Quantitative assessment of groundwater discharge using hydrograph separation and spring discharge in the Ou´em´e valley, southern Benin

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Résumé

Groundwater is the primary source of freshwater for drinking, agricultural water, and industrial in many nations worldwide. Understanding the interaction between groundwater and river is vital for the sustainable management of water resources. This study aims to evaluate groundwater discharge into two major rivers in coastal sedimentary basin. Water samples were collected from two rivers, rainfall, boreholes and springs, for the determination of stable isotopes. The streamflow of the two rivers is measured monthly. Springs have also been inventoried and their flow rates quantified. The isotopic signatures of the two rivers are of the same range and vary greatly from dry to rainy season (􀀀 1.32 ‰ to 􀀀 3.85 ‰ for 18O and 􀀀 7.56 ‰ to 􀀀 21.13 ‰ for 2H), unlike that of groundwater and springs (􀀀 3.10 ‰ to 􀀀 3.13 ‰ for 18O, 􀀀 15.51 ‰ to 􀀀 15.64 ‰ for 2H). The water level in the well is 4m above the river level in dry season, and decreases to 1.5 m in the rainy season. Groundwater discharge based on the isotopic hydrograph separation was 0–96.67 m3/s and 0.74–43.69 m3/s in Ou´em´e Stream and Sˆo River, respectively. The analysis reveals that groundwater maintains the base flow of streams in the dry season and also contributes to rivers flow in the rainy season. However, rain water is the dominant water source in rainy season. In addition, 75 springs inventoried in the valley discharge in totally more than 221 m3/h in the rivers. Hence, combining stable isotopes composition hydrometric observations and springs flow, provides an efficient method for estimating groundwater discharge rates into rivers.

Mots-clés

Stable isotopes, Hydrograph separation, Springs, Benin