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)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(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 HCO 3 , a decrease in the pH and increase in SO 4 and NO 3 concentrations of shallow groundwater. Average groundwater pH range is 3.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.
Groundwater is used by 3 million inhabitants in the coastal urban city of Douala, Cameroon, but comprehensive data are too sparse for it to be managed in a sustainable manner. Hence this study aimed to (1) assess the potability of the groundwater; (2) evaluate the spatial variation of groundwater composition; and (3) assess the interaction and recharge mechanisms of different water bodies. Hydrogeochemical tools and methods revealed the following results in the Wouri and Nkappa formations of the Douala basin, which is beneath Douala city: 30% of water samples from hand-dug wells in the shallow Pleistocene alluvium aquifer were saline and highly mineralized. However, water from boreholes in the deeper (49-92 m depth) Palaeocene aquifer was saline-free, less mineralized and potable. Water in the shallow aquifer (0.5-22 m depth) was of Na +-K +-Cl −-NO 3 − type and not potable due to point source pollution, whereas Ca +-HCO 3 − unpolluted water dominates in the deeper aquifer. Water in the deep and shallow aquifers indicates the results of preferential flow pass and evaporative recharge, respectively. Possible hydrogeochemical processes include point source pollution, reverse ion exchange, remote recharge areas and mixing of waters with different chemical signatures.
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 δ 13 C 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 δ 13 C TOC , δ 13 C DOC and δ 13 C POC of these carbon species were very close (− 30‰ to − 26‰) and typical of the forested C3 vegetation. The relationship observed between DOC and log pCO 2 and δ 13 C DIC indicated that besides the considerable CO 2 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 δ 13 C 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.
KeywordsDissolved inorganic carbon (DIC) • δ 13 C carbon isotope composition • Dissolved organic carbon (DOC) • Soil organic carbon mineralization • Partial pressure of CO 2 (pCO 2 ) • Carbon sources Electronic supplementary material The online version of this article (
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