CaractÃ©risation Hydrodynamique et QualitÃ© des eaux de lâ€™aquifÃ¨re Ã nappe libre du bassin versant du NkiÃ© (YaoundÃ©â€“Cameroun)
Authors: Ngouh Abdou Nasser , Kpoumie Amidou , Etame Ghislain Nyembe , Lebga Aloysious Kohtem , Ndjeng Esse , Ngoupayou Jules Remy Ndam 
Published: June 2020

Entre septembre 2017 et aoÃ»t 2018, une Ã©tude visant Ã  faire une caractÃ©risation physique, Ã  comprendre le comportement hydrodynamique et Ã  Ã©valuer la qualitÃ© des eaux de lâ€™aquifÃ¨re Ã  nappe libre du bassin versant du NkiÃ© a Ã©tÃ© menÃ©e. Lestravaux de terrain et de laboratoire menÃ©s conjointement ont consistÃ© au suivi bimensuel des ouvrages hydrauliques prÃ©alablement identifiÃ©es, aux mesures in situ de la permÃ©abilitÃ©, aux Ã©chantillonnages des sols et des eaux pour analyses de la granulomÃ©trie et des paramÃ¨tres physico chimiques et bactÃ©riologiques. A lâ€™issue de ces travaux, lâ€™analyse granulomÃ©trique des sols du BV rÃ©vÃ¨le une diminution de la taille des particules de la base vers le sommet du profil. Le report pondÃ©ral des diffÃ©rents niveaux de sols, permet de distinguer les sables graveleux, les sables, les graviers sableux et les argiles sableuses. Ces formations lithologiques ont des permÃ©abilitÃ©s qui varient entre 1,8.10-6 m.s -1 et 5,92.10-5 m.s -1 , illustrant ainsi quâ€™ellessont peu Ã  assez permÃ©ables. Les dÃ©bits journaliers dessources varient entre 0,69 l.s -1 (2,5 m3 .h-1 ) et 0,06 l.s -1 (0,22 m3 .h-1 ) en saison sÃ¨che (SS) et entre 0,86 l.s -1 (3,1 m3 .h-1 ) et 0,08 l.s -1 (0,3 m3 .h-1 ) en saison pluvieuse (SP). Les sources de plus grands dÃ©bits seraient situÃ©es sur les principaux axes de drainage correspondant aux axes principaux de fracturation (N10-30, N40-50, N80-100, N110-120). Lâ€™analyse des cartes piÃ©zomÃ©triques rÃ©vÃ¨le une convergence des lignes de courant dâ€™eau vers le cours dâ€™eau NkiÃ© qui en est le principal axe de drainage de lâ€™aquifÃ¨re Ã  nappe libre. Le bilan hydrique Ã©tabli, montre que le rÃ©servoir dâ€™altÃ©rites se recharge dâ€™avril Ã  novembre sous lâ€™action de lâ€™excÃ©dent pluviomÃ©trique et se vide de dÃ©cembre Ã  mars lors de lâ€™Ã©tiage. Ainsi, le comportement hydrodynamique de la nappe est Ã©troitement liÃ© aux fluctuations pluviomÃ©triques. Les eaux de sources sont acides et peu minÃ©ralisÃ©es. Ce sont les eaux de type chlorurÃ© sodique et potassique. La prÃ©sence remarquÃ©e des nitrates, des coliformes fÃ©caux et des streptocoques fÃ©caux dont les concentrations sont supÃ©rieures aux limites de toxicitÃ© dÃ©finies par lâ€™OMS (2011) induisent une pollution bactÃ©riologique et chimique de ces eaux. Elles nÃ©cessitent donc Ã  cet effet un traitement prÃ©alable avant toute consommation.
Between September 2017 and August 2018, a study aimed at carrying out a physical characterization, understanding the hydrodynamic behavior and evaluating the quality of the waters of the free water aquifer of the NkiÃ© watershed was carried out. The field and laboratory work carried out jointly consisted of the fortnightly monitoring of the hydraulic structures previously identified, in situ measurements of permeability, soil and water sampling for analysis of particle size and physico-chemical and bacteriological parameters. At the end of this work, the particle size analysis of the BV soils reveals a reduction in the particle size from the base to the top of the profile. The weight carryover of the different soil levels makes it possible to distinguish gravelly sands, sands, sandy gravels and sandy clays. These lithological formations have permeabilities which vary between 1.8.10-6 m.s-1 and 5.92.10-5 m.s-1, thus illustrating that they are little to fairly permeable. The daily flow rates of the sources vary between 0.69 ls-1 (2.5 m3 .h-1 ) and 0.06 ls-1 (0.22 m3 .h-1 ) in the dry season (SS) and between 0.86 ls-1 (3.1 m3 .h-1 ) and 0.08 ls-1 (0.3 m3 .h1 ) in the rainy season (SP). The sources of larger flows would be located on the main drainage axes corresponding to the main fracturing axes (N10-30, N40-50, N80-100, N110-120). Analysis of the piezometric maps reveals a convergence of the streamlines towards the NkiÃ© stream, which is the main drainage axis of the free aquifer. The established water balance shows that the alterite reservoir is recharged from April to November under the action of excess rainfall and is emptied from December to March during low water. Thus, the hydrodynamic behavior of the aquifer is closely linked to rainfall fluctuations. The spring waters are acidic and not very mineralized. These are waters of the sodium and potassium chloride type. The noted presence of nitrates, fecal coliforms and fecal streptococci whose concentrations are above the toxicity limits defined by the WHO (2011) induce bacteriological and chemical pollution of these waters. They therefore require prior treatment for this purpose before any consumption.