Controls on groundwater chemistry in a highly urbanised coastal area

Douala metropolitan city loated in the littoral province of Cameroon,has experienced a rapid urbaniation and industralisation. The city depends heavily on groundwater as a resource. Due to ubiquitous sources of pollution, groundwater quality and contamination has become an imprtant issue for this urban groundwater supply. This study uses the major ion chemistry of groundwater together with the minerallogical and chemical composition of sediments to investigate the chemical characteristics and contamination of groundwater. An attempt has been made to identify the different factors controlling the chemical composition of groundwater. Groundwater is acidic (4.1-6.9) and the chemistry is more influenced by atmospheric inputs and anthropogenic activities than by aquifer water reactions. The ionic content of groundwater shows a large variation with electrical conductivity ranging from 34.3-1021µs/cm. However, Cl and HCO3 are dominant anions meanwhile Na and Ca are dominant cations. Groundwater from natural low residential areas has low electrical conductivity and Ca-Na-HCO3 type where as the chemical composition of groundwater is shifted to Ca-Na-Cl ( NO3 +SO4 )downstream with maximum electrical conductivity and high nitrate levels above the guide limits occuring in the highly urbanised, settlements at the Cente of the study area.Groundwater is vulnerable to acid deposition due to the resistant nature and the low level of base forming cations in the aquifer sediments, which provides very little buffer for acid inputs.