Hydrogeological investigations in the upper Ouémé catchment in Benin, West Africa

The scope of this PhD thesis was the hydrogeological conceptualisation of the Upper Ouémé river catchment in Benin. The study area exceeds 14,500 km**2 and is underlain by a crystalline basement. At this setting the typical sequence of aquifers - a regolith aquifer at the top and a fractured bedrock aquifer at the bottom - is encountered, which is found in basement areas all over Africa and elsewhere in the world. The chosen regional approach revealed important information about the hydrochemistry and hydrogeology of this catchment. Based on the regional conceptual model a numerical groundwater flow model was designed. The numerical model was used to estimate the impact of climate change on the regional groundwater resources. This study was realised within the framework of the German interdisciplinary research project IMPETUS (English translation: "Integrated approach to the efficient management of scarce water resources in West Africa"), which is jointly managed by the German universities of Bonn and Cologne. Since the year 2000 the Upper Ouémé catchment was the principal target for investigations into the relevant processes of the regional water cycle. A first study from 2000 to 2003 (Fass, 2004, http://nbn-resolving.de/urn:nbn:de:hbz:5n-03849) focused on the hydrogeology of a small local catchment (~30 km**2). In the course of this thesis five field campaigns were underdone from the year 2004 to 2006. In the beginning of 2004 a groundwater monitoring net was installed based on 12 automatic data loggers. Manual piezometric measurements and the sampling of groundwater and surface water were realised for each campaign throughout the whole study area. Water samples were analysed for major ions, for a choice of heavy metals and for their composition by deuterium, oxygen-18 and tritium. The numerical model was performed with FEFLOW. The hydraulic and hydrochemical characteristics were described for the regolith aquifer and the bedrock aquifer. The regolith aquifer plays the role of the groundwater stock with low conductivity while the fractures of the bedrock may conduct water relatively fast towards extraction points. Flow in fractures of the bedrock depends on the connectivity of the fracture network which might be of local to subregional importance. Stable isotopes in combination with hydrochemistry proved that recharge occurs on catchment scale and exclusively by precipitation. Influx of groundwater from distant areas along dominant structures like the Kandi fault or from the Atacora mountain chain is excluded. The analysis of tritium in groundwater from different depths revealed the interesting fact of the strongly rising groundwater ages. Bedrock groundwater may possibly be much older than 50 years. Equilibrium phases of the silicate weathering products kaolinite and montmorillonite showed that the deeper part of the regolith aquifer and the bedrock aquifer feature either stagnant or less mobile groundwater while the shallow aquifer level is influenced by seasonal groundwater table fluctuations. The hydrochemical data characterised this zone by the progressive change of the hydrochemical facies of recently infiltrated rainwater on its flow path into deeper parts of the aquifers. Surprisingly it was found out that seasonal influences on groundwater hydrochemistry are minor, mainly because they affect only the groundwater levels close to the surface. The transfer of the hydrogeological features of the Upper Ouémé catchment into a regional numerical model demanded a strong simplification. Groundwater tables are a reprint of the general surface morphology. Pumping or other types of groundwater extraction would have only very local impact on the available groundwater resources. It was possible to integrate IMPETUS scenario data into the groundwater model. As a result it was shown that the impact of climate change on the groundwater resources until the year 2025 under the given conditions will be negligible due to the little share of precipitation needed for recharge and the low water needs for domestic use. Reason for concern is the groundwater quality on water points in the vicinity of settlements because of contamination by human activities as shown for the village of Dogué. Nitrate concentrations achieved in many places already alerting levels. Health risks from fluoride or heavy metals were excluded for the Upper Ouémé area.

Geochemical studies of surface sediment samples off North-West Africa

One hundred and twenty-eight surface-sediment samples collected off North-West Africa were studied geochemically to detect the expressions of different meridional climate regimes and zonal productivity gradients in the surface sediments. This geochemical multi-tracer approach, coupled with additional information on the bulk carbonate and TOC contents makes it possible to characterise the sedimentological regime in detail. Typical terrigenous elements like Al, K and Fe mirror the importance of the humid (fluvial) influence in the north of the study area and the dominance of aeolian input in the south. Furthermore, the distributions of Ti and Fe in the surface sediments serve as tracers for the supply of eolian volcanic material from the Canary Islands. The spatial variability of the TOC contents in the surface sediments closely follows the ocean surface productivity patterns, whereas the CaCO3 contents are mainly controlled by dilution with terrigenous matter. The potential productivity proxy Ba is not a reliable tracer for productivity in this region, since it is mainly supplied by terrigenous input (coupled with aluminosilicates).

Sedimentological and paleoclimatological investigation of two sediment cores off Cape Barbas, North-West Africa

Two box cores taken off Cape Barbas (North-West Africa) have been studied using three methods. The analyses of the coarse fraction, of biogenic opal and of planktonic foraminifera revealed : 1. Core GIK12310-4 penetrates Z, Y, X and upper part of W zone, whereas core GIK12379-1 penetrates Z and upper part of Y zone. 2. Holocene sedimentation rates are 2.5 cm/1000 y for core GIK12310-4 and 6.0 cm/1000 y for core GIK12379-1. During the Y zone 5 cm/l000 y were sedimented incore GIK12310-4 and > 10-20 cm/1000 y in core GIK12379-1. 3. Paleoclimatohgical results are: arid climate and relatively warm water temperatures during the Holocene (Z zone) and during X zone; humid climate and relatively cool water temperatures within the Wuerm (Y zone) (with a non-dated more arid interval found in the middle part of the Y zone) and in the upper part of the W zone. 4. Increased contents of benthos and radiolaria in the Y zone indicate upwelling. Upwelling, characterized by high content of biogenic opal and low water temperatures, was found in core GIK12310-4 at 250 to 350 cm in the lower part of the Y zone. The plankton/benthos ratio of foraminifera, the benthos/radiolaria ratio and water temperatures derived from planktonic foraminifera, differ in both cores in the Holocene, and are nearly identical during the Wuerm.

Discharge data derived from five water level gauges and discharge measurements in the Aguima and Niaou catchment, Benin, West Africa

This paper analyzes the hydrological processes and the impact of soil properties and land use on these processes in tropical headwater catchment in the sub-humid part of Benin (West-Africa), the Aguima catchment. The presented study is integrated in the GLOWA IMPETUS project, which investigates the effects of global change on the water cycle and water availability on a regional scale in Morocco and Benin. The lack of field investigations concerning soil and surface hydrology in the Benin research area necessitates detailed field measurements including measurements of discharge, soil water dynamics, soil physical properties etc. on the local scale in order to understand the dominant runoff generation processes and its influencing factors. This is a pre-requisite to be able to forecast the effects which global change has on hydrological processes and water availability in the region. The paper gives an overview over the hydrologic measuring concept of the IMPETUS-Benin project focusing on measurements concerning the soil saturated conductivity ksat and discharge behaviour of two different sub-catchment of the Aguima catchment. The results of ksat measurements revealed that interflow is the dominant runoff process on the hillslopes of the investigated catchment. Concerning the impact of land use on the hydrological processes infiltration experiments showed that infiltration rates were reduced on cultivated land compared to natural land cover. This results in significant differences in runoff behaviour and runoff ratios while comparing natural and agricultural used catchments.

Composition of recent carbonate bottom sediments from the Guiana shelf an off the West Africa

This work was based on a study of the upper layer of recent carbonate bottom sediments of the Atlantic Ocean. Biogenic carbonate of recent sediments is represented by metastable and stable minerals. In the ocean metastable phases can exist indefinitely long, but the structure of polymorphism determines inevitability of transformation of metastable phases into stable ones. This transformation occurs in the solid phase. In the absence of a critical point between the two phases of the transition process is not available for study by microscopic methods. It is estimated indirectly by studying the nature and extent of changes in mineral and chemical compositions. With aging of sediments their mineral composition alters in direction of increasing contents of resistant minerals. Fine grained sediments and fractions are subject to more intensive effects of early diagenesis processes, rather than coarse ones; this is reflected in their mineral composition. Regularities of distribution of carbonate minerals in size fractions consistent with the direction of polymorphic transformations in calcium carbonate. Such transformations can occur in a particular dimension of grains. Concrete grain size depends on environmental conditions. This situation explains presence of metastable biogenic carbonates at different depths of the ocean and suggests presence of diagenetic calcite in sediments occurring below expected for each case depth of the transition.

(Table 1) Composition of bottom sediments and pore waters from the West Africa shelf

Organic and mineral phosphorus (P_org and P_min) have been determined in pore waters of terrigenous, biogenous, as well as weakly phosphatic and phosphatic sediments from the shelf of the West Africa (in 30 samples). Concentrations of P_min in the pore waters have been examined in close relation to grain size and chemical composition (amounts of P and N_org) of solid phase of the sediments. It has been demonstrated that among sands and coarse silts, maximum concentrations of P_min (up to 1.7 mg/l) in the pore waters have been observed in weakly phosphatic and phosphatic sediments rich in organic matter of the highly productive shelf of the Southwest Africa. Concentrations of P_min in the pore waters are most clearly associated with contents of N_org in the solid phase of the sediments (correlation coefficient R = 0.71) and P_org in the pore waters (R = 0.78).

Age determination of sediment cores from the continental shelf and slope off North-West Africa

Three sediment cores from the continental shelf and slope off NW Africa (Banc d'Arguin; 52 m, 665 m and 973 m water depth) have been investigated by means of a coarse fraction analysis. The two shallower cores have been deposited during less than 10,000 years, the deeper one during the last 36,000 years. The Holocene sedimentation ( 4000 years) in the deeper part of core 79 the edge of the Banc d'Arguin is strongly influenced by reworking of Late Glacial dune sands and biogenic particles from shallower ware (<40 m), as well as eroding current influence. A decrease in grain size of silicate material and a decrease in lateral supply, correlated to a doubling of accumulation rates in the upper part of the core, indicates a more autochthonous sedimentation with less sorting influence in the youngest Holocene. The depth of provenance of the allochttonous material can be assumed in 100-300 m water depth as indicated by various biogenous particles. Small amounts of shallow water particles in the autochthonous layers indicate a supplay from shallow water, which probably occured b ythe mechanism of "particle by particle supply". None of the three cores indicates upwelling influence, although occanographers found intense upwelling in the area of the Banc d'Arguin. The Holocene climate in that area probably has been arid, small variations in terrigenous matter composition and grain size in the Early Holocene might be due to decreased wind strength or to an increase in rain fall. The Peak Glacial section (14,000-22,000 y. B.P.) of the deepest core 88 indicates a very much intensified eolian silt supply and an additional bottom supply of quartz sand In the interval 22,000-36,000 y. B.P. wind strength decreased, but probably no increase in humidity occurred. So this area in about 19° 40' N had an arid climate in the Late Holocene and in the Peak Glacial. The fragmentation of planktonic foraminifers and the abundance of aragonitic tests of pteropods in core 88 indicate an Early Holocene (8330 y. B.P.) preservation spike. Two minima in fragmentation correlated to maxima in pteropod content at about 15,700 and 21,000 y. B.P. are correlated to maxima in shallow water supply and thus do not reflect preservation conditions, but only lateral supply from the carbonate dissolution minimum zone in about 300 m water depth.