Combining Simple Field Measures of Chemistry and Hydraulics to Assess Complex Wetland Hydrogeology: 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 where available data or collectable information can be extensive, these approaches may be impossible in many coastal regions of developing countries due to the lack of available data or unavailability of sophisticated analytical techniques. In this study, Benin, West Africa, is used as a case study to show how relatively simple direct-push methods (to collect water samples for water quality analysis and penetration based soil-resistance data) can be combined with falling-head permeameter and electrical resistivity data to identify different hydrological zones for further study through additional field and numerical modeling efforts. Specific to the case study, this combination of methods demonstrated that a series of coastal wetlands in Benin demonstrated both local variability (on a scale of less than 100 meters) due to rapid changes in landscape (landuse ranges from permanent wetlands to seasonal wetlands to human impacted dry land) and regional variability (at the scale of kilometers). These data, combined with a geostatistical analysis and a MODFLOW model of the underlying aquifer system, will be used to characterize the critical zones of surface water infiltration and groundwater outflow likely to impact the water quality at the well field supply drinking water to Cotonou, Benin (a population of approximately 1.5 million). These data also demonstrate the high level of “preliminary” characterization that is possible using a carefully planned combination of simple field measures. Finally, limitations and challenges in this approach related to the analytical challenges, limited site access, and constraints on the use of the collected data to provide definitive hydrogeologic characterization are discussed.