Abstract
The largest city in Benin, West Africa (Cotonou), is reliant upon groundwater for its public water supply. This groundwater is derived from the Godomey well field which is located approximately 5 Km north of the coast of the Atlantic Ocean and in close proximity to Lake Nokoue—a shallow lake containing water with elevated concentration of chloride and other elements. Historical data indicate increased chloride concentration in a number of wells nearest to the lake, with unknown contribution from groundwater encroachment from the coastal area. Hence, there is substantial interest in better characterizing this groundwater system for the purpose of determining appropriate management practices and degree of sustainability. Among the efforts attempted to date are a series of numerical models ranging from assessment of flow to a recent effort to include density-dependent transport from the lake. In addition, substantial field characterization has been pursued including assessment of shallow water chemistry along the region of the coastal lagoon and border of the lake, characterization of hydraulic response to pumpage in the aquifer system, estimation of the distribution of electrical resistivity with depth along the coastal lagoons, and installation of multi-level piezometers at seven locations in the lake. When integrated across methods, these numerical and field results indicate that the lake remains a primary concern in terms of a source of salinity in the aquifer. Further, the coastal region appears to be more complex than previously suggested and may represent a future source of salt-water encroachment as suggested by current presence of saline waters at relatively shallow depths along the coast. Finally, hydraulic testing suggests that both natural and pumping-based fluctuations in water levels are present in this system. Substantial additional characterization and modeling efforts may provide a significantly greater understanding of the behavior of this complex groundwater system and, thereby, an improved ability to manage the potential for negative impacts from salt-water and anthropogenic contaminants entering this sole source of fresh water for southern Benin.
Highlights
Coastal groundwater systems represent both crucial water resources for growing population centers and complex hydrologic systems sensitive to changes in both land-use practices and climate
The extensive literature on coastal environments has provided the technical foundation for ongoing studies of a coastal aquifer system in Benin, West Africa
While the model has been able to reproduce the details of variation in hydraulic behavior at specific wells and piezometers (e.g., Figure 3a shows the comparison of observed and predicted variation in the screen at 65 meters within the piezometer nest, PU1), challenges remain in the modeling effort in terms of temporal/spatial distribution of hydraulic head (e.g., Figure 3b shows the predicted and observed differences in hydraulic head in the screens at 65 meters and 135 meters in the same piezometer, PU1). This model has provided initial insight into the spatial distribution of chloride as derived from Lake Nokoue (e.g., Figure 3c for well F11), providing strong support of the argument that the lake is currently a significant source of salinity to the groundwater system, and that its contribution to the salinity of groundwater is likely to increase with increased production from the Godomey well field
Summary
Coastal groundwater systems represent both crucial water resources for growing population centers and complex hydrologic systems sensitive to changes in both land-use practices and climate. The impact of hydraulic transients in coastal environments has been studied by a number of authors ranging from early work by [18] to recent work by such authors as [19] Conclusions from these studies suggest that mixing and penetration of both salt water and other contaminants will increase in the presence of tidal or other dynamic changes in groundwater hydraulics. The extensive literature on coastal environments has provided the technical foundation for ongoing studies of a coastal aquifer system in Benin, West Africa This aquifer system, accessed via a series of wells known as the Godomey well field, represents the sole source of freshwater supplying the city of Cotonou, Benin. A number of students from both Benin and the United States have been involved in this effort [20]
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