Assessing salinization of coastal groundwater by tidal action: The tropical Wouri Estuary, Douala, Cameroon

Study region: Douala, Cameroon, West Africa. Study focus: Salinity of shallow coastal groundwater in Douala, Cameroon impairs its use for drinking and industrial purposes. Previous studies suggest that salinization is from tidal flooding from the Wouri Estuary. We aimed to test the tidal origin of groundwater salinization by conducting a time series investigation of water level and salinity, and assessed the stable water isotopes (delta O-18 and delta D) and major cations (Ca2+ and Mg2+) in groundwater and adjacent estuarine tidal creek. During the time series measurements, we pumped groundwater for over 5 tidal cycles to induce flow into a test well. New hydrologic insights for the region: The time-series groundwater level mimicked water level changes in the estuary. However, the temporal salinity in groundwater did not correspond to tidal salinity changes of estuarine water, indicating that estuarine water did not intrude groundwater. The delta O-18 and delta D of the groundwater and estuarine samples are collinear and fall along the local meteoric water line of Douala and had d-excesses of >10, indicating a non-evaporated rain recharge of groundwater or lack of salinization by evapoconcentration. The salinity-delta O-18 relationship showed that the origin of salinity in the coastal groundwater aquifer is not from seawater intrusion. The Mg2+/Ca2+ ratios of <1 in groundwater compared to 6-8 in estuarine water support the non-seawater origin of groundwater salinity. We conclude that the salinity of coastal groundwater in Douala is not affected by tide-induced salinization from the adjacent Wouri Estuary but by geogenic sources in the aquifer.