Project: Sustainable Management of Groundwater Resources in the Lake Chad Basin The Transboundary Aquifer Productivity Map of the Lower Chari-Logone River 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 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