BENIN SUMMARY PROJECT SITE. Methodology

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1 BENIN SUMMARY PROJECT SITE Methodology Cotonou City whose population was estimated at approximately 700,000 inhabitants according to the general census on population and human settlement conducted in February 1992, is entirely located on a land strip separating the Lake Nokove northwards (a wide lagoon of brackish water) from the Atlantic Ocean southwards. The landstrip, which is about 5 km long, consists of a coastal plain parallel to the shoreline where offshore bars and lagoons of salt, brackish or soft water alternate. Under the actual site of the city, the underground water generally consists of shallow freshwater lenses floating above the underlying saline groundwater from the sea or the lagoons. Sometimes, the only water collecting system effectively used is the water drawn from large private wells. Under the clayey wall of this first aquifer which is about 20 m deep and practically entirely surrounded by saline water, aquifers under formation were identified though their number and respective depths were not yet well determined due to the lack of exploration and exploitation structures crossing them thoroughly. Their lateral extension might overflow on the showline and continue more or less deeply towards the open sea under the ocean. Nevertheless, Cotonou City and its suburbs are mainly supplied by water through underground water resources located at Agonkame, approximately 10 kilometers from the city center at the northwest (at the south west of Lake Nokoue). As time went by, populations have subjected the only catchment area of these water resources to a disordered land settlement. The lack of protection areas for the structures and the catchment area in general, including the concentration and intensity of pumping, results in a risk of pollution of the exploited resources, this pollution is generated from both the surface area (land-based activities) and the under ground area (saline intrusion). Thus, the analysis of urban pollution in Cotonou should take into consideration both the underground water of the actual site of the city and the aquifer system used for its modern drinking water supply network. Both of the aspects were fundamental in the selection of the project s site. Climate The Cotonou area is affected by oceanic trade winds throughout the year. The climate is typically a transitional subequatorial one consisting with an unequal spatiotemporal distribution of precipitation. The temporal distribution of the precipitation s results in four more or less distinct seasons, two rainy seasons: - A major dry season from the middle of November to the end of March; - A minor dry season from the middle of July to the middle of September; - A minor rainy season from the middle of September to the middle of November; - A minor rainy season from the middle of September to the middle of November 1

2 Rainfalls: The average rainfall levels calculated at the meteorological station of Cotonou Airport from 1952 to 1955 (43 years) were 1313 mm. The average distribution of rainfalls is comprised between 80 and 120 days. Strong effects of the relative humidity emphasize the marine influence. During the rainy seasons Cotonou city and its suburbs is actually subjected to specific conditions of the wet monsoon blowing from the ocean to the continent. While from December to February, during the dry season, it is subjected to the Harmattan a hot and dry wind blowing from the north to the coast. Table 1: Average and annual rainfall levels in mm ( ) Jan Feb Mar Apr May June July Aug Sep Coton t ou Oct Nov Dec Annua l Total Temperatures: The highest maximum daily temperature rarely exceeds 34.C. These are registered during February and March on every April, which were the hottest months of the year. The lowest temperatures (23 0 C on average are registered in August). The daily thermal amplitude oscillates about 6 0 C during the major rainy season while it exceeds 10 0 C during the dry season. The average monthly figures relating to the maximum and minimum temperatures registered at the meteorological station of Cotonou Airport are mentioned in the following table: Table 2: Average monthly temperatures (provided by Aseena Cotonou). Cotonou Airport Referenc e station Jan Feb Mar Apr May Jun Jul Au g Max min Av Se pt Oc t No v De c Yea r Relative humidity, Evaporation and Evapotranspiration: - Evaporation is a very active and important phenomenon in the Cotonou area during the period between January and April. The figures registered at Cotonou between June and July are close to 3mm per day, while the maximum figures reach 5mm per day in March. Winds Winds on the ground usually blow from the south-west to the north-east at 5m/s. They are subjected to seasonal fluctuations described as follows: - During the dry season, the wind (Harmattan) is dry and hot in day time and cool in evening time. It blows at an average speed of 2 to 3m/s; - During the major rainy season mainly influenced by the wet monsoon, the wind speed evaluated on the basis of the speed of the flying particles of dust could exceed 20m/s (72 km/h). 2

3 Hydrology Hydrologically, the Atlantic Ocean, Lake Nokove, Djonou and Todouba lagoons and the temporary or permanent water basis are the main ground water tables, which influence the human activities in the project s site. The main tributary streams of the lake Nokove include the Oveme River, the So River and the Djonou lagoon. The Todouba, Dati and Ahouhargan rivers are also tributary streams of the Djonou lagoon. All these rivers and lagoons form a drain for the water table of the plateau located northward of the project s site. The Lake Nokove is connected to the sea through the Cotonou Channel (Cotonou lagoon) which separates the eastern and western parts of the town. Otherwise, the lake Nokove is surrounded by a system of lagoons and shallows to which it was initially connected, and which presently are shut off from one another due to the urban development. The fluctuations of the lagoon water levels are related not only to the rainfalls but also to the groundwater dynamics depending on the hydraulic continuity existing among them. Geology and Hydrogeology Based on the geological nature of the superficial groundwater, it was easy to divide the project s site into three zones from the north to the south: The first zone covering the plateau area is characterized by a red argillomuddy sand with a reddish yellow shade called Ground Bar. This sand is formed under an ancient equatorial climate where the average rainfall exceeds 1400 mm, which has facilitated the lateritization process. The formation is characteristic of the Pleistocene continental period; The second zone corresponding to the yellow ochre sands of the inner bars or lower plateau; theses sands should be the eroded ground bar recovered by the offshore agents, with the elimination of the argillasilty fraction; The third area is characterized by white or brown sands and burned sands of the median bars. These were formed during the last marine oscillations of the post- Holocene period. Thus, the project s site superficially presents geological units belonging entirely to the quaternary period. The aquifer system of the southern Benin was identified through several bore-holing of water exploitation specially undertaken in the western part of the project s site on account of the National power and Water Corporation of Benin (Socièté Béninoise d Electricité et d Eau/SBEE), United Nations Development Programme (UNDP), Hydraulics Directorate (HD) and National University of Benin. A series of sections North-south and East-west undertaken in these different geological units resulted in the identification of a surficial aquifer and a lower aquifer system hydrostratigraphically complex where three major aquifer horizons more or less differentiated were determined. Surficial Aquifer: The reservoirs consist of: (1) a thin sand layer with coarse grains located just at the bottom of the ground bar on the plateau; (2) yellow sands and burned and brown sands in the coastal plain. Their thicknesses fluctuate between 10 to 20 meters at most. With its homogeneous structure and its unconfined ground 3

4 water, the surficial aquifer lies on a more or less clayey layer separating it from the lower aquifer. This separation seems discontinuous towards the northern boundary of the plain and at the level of the plateau. In these sectors of the project s site, the differentiation between a surficial aquifer and an underground aquifer became unnoticeable. In the coastal plain, the water table is very shallow and even outcropping (namely in Cotonou), while in the plateau area, it vastly becomes deeper from the south to the north where it reached 10 metres and did not exceed 20 metres. Lower Aquifer system: It consists of more or less specific horizons whose respective reservoirs were relatively heterogeneous, namely from the top to the bottom: (1) the upper horizon: its reservoir is consistent with the first course grained zone identified. Its thickness fluctuates between 40m and 20m from the north to the south; at the level of the plateau, it merges in some areas with the surficial aquifer where the clayey layer isolating them is loose; (2) The middle horizon: its reservoir consists of the second course gained zone identified. Its thickness fluctuates between 45 to 50 m; (3) the lower horizon: its reservoir is consistent with the first coarse-grained zone identified and is located between 130m and 150m. The aquifer horizons were located in sand and gravel layers whose thickness largely fluctuates from one area to another. Clayey layers more or less silty effectively separate aquifers with sandy variations performing in places as a semi porous or porous substance entailing hydraulic communications between the aquifers. All things considered, the aquifer system exploited to supply Cotonou and its suburbs consists of one surficial aquifer with a phreatic water and one underground aquifer more structurally heterogeneous. The superficial aquifer was generally caught by a multitude of large private wells, while the underground aquifer which is deeper and more loaded is mainly exploited by the National Power and Water Corporation of Benin (SBEE) through bore-holes. The underground aquifer also crossed plateau area as an unconfined groundwater. Chemical Parameters The mineralization of various aquifers is dependent on their lithological contexts. It is influenced by the carbonate dissolving, silicate hydrolysis, mixing with the sea water, absorption, exchanges and evaporation processes. During the campaigns of piezometric registrations or chemical samplings of the superficial and underground aquifers undertaken between 1991 and 1993, the physico-chemical and chemical data of the waters were measured on a quite important number of samples. The results had led to the drafting of spatial distribution maps of the different data. The water temperatures fluctuated between 26 0 C, 36 0 C according to the moment of the day, the season and the depth of the aquifer. Low ph (4.5 to 6.5) and those of the coastal plain area characterized the water of the plateau area by generally higher ph (6.5 to 8) according to the season. The complete mineralization and dry residue; the spatial distributions of these data were quite identical as they directly or indirectly expressed the chemical concentration. A globally very low and sometimes low mineralization was identified in the plateau 4

5 area (conductivity below 200 S/cm) and in the plain area, a middle mineralization and sometimes important conductivity between 200 S/cm and 600 S/cm) but, sometimes the conductivity exceeded 600 S/cm. Basically, Na + and Ca ++ were the main cautions identified. In the plateau area, the preponderance of Na + over the other cautions was practically dear in the context of the project s site, and the Iron contents were relatively high, exceeding 0.3 mg/l. The Na + contents usually fluctuated between 10 and 100mg/l and K + contents between 5 and 35 mg/l. The Ca ++ contents were generally included between 15 and 70 mg/l and Mg ++ contents between 1 and 10 mg/l, rarely exceeding 15 mg/l. The HCO 3 - contents widely oscillated between the margins of 100 to 200 mg/l throughout the year. The concentrations of C1 - ions were generally included between 20 and 100 mg/l and those of SO 4 ions usually oscillated between 10 and 50 mg/l and the phosphate (PO 4 ) s ones were generally insignificant or under the detection threshold of the analysis process implemented. 5