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.