Continental hydrosystem modelling: the concept of nested stream–aquifer interfaces

International audienceCoupled hydrological-hydrogeological models, emphasising the importance of the stream–aquifer interface, are more and more used in hydrological sciences for pluri-disciplinary studies aiming at investigating environmental is-sues. Based on an extensive literature review, stream–aquifer interfaces are described at five different scales: local [10 cm– ∼ 10 m], intermediate [∼ 10 m–∼ 1 km], watershed [10 km 2 – ∼ 1000 km 2 ], regional [10 000 km 2 –∼ 1 M km 2 ] and conti-nental scales [> 10 M km 2 ]. This led us to develop the con-cept of nested stream–aquifer interfaces, which extends the well-known vision of nested groundwater pathways towards the surface, where the mixing of low frequency processes and high frequency processes coupled with the complexity of geomorphological features and heterogeneities creates hy-drological spiralling. This conceptual framework allows the identification of a hierarchical order of the multi-scale con-trol factors of stream–aquifer hydrological exchanges, from the larger scale to the finer scale. The hyporheic corridor, which couples the river to its 3-D hyporheic zone, is then identified as the key component for scaling hydrological pro-cesses occurring at the interface. The identification of the hy-porheic corridor as the support of the hydrological processes scaling is an important step for the development of regional studies, which is one of the main concerns for water practi-tioners and resources managers. In a second part, the modelling of the stream–aquifer in-terface at various scales is investigated with the help of the conductance model. Although the usage of the temperature as a tracer of the flow is a robust method for the assess-ment of stream–aquifer exchanges at the local scale, there is a crucial need to develop innovative methodologies for as-sessing stream–aquifer exchanges at the regional scale. After formulating the conductance model at the regional and inter-mediate scales, we address this challenging issue with the de-velopment of an iterative modelling methodology, which en-sures the consistency of stream–aquifer exchanges between the intermediate and regional scales. Finally, practical recommendations are provided for the study of the interface using the innovative methodology MIM (Measurements–Interpolation–Modelling), which is graphi-cally developed, scaling in space the three pools of methods needed to fully understand stream–aquifer interfaces at vari-ous scales. In the MIM space, stream–aquifer interfaces that can be studied by a given approach are localised. The ef-ficiency of the method is demonstrated with two examples. The first one proposes an upscaling framework, structured around river reaches of ∼ 10–100 m, from the local to the wa-tershed scale. The second example highlights the usefulness of space borne data to improve the assessment of stream– aquifer exchanges at the regional and continental scales. We conclude that further developments in modelling and field measurements have to be undertaken at the regional scale to enable a proper modelling of stream–aquifer exchanges from the local to the continental scale

Polycyclic aromatic hydrocarbon transport processes on the scale of a flood event in the rural watershed of Le Bebec, France

International audienceTransport processes of polycyclic aromatic hydrocarbons (PAHs) were investigated in a rural watershed in France, which led to new quantitative results on the scale of a flood event. For that purpose, principal components analysis (PCA) combined with signal decomposition of PAH Breakthrough curves were used. The results obtained by PCA enabled us to distinguish (i) the transport of low molecular weight PAHs in dissolved form and (ii) the transport of high molecular weight PAHs in particulate form. The intermediate molecular weight PAHs are distributed in these two clusters. The application of a signal decomposition method confirms the results of PCA and makes it possible to quantify the mass PAH resulting, respectively, from resuspension (13.3 mg, i.e. 77% of the total mass) and from direct transfer (2.8 mg = 16% of the total mass). For the resuspension phase, desorption of PAH (38% of the total exported mass) could be also distinguished from sediment resuspension (39%)

Quantifying spatio-temporal stream-aquifer water exchanges along a multi-layer aquifer system using LOMOS and hydro-thermo modelling

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Designing a multiscale experimental sampling system for quantification of stream-aquifer water exchanges-the Orgeval basin case study (France)

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Influence du contexte hydrogéologique sur la connectivité nappe-rivière au niveau de la vallée incisée des Avenelles

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Detecting different water table levels in a shallow aquifer with combined P-, surface and SH-wave surveys: Insights from VP/VS or Poisson's ratios

International audienceWhen applied to hydrogeology, seismic methods are generally confined to the characterisation of aquifers geometry. The joint study of pressure-(P) and shear-(S) wave velocities (V P and V S) can provide supplementary information and improve the understanding of aquifer systems. This approach is proposed here with the estimation of V P /V S ratios in a stratified aquifer system characterised by tabular layers, well-delineated thanks to electrical resistivity tomography, log and piezometer data. We carried out seismic surveys under two hydrological conditions (high and low flow regimes) to retrieve V S from both surface-wave dispersion inversion and SH-wave refraction interpretation, while V P were obtained from P-wave refraction interpretation. P-wave first arrivals provided 1D V P structures in very good agreement with the stratification and the water table level. Both V S models are similar and remain consistent with the stratification. The theoretical dispersion curves computed from both V S models present a good fit with the maxima of dispersion images, even in areas where dispersion curves could not be picked. Furthermore, V P /V S and Poisson's ratios computed with V S models obtained from both methods show a strong contrast for both flow regimes at depths consistent with the water table level, with distinct values corresponding to partially and fully saturated sediments

Designing a multiscale experimental sampling system for quantification of stream-aquifer water exchanges - the Orgeval basin case study (France)

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Designing a multiscale experimental sampling system for quantification of stream-aquifer water exchanges - the Orgeval basin case study (France)

International audienc

Mise en place d'un dispositif expérimental multi-échelle pour quantifier les échanges nappe-rivière dans un système aquifère multicouche

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