Hydrogeological Characteristics of Shallow Hard Rock Aquifers in Yaounde (Cameroon, Central Africa)

The groundwater contained in the alterites is one of the main sources of water supply for many households in the city of Yaounde and its surroundings. Information from the field and laboratory studies was compiled and analyzed in order to understand the hydrogeological context of this superficial aquifer. Preliminary results show a staged morphology of the alteration mantle (regolith) of Yaounde migmatitic representative of the polyphase character of alteration processes observed in all granito-gneissic formations of the world. This mantle has a multilayer system whose soil sets could have a different hydrodynamic functioning. The values of the hydraulic conductivity have a normal distribution and vary over four orders of magnitude, attesting the variability of the hydraulic conductivity of the soft materials. The hydrometric and piezometric characteristics indicate that the aquifer has highly heterogeneous zones that would be related to the morphostructural character of the region. The δ18O mean values of the rain (−2.47‰) and shallow groundwater (−2.57‰) are not significantly different. They indicate that the recharge of the shallow aquifer of Yaounde Precambrian basement is recent and is done directly by infiltration of precipitation without any notable change due to evaporation

Terrestrial and fluvial carbon fluxes in a tropical watershed: Nyong basin, Cameroon

The Nyong watershed, with an area of 27 800 km2 and a mean annual discharge of 390 m3 s−1, is the second largest river in Cameroon. The Nyong watershed serves as an outstanding study area for the examination of carbon fluxes in humid tropical environments because of its limited anthropogenic impact and homogeneous silicate bedrock. Between April 2005 and April 2007, we sampled water at seven stations, from the small watershed of the Mengong (0.6 km2) to the Nyong at Edea (24 500 km2), and monitored temperature, pH, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) contents, as well as the isotopic composition of DIC (δ13CDIC)andDOC(δ13CDOC).We estimated terrestrial net ecosystemproductivity in theNyong River watershed and measured fluvial fluxes of carbon to the ocean and the atmosphere. The Nyong River basin sequesters significant amounts of carbon on an annual basis: ~7 920 000t C year−1 (300 g C m−2 year−1). The combined dissolved organic, dissolved inorganic and atmospheric fluxes of carbon from the Nyong River only export 3% of this flux fromthe basin on an annual basis. This includes a minimumCO2 outgassing of 1487 g Cm−2 year−1, comparable to 115% of the annual flux of DOC and four times greater than the flux of DIC

Assessment of the impact of gold mining on the quality of water resources in the Lom Basin: Multivariate statistical analysis, water quality index and spatial interpolation

Le bassin du Lom (Est Cameroun) est soumis à une intense exploitation minière artisanale et semi-mécanisée de l’or. Dans ces types d’exploitations, les sols sont mis à nus, fortement remaniés et le mercure est souvent utilisé pour extraire le minerai par amalgamation. Pour estimer l’impact des activités d’exploitation de l’or sur la qualité de la ressource en eau du bassin, des mesures ont été effectuées sur les eaux de surfaces de 17 stations regroupées en 2 secteurs. Ces mesures ont porté sur les concentrations des paramètres physiques (pH, OD, TDS, CE et MES), chimiques (Na+, K+, Mg2+, Ca2+, NH4+, HCO3-, NO3-, SO42- , Cl- , F- et PO43-) et du mercure total (Hg tot.). Une analyse statistique multivariée [Analyse en Composants Principaux (ACP), matrice de corrélation de Pearson] a servi à représenter les variations des concentrations des paramètres physico-chimiques, à déterminer les principaux paramètres responsables de la qualité des eaux et à confirmer l’origine de la détérioration de la qualité de la ressource en eau. Un indice de qualité de l’eau [Water Quality Index (WQI)] a été calculé afin de classer les eaux du bassin par gamme de qualité allant d’excellente à très mauvaise. Enfin, l’analyse cartographique par interpolation spatiale a permis de réaliser des cartes de dispersion spatiale des teneurs en mercure du bassin. En guise de perspective, les résultats de cette étude vont permettre d’aborder la problématique de la qualité de l’eau du bassin par une approche de modélisation.6. Clean water and sanitatio

Caractérisation hydrogéologique des aquifères de socle altéré et fissuré du bassin versant de l’Olézoa à Yaoundé, Cameroun

Les eaux souterraines du socle altéré et fissurédu bassin de l’Olézoa, dans le centre-sud du Cameroun,assurent l'alimentation en eau d’une partie importante despopulations environnantes. Cependant, les caractéristiqueshydrogéologiques de ces aquifères de socle restent malconnues. La présente étude constitue une premièreapproche pour caractériser et mieux appréhender lastructure et le fonctionnement de ces aquifères de socle. Elleest fondée sur la description des profils pédologiques,des investigations structurales, le suivi de l’hydrométrieet de la piézométrie. Les résultatspréliminaires montrent que le profil d’altérationest de type polyphasé similaire à ceux des autresformations granito-gneissiques du monde soumises aux cyclesd’altération et d’érosion. On est enprésence d’une altération ancienne (10 à 20m d’épaisseur) issue du démantèlement dessystèmes latéritiques et d’une altérationrécente (2 à 3 m d’épaisseur)développée sur substratum sain. Le profil présenteen outre des microreliefs d’altération dont ladistribution spatiale peut influencer très localement ladynamique des écoulements. Cette dynamique est similaireà ce qui est généralement observé au seindes formations de socle, sous-climat tropical humide. Lescaractéristiques hydrométriques etpiézométriques indiquent que l’aquifèreprésente, par endroits, des zones fortementhétérogènes. La structuration tectonique anciennecontrôle l’orientation du réseau hydrographique.Ainsi, le modelé du paysage, qui est façonné parle réseau hydrographique, est à l’origine desécoulements souterrains d’amont en aval conforme auxgrandes lignes de la topographie. La distribution de cesécoulements montre l’influence possible despropriétés physiques et hydrauliques des terrains enamont du puits sur les systèmes d’écoulementssouterrains. Cette étude contribue à la connaissance dela structure et du fonctionnement hydrodynamique des aquifèresdu socle. Elle montre des liens structurels avec les autresaquifères décrits dans le monde et desspécificités régionales qui influencentl’hydrodynamisme du socle altéré et fissuré.The ground water of the altered and fissured basement of the Olezoawatershed in Centre-South Cameroon provides water to its surroundingpopulation. However, the hydrogeological characteristics of thesebasement aquifers have not yet been well studied. The present study isan attempt to better comprehend the structure and functioning of thesebasement aquifers. It mainly focuses on the description of soilprofiles, structural investigations, as well as hydrometric andpiezometric follow-ups. Preliminary results indicate that the soilprofile is of a polyphase type similar to others observed in otherparts of the world, particularly those developed on granito-gneissicformations, exposed to weathering and erosion. This is an ancientweathering system (10-20 m thick) derived from the degradation oflateritic systems and a more recent weathering (2-3 m thick) developedon the fresh rock. In addition, the profile contains alterationmicroreliefs, the spatial distribution of which can locally influenceflow dynamics. These dynamics are similar to those that commonly occurin basement rocks under wet tropical climates. The hydrometric andpiezometric characteristics indicate that the aquifers are quiteheterogeneous in some areas. The old structural tectonics control theorientation of the drainage pattern. As such, the nature of thelandscape designated by the drainage pattern is at the origin of thegroundwater flow from the upstream towards the downstream. Thedistribution of water flows from the upstream to the downstream showsthe possible influence of physical and hydraulic properties of theaquifer up-gradient from the well. This study is a contribution to theunderstanding of structures and the hydrodynamic functioning ofbasement aquifers. It shows a structural relationship with otheraquifers in other parts of world and the regional specificities whichinfluence the hydrodynamics in the weathered and fissured basement

Floods of 18 and 19 November 2016 in Batouri (East Cameroon): Interpretation of the Hydro-Meteorological Parameters and Historical Context of the Post-Event Survey Episode

During the night of November 18 to 19, 2016, many stormy cells are not very mobile organized on the east of the southern plateau of Cameroon and dumped up to 260 mm of rain in 4 hours. Occurring on a relatively saturated soil, these rains caused strong floods of Kadey and Doumé. The floods were particularly damaging in the city of Batouri, where a subdivision was submerged by the Boumbé (tributary of the Sangha) with water heights in the houses reaching 1.75 m, despite the presence of a dam allowing clipping floods upstream of the basin. In this article, we present the results of the analysis of the postevent survey generated on this event with flow rates estimated on 15 sections of ungauged subbasins. These flows are then compared with those obtained from other recent postevent survey and those estimated by various regional estimations. The inventory of heavy rains around Batouri during the period 1970-2016 has led to the revision of current development standards in the region, which seem to underestimate rainfall and infrequent flows

Soil Carbon Stock and River Carbon Fluxes in Humid Tropical Environments: The Nyong River Basin (South Cameroon)- Chapter 18

This study investiged the concentrations of carbon (C) in different water reservoirs in different forms in relation to the weathering rate and the C content in the soils of five nested catchments belonging to the Nyong basin in South Cameroon. This forested granitic watershed is representative of the humid tropical domain. The five nested catchments (from upstream to downstream) are : the experimental Mengong catchment near Nsimi village, the Awout watershed (tributary of the So'o) at Messam, the So'o watershed (tributary of the Nyong) at Pont So'o, the upper Nyong basin at Mbalmayo, and at Olama station after the Nyong and So'o confluence. Soil samples were collected and analysed only in the Mengong catchment for different horizons. The C contents exhibited a very heterogeneous distribution in the different soil profiles. The organic C stock was estimated at 32 000 t in soils of this catchment. For the hydrolic year 1998/99, the mean annual DOC concentration was 0.31 mg/litre in the spring and 14.3 mg/litre in the stream at the outlet of the Mengong catchment. the mean dissolved organic C (DOC) river fluxes were between 4700 and 9300 kg/km2 per year. In coloured river and swamp waters, DOC fluxes were between 4700 and 9300 kg/km2 per year, whereas POC fluxes were between 600 and 1400 kg/km2 per year. In contrast, clear spring waters or groundwaters contain low DOC contents. The mean annual bicarbonate concentrations were 3.6 mg/litre in the spring water, and between 3.4 and 7.7 mg/litre in different rivers. In a non-carbonated environment, the dissolved inorganic carbon fluxes are normally low. Whereas for the Nyong River basin, these fluxes are lower than for other tropical silicatedd catchments due to low chemical weathering rate of silicate rocks for the upper Nyong Basin at Olama. Particulate organic C (POC) was measured (17 samples) during the flood event of 28 and 29 April 2002 on the Mengong River. These measurements indicated that POC content represented on average 25.3% if the TSS content, with relatively low variations compared to water discharge fluctuations

Present weathering rates in a humid tropical watershed: Nsimi, South Cameroon

International audienceThe study of biogeochemical and hydrological cycles in small experimental watersheds on silicate rocks, common for the Temperate Zone, has not yet been widely applied to the tropics, especially humid areas. This paper presents an updated database for a six-year period for the small experimental watershed of the Mengong brook in the humid tropics (Nsimi, South Cameroon). This watershed is developed on Precambrian granitoids (North Congo shield) and consists of two convexo-concave lateritic hills surrounding a large flat swamp covered by hydromorphic soils rich in upward organic matter. Mineralogical and geochemical investigations were carried out in the protolith, the saprolite, the hillside lateritic soils, and the swamp hydromorphic soils. Biomass chemical analyses were done for the representative species of the swamp vegetation. The groundwater was analysed from the parent rock/saprolite weathering front to the upper fringe in the hillside and swamp system. The chemistry of the wet atmospheric and throughfall deposits and the Mengong waters was monitored. In the Nsimi watershed the carbon transfer occurs primarily in an organic form and essentially as colloids produced by the slow biodegradation of the swamp organic matter. These organic colloids contribute significantly to the mobilization and transfer of Fe, Al, Zr, Ti, and Th in the uppermost first meter of the swamp regolith. When the organic colloid content is low (i.e., in the hillside groundwater), Th and Zr concentrations are extremely low (<3 pmol/L, ICP-MS detection limits). Strongly insoluble secondary thorianite (ThO2) and primary zircon (ZrSiO4) crystals control their mobilization, respectively. This finding thus justifies the potential use of both these elements as inert elements for isoelement mass balance calculations pertaining to the hillside regolith. Chloride can not be used as a conservative tracer of hydrological processes and chemical weathering in this watershed. Biogenic recycling significantly influences the low-Cl input fluxes. Sodium is a good tracer of chemical weathering in the watershed. The sodium solute flux corrected from cyclic salt input was used to assess the chemical weathering rate. Even though low (2.8 mm/kyr), the chemical weathering rate predominates over the mechanical weathering rate (1.9 mm/kyr). Compared to the Rio Icacos watershed, the most studied tropical site, the chemical weathering fluxes of silica and sodium in the Mengong are 16 and 40 times lower, respectively. This is not only related to the protective role of the regolith, thick in both cases, but also to differences in the hydrological functioning. This is to be taken into account in the calculations of the carbon cycle balance for large surfaces like that of the tropical forest ecosystems on a stable shield at the global level