INVESTIGATING THE EFFECTS OF ESTUARINE WATER ON GROUNDWATER QUALITY IN A POPULATED NEIGHBORHOOD IN THE WOURI ESTUARY, CAMEROON

The city of Douala (210 km²) in the Wouri estuary is home to 3 million people who mostly depend on groundwater for domestic and industrial activities. The importance of groundwater as a resource poses a critical need for understanding how the estuary affects groundwater chemistry. The goal of this study is to assess the influence of tidal regime on groundwater quality. Our objectives are to (1) document the temporal variations in the physicochemical parameters of groundwater, (2) determine the extent of tidal influence on the interaction between groundwater and estuarine water, and (3) examine the effect of the estuarine water on groundwater quality. Measurements of physicochemical parameters were made in 3 wells located between 150-300 m from the shoreline. Physicochemical parameters were logged at minute intervals for 48 h using a levelogger. Temperature, pH, salinity, TDS, ORP and DO were recorded using a multiparameter probe. Alkalinity was determined by titration and ionic concentrations were obtained by spectrophotometry. Samples were collected for DIC, DOC, POC, cation and anion analyses. Water levels fluctuated by 0.4 m, pH ranged between 5.9 and 6.7 and conductivity varied between 780 and 820 µS/cm. The mean salinity in the groundwater was 1.26 (±0.05) and DO varied between 27 and 66%. Fe, NO₃^-, PO₄ and NH₄⁺ concentrations varied between 20 and 24 mg/l, 15 and 21 mg/l, 0.3 and 0.5 mg/l and 1.5 and 3.9 mg/l, respectively. Fluctuations in groundwater levels were concomitant with tidal cycles, but not with conductivity trends. Although concentrations of ions and metals in groundwater are high, they do not appear to be affected by estuarine water incursion. We infer that groundwater levels were responding to tidal energy but not to sea flow into groundwater. We suggest that the contribution of ions is from other sources such as aquifer geology and anthropogenic sources.