Coupling hydrodynamic, geochemical and isotopic approaches to evaluate oxbow connection degree to the main stream and to adjunct alluvial aquifer

Abstract

Wetlands as oxbows play a fundamental socio-economic and environmental role. They behave as efficient tools in regards to hydrological and ecological concerns: they contribute to control rivers discharge (flood prevention and contribution to drought flow) as well as to manage water quality (excess nutrients retention) and they constitute refuge habitats for flora and fauna. They may so become more and more determinant as nature-based solutions in a near future. However, the establishment of complete and reliable functioning models is complicated by the different connection degrees that can present these specific wetlands to the main stream and to the adjunct alluvial aquifer. Moreover, monodisciplinary approaches usually conducted can underestimate or even neglect sources of water supply. The present study offers a multidisciplinary approach coupling hydrodynamic (surface water/groundwater levels), geochemical (T, EC, pH, major ions) and isotopic (δ2H-δ18O) characterization in order to properly assess the dynamic of these specific hydrosystems and so be able to establish a complete and reliable hydrodynamic conceptual model. The coupling approach has been tested on the Auzon Oxbow, one of the fluvial annexes of the Allier River (Massif Central, France). The resulting conceptual model testifies for connection degree higher that expected between the oxbow and both the main stream and alluvial aquifer. Indeed, a non-negligible supply of the oxbow by the alluvial groundwater is observed, especially in its downstream part. As this part of the oxbow is the site of its confluence to the Allier River, groundwater supply could have been underestimated. The geochemical approach shows that Allier River also supply the oxbow through the upstream paleochannel, result of a channel migration 30 years ago. Hydrodynamic and isotopic approaches complete the understanding showing that the paleochannel pathway is only active during high flow periods (from November to June). Based on these results, the strategic utility of the proposed approach to study oxbow’s hydrosystems has been proved.