Chemical Measures to Characterize Coastal Groundwater: A Case Study in Benin, West Africa

Understanding coastal hydrogeology can be important for assessing the sustainability of groundwater resources for urban centers in coastal environments. While a number of advanced methods based on isotopic analysis, detailed modeling, and detailed characterization of hydraulics can be applied in coastal regions of developed countries, these approaches may be impossible in many coastal regions of developing countries due to the lack of available data or access to sophisticated analytical techniques. In this study, Benin, West Africa, was used as a case study to show how a field measureable and relatively simple water quality analysis of shallow groundwater samples can be used to identify different hydrological zones for further study. Various chemical methods (including ion specific electrodes, a conductivity probe, and ion chromatography) were used to determine the chloride, sodium, bromide, and electrical conductivity of the samples, with additional parameters of sulfate, nitrate, and phosphate determined through the use of ion chromatography for bromide. These data, through the combination of a suite of statistical and geostatistical analysis techniques (including parameter comparisons, indicator plots, cluster analysis, and variogram analysis), were used to identify potential critical zones based on the current conditions of the coastal wetlands that indicate saline influence and changes in water quality that are likely to impact the water quality at the well field supplying drinking water to Cotonou, Benin (a population of approximately 1.5 million). Specific to the case study, this combination of chemical and geostatistical methods demonstrated that a series of coastal wetlands in Benin demonstrated both local variability due to rapid changes in landscape and regional variability. These data also demonstrated the high level of 'preliminary' field characterization that is possible using a carefully planned combination of the field sampling design, water quality chemical analysis, and geostatistical analysis. This preliminary characterization has identified a coastal zone of saline influence intruding into the wetlands as well as several areas with other interesting chemical characteristics (including an altered Cl/Br ratio and higher nitrates). Finally, the limitations and challenges in this approach related to the analytical challenges, limited site access, and constraints on the use of data to provide definitive hydrogeologic characterization are discussed.