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

Abstract

Study regionDouala, Cameroon, West Africa. Study focusSalinity 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 (δ¹⁸O and δD) 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 regionThe 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 δ¹⁸O and δD 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-δ¹⁸O 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.