Caractérisation des relation eaux de surface - eaux souterrain en milieu tropical sec : exemple du bassin de la Nema (sine Saloum, Sénégal).

Les nappes d'eau phréatiques, facilement accessibles, sont les réserves les plus sollicitées dans les régions tropicales sèches. Une étude de leurs interactions avec les eaux de surface a été entreprise conjointement par le département des eaux continentales de l'ORSTOM - Dakar et le laboratoire d'hydrogéologie de l'université Cheikh Anta Diop. Cet article présente les principaux résultats obtenus dans le Sine - Saloum(Sénégal). La nappe phréatique est contenue dans les formations argilo sableuses du continental terminal. Sa profondeur augmente considérablement du bas-fond marécageux vers les zones de plateau. La surface piézométrique épouse les formes de la vallée. Les lignes de courant indiquent un écoulement radial convergeant vers la rivière. Pendant la saison de pluie (juillet à octobre), la nappe emmagasine importante quantité d'eau avec une vitesse d'infiltration- d'environ 5 m/j. Au cours de la longue saison sèche (novembre à juin), la réserve souterraine régulatrice s'amenuise progressivement pour s'annuler à la fin de la saison. Le bilan de l'hydrosystème montre deux sources de vidange de la nappe superficielle : - la résurgence sur les berges commence au milieu de là saison de pluie et se poursuit pendant la saison sèche. Elle draine annuellement 4 à212 % de la réserve. - la reprise évaporatoire, épisode: clique pendant la saison de pluie et continue en saison sèche, est le principal facteur d'épuisement de la réserve en eau du bassin. Elle reprend annuellement environ 90 % de la masse d'eau emmagasinée. Aussi les mouvements d'eau sont dominés par des transferts verticaux entre la nappe et l'atmosphère, conditionnés par le degré hygrométrique de l'air

Water supply, sanitation and health risks in Douala, Cameroon

With a view to contribute to the hygiene condition and improvement in the urban zones of Cameroon, a survey was carried out in March 2007 among 1400  households with respect to the water supply, sanitation and health risks in seven quarters of spontaneous habitat of Douala town. It results from this study that the majority of participants were married (76%). The populations are supplied with of  water especially by CAMWATER network (49% of households) and wells (50% of  households). The majority of participants evacuate solid waste in public refuse vats (56% of households). Household’s wastewater is especially discharged around the houses (21% of households) and in rivers (20% of households). The companies  present in the quarters discharge their wastewater in the drains. 52% of households deposit their excrements in latrines. We noticed in these quarters a prevalence of residences made out of hard materials (43% of households). Cholera (88.5% of households), Diarrhoea (70% of households), Dysenteries (74% of households), Typhoid fever (72% of households), Malaria (32% of households) and skin diseases (76% of households) were the most frequent diseases. These results highlight problems for which, it would firstly be necessary to attack in the research of the  improvement strategies for hygiene conditions in the populations of an urban  environment.Key words: Environment, sanitation, water, diseases, Douala

Estimation of the surface runoff and its suspended load in some major world river basins

In 26 major world rivers, the estimation the early average contribution of the surface runoff to the river discharge, using the spectral analysis of Fourier applied to the time series of mean monthly discharges. The river-suspended sediment transports are used to calculate the concentration in surface runoff. On aglobal scale, these results allow assessing the surface runoff intensity over the entire world and to calculate its suspended load

La plaine du Yaéré dans le Nord-Cameroun

Située entre les 10° et 13° de latitude Nord et les 14° et 16° de longitude Est, la province de l’extrême-nord du Cameroun est occupée par une grande plaine qui s’étend des pieds des monts Mandara au sud-ouest jusqu’au lac Tchad dans le nord, les bourrelets de berge du fleuve Logone en formant la limite est. Elle fait partie d’une vaste unité géomorphologique dénommée « cuvette tchadienne », qui s’étend entre le Cameroun, le Nigeria, le Niger et le Tchad. Une grande partie de la plaine camero..

Rainwater chemistry and wet deposition over the Equatorial Forested ecosystem of Zoétélé (Cameroon)

Within the framework of IDAF (IGAC DEBITS AFRICA: International GlobalAtmospheric Chemistry/DEposition of Biogeochemically Important TraceSpecies/Africa) network, data analysis is realised on precipitation chemical composition collected in Zotl, in Southern Cameroon. This station, located atabout 200 km from the Atlantic Ocean, is representative of a so-called `Evergreen Equatorial Forest' ecosystem. An automatic wet-only precipitation collector was operated at the station from 1996 to 2000. The rainfall regime, associated with eastward advection of moist and cool monsoon air masses, amounts to an average of 1700 mm/year. Inorganic and organic content of the precipitation were determined by IC in 234 rainfall events, representing a total 4,583 mm of rainfall from an overall of 7,100 mm.The mean annual precipitation chemistry and wet deposition fluxes characteristic of an African equatorial forest are quantified. Typical atmospheric gases and particles sources influence the precipitation chemical content and the associated deposition of chemical species. Indeed, hydrogen concentration is the highest (12.0 eq.L–1) of the IDAF measurements, leading to acid rains with a low mean pH 4.92. The mineral species are dominated by nitrogenous compounds (NH4 +:10.5 and NO3 –: 6.9 eq.L–1), Ca2+ (8.9 eq.L–1) and SO4 2 – 5.1 eq.L–1. Relationship between Ca2 + and SO4 2 – indicated aterrigeneous particulate source and an additional SO4 2 – contributionprobably due to swamps and volcano emissions. Na+ and Cl–concentrations, around 4.0 eq.L–1, seem very low for this site,accounting for the marine source. Besides, strong correlations between NH4 +/K+/Cl– indicate the biomass burning originof these species. Accordingly, precipitation chemistry in Zotlis influenced by three major sources: biogenic emissions from soil and forest ecosystems, biomass burning from savannah, and terrigenous signature from particles emissions of arid zones; and three minor sources: marine, volcano and anthropogenic. In spite of the relatively low concentration of all these elements, the wet deposition is quite significant due to the high precipitation levels, with for example a nitrogenous compounds deposition of 34 mmol.m–2.yr–1

Chemical composition and sources of atmospheric aerosols at Djougou (Benin)

International audienceIn the framework of the INDAAF (International Network to study Deposition and Atmospheric chemistry in AFrica) program, atmospheric aerosols were collected in PM2.5 and PM10 size fractions at Djougou, Benin, in the West Africa, from November, 2005 to October, 2009. Particulate carbon, ionic species, and trace metals were analyzed. Weekly PM2.5 and PM10 total mass concentrations varied between 0.7 and 47.3 µg m-3 and 1.4-148.3 µg m-3, respectively. We grouped the aerosol chemical compounds into four classes: dust, particulate organic matter (POM), elemental carbon (EC), and ions. We studied the annual variation of each class to determine their contribution in the total aerosol mass concentration and finally to investigate their potential emission sources. On an annual basis, the species presented a well-marked seasonality, with the peak of mass concentration for both sizes registered in dry season, 67 ± 2 to 86 ± 9 versus 14 ± 9 to 34 ± 5% in wet season. These values emphasized the seasonality of the emissions and the relative weak interannual standard deviation indicates the low variability of the seasonality. At the seasonal scale, major contributions to the aerosol chemistry in the dry season are: dust (26-59%), POM (30-59%), EC (5-9%), and ions (3-5%), suggesting a predominance of Sahelian and Saharan dust emissions and biomass burning source in this season. In the wet season, POM is predominant, followed by dust, EC, and ions. These results point out the contribution of surrounded biofuel combustion used for cooking and biogenic emissions during the wet season

Satellite mapping of rain induced nitrite oxide emission form soils

We use space-based observations of NO 2 columns from the Global Ozone Monitoring Experiment (GOME) to map the spatial and seasonal variations of NO x emissions over Africa during 2000. The GOME observations show not only enhanced tropospheric NO 2 columns from biomass burning during the dry season but also comparable enhancements from soil emissions during the rainy season over the Sahel. These soil emissions occur in strong pulses lasting 1–3 weeks following the onset of rain, and affect 3 million km 2 of semiarid sub-Saharan savanna. Surface observations of NO 2 from the International Global Atmospheric Chemistry (IGAC)/Deposition of Biochemically Important Trace Species (DEBITS)/Africa (IDAF) network over West Africa provide further evidence for a strong role for microbial soil sources. By combining inverse modeling of GOME NO 2 columns with space-based observations of fires, we estimate that soils contribute 3.3 ± 1.8 TgN/year, similar to the biomass burning source (3.8 ± 2.1 TgN/year), and thus account for 40% of surface NO x emissions over Africa. Extrapolating to all the tropics, we estimate a 7.3 TgN/year biogenic soil source, which is a factor of 2 larger compared to model-based inventories but agrees with observation-based inventories. These large soil NO x emissions are likely to significantly contribute to the ozone enhancement originating from tropical Africa