Stable Carbon Isotopes δ13C as a Proxy for Characterizing Carbon Sources and Processes in a Small Tropical Headwater Catchment: Nsimi, Cameroon

Stream carbon fluxes are one of the major components in the global C cycle, yet the discrimination of the various sources of stream carbon remains to a large extent unclear and less is known about the biogeochemical transformations that accompany the transfer of C from soils to streams. Here, we used patterns in stream water and groundwater δ13C values in a small forested tropical headwater catchment to investigate the source and contribution from the soil carbon pools to stream organic and inorganic carbon behavior over seasonal scales. Stream organic carbon (DOC and POC) comes mainly from the upper rich soil organic carbon horizons and derived from total organic carbon (TOC) of biogenic source. The isotopic compositions δ13CTOC, δ13CDOC and δ13CPOC of these carbon species were very close (− 30‰ to − 26‰) and typical of the forested C3 vegetation. The relationship observed between DOC and log pCO2 and δ13CDIC indicated that besides the considerable CO2 evasion that occurs as DIC is transported from soils to streams, there were also other processes affecting the stream DIC pool. In-stream mineralization of DOC and mixing of atmospheric carbon had a significant influence on the δ13CDIC values. These processes which varied seasonally with hydrological changes represent the main control on DOC and DIC cycling in the wet tropical milieu. The rapid turnover of carbon on hillside soils, the transformation of TOC to DOC in wetland soils and further mineralization of stream DOC to DIC favor the evasion of C, making the zone a source of carbon to the atmosphere

Apport de l’hydrochimie et de l’isotope de l’environnement à la connaissance des ressources en eaux souterraines de Yaoundé, Cameroun

Introduction : L’eau souterraine constitue la principale source d’approvisionnement en eau potable de la population de la région de Yaoundé. Cependant, le manque de suivi de l’évolution de la qualité de cette eau et la méconnaissance des caractéristiques hydrochimiques est à noter.Objectif : Cette étude se propose d’améliorer les connaissances sur ces ressources en eaux les plus exploitées. Il est question de déterminer l’origine de la minéralisation des eaux souterraines et le mécanisme de recharge de la nappe à partir d’une combinaison des méthodes hydrochimiques et isotopiques.Méthodologie et résultats : Les travaux ont nécessité des campagnes d’échantillonnages, des mesures in situ, des dosages des éléments chimiques et isotopiques et enfin des analyses statistiques. Les résultats montrent que les eaux sont acides (pH= 4,9 ± 0,2) et faiblement minéralisées. Le faciès chimique est chloruré sodique et potassique. Les principaux ions à l’origine de la minéralisation des eaux seraient issus de l’hydrolyse des minéraux silicatés, du phénomène d’échange de base et de la pollution anthropique. Les fortes teneurs en CO2 dissous (8,6.10-4 ± 3,1 atm > 3,16.10-4 atm) dans les eaux souterraines témoignent qu’elles circulent ensystème ouvert par rapport au CO2 gazeux du sol. Les teneurs moyennes s’établissent pour l’oxygène-18 à -2,57 ± 0,23‰ et pour le deutérium à -10,93 ± 5,6 ‰ avec un excès en deutérium de 8,64± 5,2‰.Conclusion et application des résultats : La recharge de la nappe est récente et se fait directement par infiltration des précipitations sans aucune modification notable due à l'évaporation. Cet apport récent d’eau est confirmé par les faibles valeurs du bicarbonate. Les fortes teneurs en nitrate dans les eaux souterraines montrent que celles-ci sont chimiquement non appropriées à la consommation humaine et nécessite un traitement préalable.Mots clefs : Hydrochimie, isotope, recharge, minéralisation, Yaound

Macrophytes and water quality of the Nkoup River System (Foumbot, Cameroon)

No Abstract.Journal of the Cameroon Academy of Sciences Vol. 7 (2) 2007: pp. 59-7

Stable carbon isotopes delta C-13 as a proxy for characterizing carbon sources and processes in a small tropical headwater catchment : Nsimi, Cameroon

Stream carbon fluxes are one of the major components in the global C cycle, yet the discrimination of the various sources of stream carbon remains to a large extent unclear and less is known about the biogeochemical transformations that accompany the transfer of C from soils to streams. Here, we used patterns in stream water and groundwater delta C-13 values in a small forested tropical headwater catchment to investigate the source and contribution from the soil carbon pools to stream organic and inorganic carbon behavior over seasonal scales. Stream organic carbon (DOC and POC) comes mainly from the upper rich soil organic carbon horizons and derived from total organic carbon (TOC) of biogenic source. The isotopic compositions delta C-13(TOC), delta C-13(DOC) and delta C-13(POC) of these carbon species were very close (-30 parts per thousand to -26 parts per thousand) and typical of the forested C3 vegetation. The relationship observed between DOC and log pCO(2) and delta C-13(DIC) indicated that besides the considerable CO2 evasion that occurs as DIC is transported from soils to streams, there were also other processes affecting the stream DIC pool. In-stream mineralization of DOC and mixing of atmospheric carbon had a significant influence on the delta C-13(DIC) values. These processes which varied seasonally with hydrological changes represent the main control on DOC and DIC cycling in the wet tropical milieu. The rapid turnover of carbon on hillside soils, the transformation of TOC to DOC in wetland soils and further mineralization of stream DOC to DIC favor the evasion of C, making the zone a source of carbon to the atmosphere

Acidification of shallow groundwater in the unconfined sandy aquifer of the city of Douala, Cameroon, Western Africa: implications for groundwater quality and use

The alluvial aquifer underlying the city of Douala comprises shallow Quaternary deposits where groundwater is the main source for domestic and drinking purposes. Shallow groundwater in the area show signs of acidification with average pH range of 3.8–6.8. Long-term groundwater chemistry data (1998–2013), hydrogeochemical and R mode factor analysis were used to establish the acidification process of shallow groundwater and also determine possible origin and implications for water quality and use in the area. Twenty-six groundwater sample points, three streams and three rain sample points were studied seasonally in the 2006–2008 and the 2010–2013 study periods. The data were compared with result of a study, 7 years earlier (1998–1999). The results show evidence of acidification manifested by depletion of HCO3, a decrease in the pH and increase in SO4 and NO3 concentrations of shallow groundwater. Average groundwater pH range is 3.94–7.70 (1998–1999), 3.8–6.91 (2006–2008), and 3.7–6.8 (2010–2013). Only approximately 16 % (1998–1999), 14 % (2006–2008) and 11.11 % (2010–2013) of water samples fall within the range of pH (6.5–8.5) for potable water according to WHO (1993). The alkalinity/acid neutralizing capacity of the shallow groundwater has decreased significantly coupled with increase in the number of zero alkalinities recorded in the 2010–2013. The shallow groundwater is generally undersaturated with common carbonate minerals (calcite, dolomite), therefore providing insufficient acid buffer. Principal component analysis in combination with hydrogeochemical studies revealed that four main factors are responsible for the groundwater chemistry and acidity: (1) acid atmospheric deposition, (2) anthropogenic activities (industrial effluent discharges and acid spill, (3) chemical weathering, and (4) coastal atmospheric deposition/cation exchange. In general, the shallow groundwater is not suitable for drinking and domestic purposes with respect to the low pH and elevated nitrate concentration. In view of the implications such as increase in corrosion and increased mobilization of toxic elements (e.g. Al, Pb, Cu, Zn, Mn) as well as their possible harmful effect on health, it is recommended that the causes, rate of acidification and the mobility of trace elements be investigated with more details