Influence of groundwater-surface interactions on groundwater salinity in the Senegal River Delta

The Senegal River Delta is a strategic region for the development of irrigated agriculture. However, the existence of a shallow saline aquifer, in conjunction with intensification of irrigated agriculture, has led to degradation of cultivated soils. At the end of the XXth Century, the construction of dams on the river has secured water availability throughout the year and induced a rise in surface water levels, inducing a lateral recharge of groundwater in the alluvial aquifer. Because of these major environmental changes, groundwater freshening was expected to occur. Using a combination of regional piezometric and hydrogeochemical surveys as well as local geophysical and hydrogeochemical surveys, groundwater–surface water interactions were characterized to identify the impact of artificial river management and agricultural intensification (among other rice cultivation) on the evolution of groundwater dynamics and chemistry. Results show that groundwater-surface water interactions are mainly visible near rivers where freshwater lenses have developed, with a groundwater salinity that is lower than seawater and groundwater mineralization that seems to evolve in the direction of softening through cationic exchanges related to permanent contact with fresh water. Groundwater far away from rivers and outside irrigated plots has evolved from marine water to brines under the influence of evapotranspiration. In the cultivated parcels, despite large volumes of water used for rice cultivation, groundwater does not show real softening trend. In general, mechanisms that contribute to repel salt water from the sediments correspond to a lateral flush near permanent surface water streams and not to vertical drainage and dilution with rainfall or irrigation water. It is however difficult to estimate the time required to return to more favorable conditions of groundwater salinity

Hydrodynamic and Hydro-geochemical Processes in the Catchment Area of Lake Retba and Their Implications in Relationship between Groundwater, Lake and Ocean

Abstract The drought that began in the Sahel in the late 70s has led to a drying up of most surface water bodies and a decrease of ground water level. In the north coast of Cap-Verde, the Retba lake is one of the few depressions that have preserved its water and experienced all the same, a very important salinization. This saline ecosystem plays an important role in the economy of the area due to the exploitation of salt deposits. However, it is now threatened by a significant reduction of the water surface. Its backup requires a good knowledge of the environment and the interactions between the different reservoirs, namely the lake, groundwater and ponds. To study the relationship between different parts of the hydro system, a network, monitoring the groundwater table including 41 village wells across the watershed, has been established. All observation points and the level of the lake were leveled with the reference 0 IGN. Water samples were taken at the wells, lake and ponds and source for chemical analysis. The results of the altimetric study indicated that the lake is below sea level, at odds of -5.26 m and a good part of the watershed is under the level of the sea. Hence its vulnerability of being invaded by the ocean water due to the difference in altitude. Water table monitoring shows different movements from one point to another due to the interference of several factors in the process of charging and discharging of the water table. Thus, some wells are only under the influence of climate parameters (rain and evaporation), while others are fed or drained by lateral flow. Water chemistry is dominated by two chemical facies: calcic and chloride facies (Ca-cl) and sodium chloride-facies (Na-Cl). Na and Cl ions control mainly the mineralization water and cationic exchange is the main factor controlling water chemistry even if other processes such as anthropic pollution could occur. These cationic exchanges reflect the intrusion of seawater into the sand aquifers. These results also show that in the late dry season, water ponds have a chemical signature very close to seawater. This shows that the ponds that are in the north of the lake is an outcrop of the Sea that feeds the lake permanently. The lake water is a mixture of seawater and groundwater whose evaporation causes sedimentation of salt exploited by local people. Monitoring of the groundwater level, the lake level and the chemical analysis shows a connection between the lake, the groundwater, the ocean and ponds

Impacts of Irrigation Water on the Hydrodynamics and Saline Behavior of the Shallow Alluvial Aquifer in the Senegal River Delta

peer reviewedThe Senegal River Delta located in north-western Senegal is a strategic region for the development of irrigated rice cultivation for achieving rice self-sufficiency. The presence of a shallow salty water table is often considered as a brake to the development of irrigation, by causing salinization of the soil, although the mechanisms of operation are not well known. An experimental study was carried out in a rice paddy located in the village of Ndiaye, 35 km north from Saint Louis, to characterize the water and solute flux processes below the irrigated plots. The objective was to better understand the irrigation-driven dynamics of soil salinization processes. An experimental monitoring network was installed for monitoring the transit of water at the plot level, in the unsaturated zone and in the aquifer. The results show that the supply of water by irrigation contributes to significantly recharging the water table, as shown by the rise in piezometric level, with a concomitant dilution of the water salinity in the soil zone and in the shallow groundwater. However, when irrigation is stopped, the groundwater level and salinity return within a month to their initial level and salinity status because of the evaporative recovery, which strongly governs these processes. Thus, water flow and solute transfers operate in the delta following a recharge–discharge and dilution–concentration cycle controlled by the water balance, and we do not expect to observe in the short- to middle-term any significant reduction in soil salinization processes by drainage.Maîtrise de la gestion de l’eau pour une agriculture durable dans le delta du fleuve Sénégal : appui à la mise en œuvre de la Grande Offensive Agricole pour la Nourriture et l'Abondance (GOANA

Factors controlling the evolution of groundwater dynamics and chemistry in the Senegal River Delta

tStudy region: Senegal River Delta. Study focus: The Senegal River Delta is a strategic region for the development of irri-gated agriculture. Despite a Sahelian climatic context, the management of the river withdams ensures water availability throughout the year. With the intensification of agri-culture, degradation of cultivated soils is observed, mostly linked to the existence of ashallow salty aquifer. In this context, regional surveys were performed to characterizegroundwater–surface water interactions and to identify the impact of artificial river man-agement and agricultural intensification on the evolution of groundwater dynamics andchemistry.New hydrological insights for the region: Results show that groundwater far away from riversand outside irrigated plots has evolved from marine water to brines under the influenceof evapotranspiration. Near rivers, salinity of groundwater is lower than seawater andgroundwater mineralization seems to evolve in the direction of softening through cationicexchanges related to permanent contact with fresh water. Despite large volumes of waterused for rice cultivation, groundwater does not show any real softening trend in the culti-vated parcels. Results show that the mechanisms that contribute to repel salt water fromthe sediments correspond to a lateral flush near permanent surface water streams and notto vertical drainage and dilution with rainfall or irrigation water. It is however difficultto estimate the time required to come back to more favorable conditions of groundwatersalinity