with weak abundance compared to lacustrine deposits [l2] (Pias, 1970a). Torrent (1966) describes the lacustrine deposits in the Chadian part of the study area as sediments composed of clays, sandy clays and silts. In the inundation plains of the Logone, the lacustrine deposits [l3] overlie the lacustrine deposits [l2] forming together a clayey sequence of 3 m to 4 m, which in some parts is separated by the “recent sands” [a2] (Pias, 1970a). Following Biscaldi (1970b), lacustrine deposits [l3] are of relatively low thickness (1.5 m and less) in the Yaéré floodplain and even lower thicknesses further north. Morin (2000) refers to a thickness of up to 7 m within the Yaéré floodplain.

Beach ridge deposits of Bama Ridge [a3(c2)] – Holocene –

Beach ridge deposits [a3(c2)], known as “cordon sableux” or “Bama Ridge”, indicate the former coasts of the Lake Chad with a coastline between 310 and 320 m a.s.l.. They were deposited in association with the 3rd transgression of Lake Chad and appear from Niger to Chad (Pias, 1970b). In Cameroon the orientation of the structure is SE-NW (“cordon sableux de Yagoua-Limani”) and ranges from Yagoua in the southeast to Limani in the northwest, where it continues into the Nigerian part of the study area. In the Chadian part, the structure is found at Bongor and continues with a SW-NE orientation, which then turns into a S-N orientation between the Chari River and Dourbali (Figure 7). The structure is interrupted by several crossing streams. Especially in the Chari-Logone depression north of Bongor, the structure is very fragmented (Pias, 1970a). The “cordon sableux” locally splits up into three parallel structures with a width of 0.5 km to 4 km and sediment thickness of about 7 - 20 m. Sediments consist of coarse sands on the bottom, while their composition gets finer up to the top of the structure where fine sands confirm an aeolian origin of the deposits (Morin, 2000). Following Pias (1970a), the composition of these mostly sandy deposits is relatively heterogeneous due to the high variety of sediment sources originating from reworked material of different streams. In Nigeria, the beach ridge deposits known as “Bama Ridge” are described as sands of very high permeability (Department of Hydrogeology and Hydrology, 1992).

Alluvial deposits [a3] – Holocene “recent sandy series” –

Alluvial deposits [a3] during the 3rd transgression of Lake Chad are mainly documented in the Chadian part of the study area and are characterized by a complex alternation of sands and clays, while sands are dominating (Schneider & Wolff, 1992). Pias (1967,1970a/b) describes this series as “recent sandy series” (“série sableuse récente”) which is associated with the 2nd Chari delta (Figure 7) and which is constituted of river channels and levees disrupting the extension of preceding lacustrine sediments [l2,l3]. Sediments of this series originate from reworking of the alluvial sediments [a1-a2] and from the weathering of the granitic basement. The thickness of “recent sandy series” in the Logone-Chari-Basin generally varies between 4 m and 5 m (Pias, 1970a). In Cameroon, Biscaldi (1970b) describes these sediments as “recent alluvial sediments” (“alluvions récentes”) characterized by clayey to silty stream banks.

Alluvial deposits of Mandara Mountains piedmont plain [a3(p)] – Holocene –

The piedmont plain spreading N and SW of Maroua, between the Mandara Mountains and the Bama Ridge, is characterized by alluvial quaternary sediments originating from paleo-rivers (“mayos”) and derived from the Mandara Mountains. These deposits can be related to the “recent quaternary sedimentation” (“Quaternaire recent”) (Detay, 2000). The thickness of the alluvial series above the basement varies between 0 m and 50 m. It is characterized by a strong heterogeneity of facies and a domination of sandy clays (Tillement, 1969). Paleo-channels and lenses with coarse grained sands can be observed (Detay, 2000). In Nigeria, the deposits are described as alluvial fans of sandy, silty and partly clayey composition (Department of Hydrogeology and Hydrology, 1992).

Beach ridge deposits [a3(c1)] – Holocene –

North of N’Gouma (Cameroon) and in the southeast of Tourba (Chad) another discontinuous sandy barrier (“cordon sableux”) with W-E orientation can be found (Figure 7 and Figure 8). It is associated with the 4th Lake Chad transgression (2500 to 1500 B.P.) with maximum lake levels around 287-290 m a.s.l. (Pias, 1967) or 290-300 m a.s.l. (Biscaldi (1970a). The sediments of this sandy barrier exhibit a very low percentage of clay (4-6 %) and silt (2 %) indicating mainly aeolian reworking of alluvial deposits [a1-a2] (Pias, 1970a).

Beach deposits (reworked dunes) [d1] – Holocene –

Beach deposits north of the Beach ridge deposits [a3(c1)] formed during the 4th Lake Chad transgression. They are composed of more or less clayey fine sands in the Cameroonian part of the study area and reach a thickness of more than 3 m at Makari (Morin, 2000). Schneider (1966) and Biscaldi (1970a) describe these sands as dune sands which were reworked during the lacustrine stages of the Lake Chad.

Alluvial and lacustrine deposits [a4] – Holocene “Sub-present to present series” –

Alluvial and lacustrine deposits [a4] deposited since the 4th lake Chad transgression in the Logone- Chari plain. They are denominated “sub-present to present series” (“série subactuelle à actuelle”) comprising sediments of the 3rd and the 4th Chari delta (Figure 7) (Pias, 1967; Pias, 1970a,b). This series consists of heterogeneous sediments forming the river banks of the present rivers (e.g. El Béid, Serbéwel) and filling depressions with dominantly fine grained deposits (fine sands, silts and clays). Purely sandy facies are rare but can be observed north of N’Djamena. The thickness of the river banks reaches 3 m to 4 m. In the depressions, silty facies dominate and the deposits exhibit low thicknesses of generally 0.2 m to 0.4 m. These deposits overlie either the alluvial deposits [a1-a2] or the lacustrine deposits [l2,l3]. Recent lacustrine series (“formations lacustres actuelles”) can be observed in the marshland of the Lake Chad. They are clayey-silty at the surface and become more clayey and laminated with depth (Pias, 1970a,b).

The alluvial and lacustrine deposits [a4] comprise modern sediments which are controlled by a semi- arid climate, relatively low flow velocities and external aeolian sediment input (silts, sands) (Pias, 1970a). Modern rivers are mainly deposing sands in their riverbeds and clayey to silty material in pools (Biscaldi, 1970a).

Figure 7: Chari deltas and beach ridge deposits (“Cordons sableux”) (Pias 1967).

3.4 Attribution of the lithological units in the harmonized lithological map Based on the lithological description in the original maps and further literature, the 12 lithological units were subdivided into 19 lithological descriptions (see Table 1 column “Lithological unit and column “Lithology IHME_lvl1”). In order to establish a harmonized degree of lithological description, the aggregation scheme after Duscher et al. (2015) has been applied, resulting in a level 2 description (main component plus major side component) and a level 3 description (main component) (see column “Lithology IHME_lvl2” and “Lithology IHME_lvl3”). On the harmonized transboundary lithological map (Figure 8) the level 2 description is used with the following restrictions:

- “Undifferentiated volcanic rocks” and “volcanic rocks (acid)” were summarized in the category “undifferentiated volcanic rocks” - “Undifferentiated metamorphic rocks” and “undifferentiated metamorphic/magmatic rocks” were summarized in the category “undifferentiated metamorphic rocks”.

The sources of lithological description and the original map symbols are documented in the last two columns of Table 1.

Table 1: Attribution of harmonized lithological units.

Lithology Lithology Lithology Lithological unit Source map Orig. symbol IHME_lvl1 IHME_lvl2 IHME_lvl3 Silts, clays, Silts, clays Silts FMARD_1992/93 26, 34, 44 sands Schneider_1966 a4

” Alluvial and lacustrine es deposits Torrent_1966 a4 Sands, silts, Sands, seri [a4] Sands clays clays

present to present

- Biscaldi_1970 a4

sub „ FMARD_1992 54, 63, 64

Beach deposits Schneider_1966 d1 Sands, (reworked dunes) Sands, clays Sands clays [d1] Biscaldi_1970 d1 Schneider_1966 a3 Beach ridge deposits Sands Sands Sands Biscaldi_1970 a3(c1) [a3(c1)] FMARD_1992 71

Alluvial deposits of Péronne_1968 a “ Sands, Mandara Mountains Sands, clays Sands clays piedmont plain [a3(p)] FMARD_1992 42a

Holocene Schneider_1966 a3 Alluvial deposits Sands, Sands, clays Sands Torrent_1966 a3 [a3] clays

recent sandy series „ Biscaldi_1970 a3 Schneider_1966 a3 Torrent_1966 - Beach ridge deposits Sands Sands Sands Biscaldi_1970 a3(c2) [a3(c2)] Péronne_1968 ds FMARD_1992 71a Torrent_1966 l3

Biscaldi_1970 l3 Lacustrine deposits Clays, silts, Clays, silts Clays [l3] sands series“ Péronne et Dumort_1968 a1

„recent clayey FMARD_1992 15

Lithological unit IHME_lvl1 IHME_lvl2 IHME_lvl3 Source map Orig. symbol Ancient dunes Sands Sands Sands Péronne_1968 ds [ds]

Torrent_1966 l1

Lacustrine deposits Clays, Clays, sands Clays [l2] sands

series“ Biscaldi_1970 l2

„ancient clayey

Alluvial deposits Biscaldi_1970 a1-2(d1)

[a1-a2] Sands Sands Sands ” (SW of Bama Ridge) Péronne_1968 s

Pleistocene

series Schneider_1966 a1

Torrent_1966 a1-2

reworked Alluvial deposits Sands, [a1-a2] Sands, clays Sands Biscaldi_1970 a1-2(d1) clays (NE of Bama Ridge) Péronne_1968 s

„ancient 33, 42, 44a, FMARD_1992 53, 53a, 74 Plutonic rocks Plutonic o, γ, γ1, γ2, Plutonic (acid to rocks Péronne_1968 γ3, γ3G, σ2, rocks intermed.) (acid) σ3 Undiff. Undiff. Volcanic volcanic Péronne_1968 Θ, α volcanic rocks rocks rocks Volcanic Volcanic rocks Volcanic rocks Péronne_1968 τ, δ Crystalline basement (acid to rocks (acid) (Magmatic and intermed.) metamorphic rocks) Quartzite Quartzite Quartzite Peronne_1965 ζ2, ξe, Q [b] Gneiss Gneiss Gneiss Peronne_1965 ζ, Q2, M2 Undiff. Undiff.

Precambrian and Cenozoic Precambrian Undiff. met. met. met. Peronne_1965 A, So, M1, γA rocks rocks rocks Undiff. Undiff. Undiff. met. / met. / met. / Biscaldi_1970 Aff_du_socle magm. rocks magm. magm. rocks rocks

Figure 8: Harmonized lithological map of the lower Chari-Logone River Basin.

4. Aquifer productivity mapping

4.1 Methodology The Standard Legend for Hydrogeological Maps by Struckmeier & Margat (1995) provides a qualitative classification of aquifer productivity based on aquifer type and flow type (Figure 9). For the present analysis, to add a quantitative component to the qualitative differentiation, the classification scheme after Krásný (1993) was translated into the Struckmeier & Margat (1995) categories (Table 3).

Krásný (1993) defined a logarithmic index Y which can be calculated either from the transmissivity, the specific capacity or the yield as follows:

3 푌푇 = log⁡(10 ∗ 푇) T in [m²/d] ( 1 )

6 푌푆푐 = log⁡(10 ∗ 푆푐)⁡ Sc in [L/sm] ( 2 ) 5 푌푄 = log⁡(2 ∗ 10 ∗ 푄) Q in [L/s] ( 3 )

with T = Transmissivity [m²/d], Sc = Specific Capacity [L/sm] and Q = Yield [L/s].

According to Krásný (1993), the index Y is calculated for each borehole of a respective lithological unit. For the whole set of boreholes x̅ (arithmetic mean of the index Y), s (standard deviation) and the 1- sigma confidence interval I [x̅ +s, x̅ -s] is calculated (Chapter 4.3.1). The index Y values lying within the confidence interval are then related to the classes of transmissivity magnitude (Table 3). Based on the percentage of index Y values falling into one of the respective classes, following nomenclature scheme is applied to classify the aquifer:

 > 70 % of index Y values fall into one class: The class is listed.  Index Y values are distributed over several classes, in each of which they are represented by 30 - 70 %: Both (all three) classes are listed according to their rank.  10 - 30 % of index Y values fall into one class: The class is added in parentheses.  < 10 % of index Y values fall into one class: The class is not listed.

E.g.: Class II = 45 %, Class III = 35 %, Class IV = 20 %  Nomenclature: II-III (-IV) Class II = 80%, Class III = 15 %, Class IV = 5%  Nomenclature: II (-III)

Based on the range of the standard deviation s of the index Y, the hydraulic heterogeneity of the lithological units is determined (Table 2):

Table 2: Classification of the hydraulic heterogeneity of a lithological unit, modified after Krásný (1993).

Standard deviation s of the index Y Class Hydraulic heterogeneity < 0.2 a Homogeneous 0.2 – 0.4 b Slightly heterogeneous 0,4 – 0.6 c Fairly heterogeneous 0.6 – 0.8 d Considerably heterogeneous 0.8 – 1.0 e Very heterogeneous > 1.0 f Extremely heterogeneous

Figure 9: Aquifer classification after Struckmeier & Margat (1995), modified after Bäumle (2011).

Table 3: Aquifer productivity classification based on transmissivity, specific capacity and yield magnitude, adapted from Krásný (1993) and Struckmeier and Margat (1995).

Classes of Index Trans- Specific Transmissivity Category Y missivity Capacity Yield Supply Magnitude (Struckmeier Potential and Margat, (Potential [-] [m²/d] [L/sm] [L/s] 1995) Productivity)

Withdrawals of great regional I: Very high > 7 > 1000 > 10 > 50 importance A/C Withdrawals of regional II: High 7 - 6 1000-100 10 - 1 50 - 5 importance

Withdrawals for local water III: Intermediate 6 - 5 100 - 10 1 - 0.1 5 - 0.5 supply (smaller communities, B/D small scale irrigation etc.) Smaller withdrawals for local water supply (supply through IV: Low 5 - 4 10 - 1 0.1 - 0.01 0.5 - 0.05 hand pump, private consumption) E Withdrawals for local water V: Very low 4 - 3 1 - 0.1 0.01 - 0.001 0.05 - 0.005 supply with limited consumption Sources for local water VI: Imperceptible < 3 < 0.1 < 0.001 < 0.005 F supply are difficult to ensure

4.2 Data sources and quality The main source for borehole data was the Chadian database SITEAU (MHUR/DCDH, 2014), hosting geological profiles and hydrogeological measurements at well locations. Furthermore, Detay (1987) and Ngounou Ngatcha (1993) published specific capacity and yield measurements for the Cameroonian regions Nord and Extrême-Nord.

Several data quality issues were detected and had to be dealt with:

Coordinates

As boreholes in Detay (1987) are listed without coordinates, some locations were digitized from an enclosed map with a scale of 1 : 500 000. Considering an assumed accuracy of 0.2 cm within the cartography and digitizing process, digitized coordinates have an approximate accuracy of +/- 1 km.

In addition, boreholes of Detay (1987) for which coordinates were missing but the village name was available, were linked to the coordinates of villages found in topographic maps and department information. Here accuracy is highly variable, depending on whether the borehole is located in the centre of the village or not. For 41 villages more than one boreholes were identified, causing the attribution of up to 12 boreholes to the same coordinates (a list of villages with the associated number of boreholes is given in Appendix 3). Therefore, the productivity map shows less borehole locations than the number of boreholes used in the statistics. None of the concerned boreholes were excluded from the statistics as in reality they represent different borehole locations with different depths and therefore contribute to a differentiated analysis of the aquifer parameters.

Temporal distribution

The available data cover the period from 1950 to 2014 and do not necessarily reflect the current status. The temporal distribution of the data was derived from the dates of pumping tests at the respective wells. Where no date was available for the pumping test, the well construction date was assumed to indicate the date of the pumping test (25.7 % of the data set). The temporal distribution of the pumping test data is shown here below (Figure 10):

Figure 10: Temporal distribution of pumping test data.

Tapped Aquifer

The tapped aquifers are rarely known and also information on well screen depths are often missing. However, based on the general thicknesses of lithological units derived from the literature and based on documented borehole depths, it can be assumed that most of the boreholes northeast of Bama Ridge tap the Pleistocene alluvial deposits [a1-a2]. This assumption is based on the following arguments:

- The thickness of the quaternary sequences in the study area generally varies between 30 m and 70 m (Schneider & Wolff, 1992; Biscaldi, 1970). - 96 % of the boreholes which were used for the productivity mapping exhibit total depths between 20 m and 80 m. Thus, an error introduced by assigning Upper Pliocene series or Continental Terminal series to the alluvial deposits [a1-a2] is considered to be negligible. The maximum thickness of the quaternary cover above the alluvial deposits [a1-a2] is about 20 m (thicknesses of overlying quaternary deposits: [l2]= 3-6 m; [a3]= 4-5 m; [l3] = 1,5-7 m; [d1] = ~3 m, [a4]= 3-4 m). Thus, an error introduced by assigning younger Quaternary series to the alluvial deposits (a1-a2) is considered to be negligible too. - Quaternary deposits overlying the alluvial deposits [a1-a2] generally represent aquicludes ([l3], [l4]) or minor aquifers with very limited extent ([a3(c1)], [a3(c2)], [ds]) or with low thicknesses ([a3], [d1], [a4]). Therefore, boreholes northeast of Bama Ridge are supposed to tap the alluvial deposits [a1-a2] representing the only continuous surface near aquifer in the study area with important groundwater reserves.

Southwest of Bama Ridge the boreholes were assigned to the respective lithological units they are tapping by means of spatial join with the harmonized lithological map. In summary four tapped lithological units were identified in the study area:

- Alluvial deposits [a1-a2] NE of Bama Ridge, - Alluvial deposits [a1-a2] SW of Bama Ridge, - Alluvial deposits of Mandara Mountains piedmont plain [a3(p)], - Crystalline basement [b].

Spatial distribution

A lack of data for wide parts of Nigeria, and reduced information in Cameroon led to a reduced reliability of the result for regions with low data density. The spatial coverage of borehole data is about 52 % of the study area, assuming that one borehole is representative for a hexagon with a diameter of 10 km (see Appendix 1, side map).

In total, 1771 boreholes located in the study area and with known measurements of specific capacity and yield were available for analysis. This number includes 46 boreholes where either a specific capacity of 0 Ls-1m-1 was documented or - based on the fact that the well drilling was reported as unsuccessful – 0 Ls-1m-1 was assumed. As the logarithmic Index Y is not defined for the value 0, a dummy value of Index Y equal 2.5 was assigned for unsuccessful drillings – which refers to the classes “VI: imperceptible” after Krásný (1993) or “F: no groundwater” after (Struckmeier & Margat, 1995).

4.3 Analysis The aquifer productivity was determined following the Krásny (1993) and Struckmeier & Margat (1995) approaches for each lithological unit representing an aquifer:

- Alluvial deposits [a1-a2] NE of Bama Ridge, - Alluvial deposits [a1-a2] SW of Bama Ridge, - Alluvial deposits of Mandara Mountains piedmont plain [a3(p)], - Crystalline basement [b]).

The inhomogeneous spatial distribution of boreholes which indicate low aquifer productivities within the Alluvial deposits [a1-a2] NE of Bama Ridge revealed the need for subdividing the lithological unit into two aquifer productivity domains. This was realised based on the available water table depth (WTD) measurements serving as indicator for the saturated aquifer thickness. The classification result following the Krásný approach is shown in Appendix 1 while the classification result after Struckmeier and Margat is given in Appendix 2.

4.3.1 Classification results following the approaches of Krásný (1993) and Struckmeier & Margat (1995)

Following the approach of Krásný (1993), the index Y values and the respective classes of transmissivity magnitude were determined for all 1771 boreholes based on available specific capacity and yield values. Specific capacity was derived from pumping test data which was available for 97.7 % of the data set. Where no pumping test data was available, the well yield was used to calculate the Index Y value. For reasons of data set homogeneity, transmissivity values were not taken into account as they were only available for 1 % of the dataset and the specific capacity values for the respective wells were available. Index Y values of the boreholes were then assigned to the corresponding lithological units. For each borehole set assigned to one lithological unit the mean (x̅ ) and the standard deviation (s) of the index Y value and the confidence interval I (x̅ -s, x̅ +s) were calculated. The respective percentage of index Y values falling into the different aquifer productivity classes was determined and the productivity classes and descriptions were defined following the quantitative approach of Krásný (1993) and the qualitative classification after Struckmeier & Margat (1995). The results are discussed here below for each tapped lithological unit and they are summarized in Figure 11 and in Table 4.

The alluvial deposits [a1-a2] NE of Bama Ridge are dominated by sandy sediments with an intermediate aquifer productivity and considerable heterogeneity. Most of the sediments plot in the Struckmeier & Margat category B representing an extensive but only moderately productive aquifer. In general the aquifer fits for withdrawals for local water supply (smaller communities, small scale irrigation etc.) but in the vicinity of rivers most boreholes exhibit intermediate (to high) productivities allowing abstractions for larger infrastructures (e.g. city of N’Djamena). In order to take into account this inhomogeneous spatial distribution of aquifer productivity within the lithological unit, it was subdivided into two aquifer productivity domains (see detailed description in Chapter 4.3.2). The results for this lithological unit are only representative for the Chadian part of the study area and few areas in Cameroon as sufficient borehole data for reliable statistics is missing for entire Nigeria and for large areas in Cameroon. In the Cameroonian part of the study area, between Lake Maga and Kousseri, only 20 data points are available within the 1-sigma confidence interval on a surface of about 8500 km² (see Appendix 1, side map). All other data points (n=53) in this area lie below the 1-sigma confidence interval, indicating that the classification result overestimates the productivity for that specific part of the study area.

The alluvial deposits [a1-a2] SW of Bama Ridge exhibit a low to intermediate aquifer productivity and an extreme hydraulic heterogeneity. In general, these sediments represent only a minor aquifer with local and limited groundwater resources which fit for smaller withdrawals for local water supply (supply through hand pump, private consumption). Intermediate productivities of this lithological unit are mainly documented in the “Bec de Canard” of the Cameroonian border where local water supply for smaller

20 communities and small scale irrigation might be feasible. According to the aquifer properties described by Tillement (1969) and Detay (1987) low aquifer productivities and non-water-bearing sedimentary units in this area can be found in the interfluves and where elevations of the underlying basement reduce the sediment thickness. In the eastern part of this lithological unit (S and SW of Bongor) intermediate and high aquifer productivities can be found, indicating that the classification result underestimates the productivity in that specific area.

Alluvial deposits of Mandara Mountains piedmont plain [a3(p)] are composed of very heterogeneous sandy and clayey sediments with a low to intermediate aquifer productivity. Locally, very low aquifer productivity occurs. Following the Struckmeier & Margat classification these sediments represent only a minor aquifer with local and limited groundwater resources which fits for smaller withdrawals for local water supply (supply through hand pump, private consumption). This classification result is in accordance with the hydrogeological setting described by Tillement (1969) and Detay (1987) for this region: The alluvial sediments exhibit highly alternating thicknesses and an important portion of clay layers and lenses favouring small and partially perched aquifers. The occurrence of productive aquifers is limited to recent and ancient riverbeds/-channels which locally form important reservoirs (e.g. Mayo Tsanaga NE of Maroua). Same as for the alluvial deposits [a1-a2] SW of Bama Ridge non-water- bearing sedimentary units can be found in the interfluves and where elevations of the underlying basement reduce the sediment thickness.

The crystalline basement [b] exposed in the Mandara Mountains in Cameroon has very low to low aquifer productivities and a significant portion of imperceptible productivity. The category “imperceptible” (Y = 2.5) was assigned for unsuccessful drillings representing 38 % of the crystalline basement data set (see Figure 11). The wide range of the values documents an extreme heterogeneity of the aquifer. Following the Struckmeier & Margat classification the crystalline basement represents a fissured aquifer with local and limited groundwater resources. The supply potential of this unit ranges from smaller withdrawals for local water supply (supply through hand pump, private consumption) and local water supply with limited consumption to essentially no supply potential. This classification result fits with the observations made by Tillement (1969) who describes a very low thickness of the alteration zone (in general less than 5 m) in the Mandara Mountains and the occurrence of non-weathered rocks where slopes are steep. Therefore, the prospection of groundwater resources in this area mainly focusses on mountain slope debris and alluvial/colluvial deposits in the valleys.

Figure 11: Cumulative relative frequency of specific capacity (Sc) and yield (Q) values, where T ≜ transmissivity; index Y ≜ logarithmic conversion of Sc and Q; x̅ ≜ arithmetic mean; s ≜ standard deviation; (++A), +A, -A, (--A) ≜ fields of positive and negative anomalies (extreme anomalies); WTD ≜ water table depth. Adapted from Krásný (1993).

Table 4: Parameters and results of productivity classification after Krásný (1993) and Struckmeier & Margat (1995).

Confidence Percentage of index Y values falling into the respective class Productivity Category Lithology x s n Classification Lithological unit ̅ Interval I and description after (IHME level 1) (Index Y) (Index Y) (Index Y) High Intermediate Low Very Low Imperceptible after Krasny [x̅ -s, x̅ +s] (II) (III) (IV) (V) (VI) after Krasny Struckmeier Alluvial deposits Intermediate (NE of Bama Ridge) (to high), Sands, clays 5.60 0.63 925 4.96 – 6.22 17.60 78.97 3.44 0 0 III (-II) d B [a1-a2 NE] considerably WTD < 27 heterogeneous Alluvial deposits Intermediate (NE of Bama Ridge) to low, Sands, clays 5.30 0.76 663 4.54 - 6.06 4.26 65.75 29.99 0 0 III -IV-d B [a1-a2 NE] considerably WTD > 27 heterogeneous Low to Alluvial deposits intermediate, (SW of Bama Ridge) Sands 4.95 1.02 28 3.92 - 5.97 0 47.36 48.80 3.84 0 IV-III f E extremely [a1-a2 SW] heterogeneous Alluvial deposits of Low to Mandara Mountains intermediate (to Sands, clays 4.91 0.92 105 3.99 - 5.83 0 45.24 54.24 0.52 0 IV -III (-V) e E piedmont plain very low), very [a3(p)] heterogeneous Very low to low Undifferentiated (to Crystalline basement metamorphic 3.81 1.14 50 2.67 - 4.96 0 0 41.98 43.71 14.31 V-IV (-VI) f imperceptible), E [b] and magm. rocks extremely heterogeneous

4.3.2 Subdivision of alluvial deposits NE of Bama Ridge into two aquifer productivity domains

The lithological unit “alluvial deposits [a1-a2] NE of Bama Ridge” covers ~80 % of the study area and cannot be subdivided into further spatial units based on the available lithological information. However, further subdivision is needed to establish a differentiated aquifer productivity map which can serve as a tool for spatial planning. The subdivision must display the inhomogeneity of the aquifer productivity distribution within that lithological unit. This inhomogeneity becomes evident when looking at the spatial distribution of boreholes which indicate a low aquifer productivity (Appendix 1). West of Logone River and about 15-25 km east of Chari River the proportion of boreholes indicating low aquifer productivities is exceeding 30 %. This can be caused by a change of the hydraulic properties of the aquifer and/or by the change of the saturated aquifer thickness. In the conceptual cross-sections of Detay et al. (1991) and Schneider and Wolff (1992) these areas are characterized by a drop in the groundwater level, causing a decrease of the saturated aquifer thickness (see Figure 12 between Bougoumene and Guirlie and Appendix 5 & 6 between Kousseri and Tourba, between N’Djamena and Abou Bazan and between Mandara Mountains and Yaérés Floodplain). However, detailed studies of hydraulic properties and aquifer thickenss are not available. It was therefore tested whether the water table depth (WTD), which partly controls the saturated thickness of the aquifer and which is available for 97.5 % of the data set, can serve as an indicator of aquifer productivity (Figure 13). The results show that there is no correlation between WTD and high aquifer productivities (including the very upper range of intermediate aquifer productivities). This result fits with the observation that boreholes indicating high aquifer productivities are scattered all over the study area. For these locations, hydraulic parameters other than the WTD are more important to describe the aquifer productivity (e.g. permeability, thickness of the quaternary aquifer, transmissivity). In the range between low and intermediate aquifer productivity (~70 % of the data set), however, a negative correlation with the WTD can be observed, so that the proportion of low aquifer productivity becomes > 30 % at a certain WTD. This negative correlation confirms the existence of areas where the WTD is a good indicator for aquifer productivity. Following the logic of the Krásný classification nomenclature, the low aquifer productivity class is listed as major component when exceeding 30 %. Therefore, it was tested which WTD contour line can serve as outline to produce an area where > 30 % of boreholes indicate a low aquifer productivity. The testing started with the 50 m WTD contour line and then was continued for lower WTDs (Figure 14). The simulation result shows that almost all contour lines > 27 m result in areas with a proportion of low aquifer productivity > 30 %. A clear drop below the 30 % limit can be observed at the 27 m contour line, which finally was used to subdivide the lithological unit “alluvial deposits [a1-a2] NE of Bama Ridge” into two aquifer productivity domains:

1. Alluvial deposits [a1-a2] NE of Bama Ridge with WTD > 27 m and > 30 % of boreholes indicating a low aquifer productivity. Classification result: Intermediate to low [III-II] aquifer productivity, 2. Alluvial deposits [a1-a2] NE of Bama Ridge with WTD < 27 m and < 30 % of boreholes indicating a low aquifer productivity. Classification result: Intermediate (to high) [III (-II)] aquifer productivity.

In the Chadian part of the study area, the WTD contour line was drawn based on the measurements of the present data set. In the Cameroonian part, the WTD contour lines of Biscaldi (1970a) were used to supplement the present data set. Due to the unequal spatial distribution of data points between Chad and Cameroon the result of this approach mainly reflects the hydraulic conditions in the Chadian part of the study area. For Cameroon it is questionable if the 27 m WTD contour line can serve to identify areas where the proportion of low aquifer productivity exceeds 30 %. Here, the result rather gives a rough idea of the “dry wedge” geometry between Mandara Mountains and Yaérés Floodplain which is characterized by the occurrence of zones where the whole quaternary sedimentary sequence remains dry (see boreholes with productivities classified as “imperceptible” in Appendix 1).

Figure 12: Conceptual hydrogeological cross section based on Schneider and Wolff (1992) lithological profiles, digital elevation model of Jarvis et al. (2008) and water table depth (WTD) data derived from Biscaldi (1970), Detay (1987) and MHUR/DCDH (2014).

Figure 13: Correlation between Index Y value and water table depth (WTD).

Figure 14: Simulation results for the testing of different water table depth (WTD) contours as outlines for an area where > 30% of the boreholes indicate low aquifer productivities.

5. Conclusions

An aquifer productivity map for the lower Chari-Logone River Basin has been developed based on the geometries of the lithological units derived from four different analogue maps. These maps vary in their topic (hydrogeology, geology and hydrology), scale and degree of detail causing gaps and uncertainties at the map sheet borders. In that areas, manual corrections were realised based on satellite imagery and literature comprising the filling of gaps, the adding of structures and the thematic adjustment where the lithological description differed between adjacent maps. The harmonized lithological map shows more lithological units than the aquifer productivity map because only four lithological units were identified being an aquifer.

The aquifer productivity of the respective lithological units was determined following the quantitative approach of Krásný (1993) and the qualitative classification of Struckmeier & Margat (1995). The significance of the results of the quantitative approach of Krásný (1993) mainly depends on the data point density and distribution. In the Chadian part of the study area sufficient data points are available for reliable statistics. In contrast, the significance of the classification results for Nigeria and large areas of Cameroon is strongly reduced due to the lack of data. Nevertheless, compared to the Struckmeier & Margat (1995) classification, the approach of Krásný (1993) gives a more differentiated classification result for the aquifers southwest of Bama Ridge, where Tillement (1969) and Detay (1987) also observed different aquifer domains. The classification result for the alluvial sediments NE of Bama Ridge is biased by the large number of data points in the Chadian part compared to the Nigerian and Cameroonian parts of the study area and therefore neglects probable hydrogeological inhomogeneity of the aquifer in Nigeria and Cameroon. West of Logone River and about 15-25 km east of Chari River the portion of boreholes which indicate low aquifer productivities exceeds 30 %. This observation was used to subdivide the alluvial sediments NE of Bama Ridge into two aquifer productivity domains. This subdivision was realised based on water table depth measurements available for 97.5 % of the data set. It was shown that the portion of bore wells which indicate low aquifer productivities exceeds 30 % at water table depths > 27 m. More accurate parameters for the subdivision of this aquifer into different aquifer productivity domains would be the saturated aquifer thickness or the transmissivity, two parameters which strongly control the aquifer productivity, but which are barely available for the study area.

In the areas, where sufficient data was available for reliable statistics, the presented aquifer productivity map of the lower Chari-Logone River Basin represents a tool for groundwater management and spatial planning, highlighting areas with limited groundwater resources and pointing out areas which may serve to meet the increasing water demand. Further efforts are necessary to overcome data gaps in order to establish a reliable analysis for the entire region. In the case that further borehole information becomes available, the adopted approach will allow a continuous and comprehensible update of the presented map. Both, the harmonized transboundary lithological map and the aquifer productivity map set the basis for further analysis of the groundwater resources in this region, such as aquifer vulnerability or feasibility for manual drilling.

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Appendix

Appendix 1: Transboundary aquifer productivity map of the lower Chari-Logone River Basin

In the online version of this report the aquifer productivity map is provided in a separate file:

“BOSCH, K. & RUECKL, M (2020)_The Transboundary Aquifer Productivity Map of the Lower Chari-Logone River Basin. APPENDIX I, prepared by LCBC & BGR; Berlin.pdf”.

Appendix 2: Classification result following the Struckmeier & Margat (1995) approach

Appendix 3: List of villages where more than one boreholes were attributed from Detay (1987)

Village Longitude Latitude N° of boreholes Lithological Unit [dec. degrees] [dec. degrees] Kolofata 14,00896 11,16128 5 Limani 14,17371 11,23458 3 Fotokol 14,22384 12,36930 2 Dougoumsilio I 14,38476 12,58711 2 Tagawa 14,46666 11,33333 2 Farch 14,48333 12,36666 2 Massaki 14,53388 12,72972 2 Sedek 14,71361 10,67464 2 Alluvial deposits Maltam 14,81527 12,1825 3 [a1-a2] Amfara 14,85 12,21666 2 (NE of Bama Ridge) Moulvoudaye 14,86899 10,40665 3 Goulfey 14,89491 12,37282 6 Maga 14,94361 10,83666 9 Kay Kay 15,02972 10,67538 2 Kousseri 15,03083 12,07833 5 Pouss 15,05587 10,85319 4 Yagoua 15,24055 10,34277 3 Yoldéo 14,90628 10,16363 2 Kalfou 14,93406 10,28752 3 Alluvial deposits Madalam 14,94579 10,15768 2 [a1-a2] Doukoula 14,96570 10,11253 3 (SW of Bama Ridge) Gabo 15,42605 10,00304 2 Takomari 14 11,08333 2 Tarmoua 14,02575 11,08112 2 Gansé 14,06630 11,17119 2 Kourgui 14,10463 11,09540 2 Amtchidé 14,11414 11,21972 2 Alluvial deposits of Mandara Mountains Gazaoua 14,15 10,53333 2 piedmont plain Katoual 14,2 10,51666 2 [a3(p)] Sava 14,2 11,01666 6 Manawatchi Mémé 14,21270 10,96401 2 Salak 14,24330 10,45731 3 Djoulgouf 14,46666 10,63333 2 Bogo 14,61083 10,73611 3 Ndoukoula 1 14,02976 10,27214 2 Meri 14,12277 10,77666 4 Zongoya 14,13694 10,47989 3 Crystalline Basement (Magmatic and Mora 14,14472 11,0425 12 metamorphic rocks) Mouda 14,22429 10,37085 4 [b] Djoundé 14,29047 11,04161 2 Waza 14,56916 11,39472 4

Appendix 4: Traces of lithostratigraphic cross sections of the study area (Schneider & Wolff 1992)

Appendix 5: Lithostratigraphic cross sections of the study area (Schneider & Wolff 1992)

See trace of cross section N° 7 in Appendix 4

See trace of cross section N° 9 in Appendix 4

Appendix 5: Lithostratigraphic cross sections of the study area (Schneider & Wolff 1992)

See trace of cross section N° 9 in Appendix 4

Appendix 5: Lithostratigraphic cross sections of the study area (Schneider & Wolff 1992)

See trace of the cross section in Appendix 4

Appendix 6: Conceptual hydrogeological cross sections of the transition zone between Mandara Mountains and Yaérés floodplain (Ngounou Ngatcha 1993, Detay et al.1991 and Detay 2000)

Schematic cross section of the Mandara Mountains piedmont domain (Detay, 2000 modified after Mathiez et Huot (1966))

Groundwater circulation scheme in the Quaternary aquifer (Ngounou Ngatcha, 1993)

Schematic cross section of the Lake Chad Basin SW boundary (Detay et al., 1991)