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Gas exchanges between streams and atmosphere strongly impact biogeochemical cycles, yet their quantification in space and time remains challenging. We propose a new method to measure gas exchange rate coefficients in headwater streams. The method is based on simultaneous slug injections of a conservative tracer and a volatile tracer, coupled to in situ semicontinuous measurements. Its originality lies in the mathematical exploitation of the tracers' breakthrough curves. Taking advantage of the entire breakthrough curves, we infer the gas exchange rate coefficient from a comparative analysis of the shape of the volatile and the conservative breakthrough curves. Tests on synthetic datasets confirmed the validity of the mathematical framework for the whole range of gas exchange rate coefficients found in headwater streams. Field experiments further attested to the real-world applicability of the method. Salt and helium injections were performed in a first-order stream and the helium breakthrough curves were obtained using in situ membrane inlet mass spectrometry. This new method allows to determine the gas exchange rates directly in the field, without any external calibration of the data. Its fast implementation enables a wider spatial coverage of the measurements of gas exchange rates, and thus offers an opportunity to improve the large-scale estimates of CO 2 emissions from headwater streams. More generally, our study highlights the informative potential of semicontinuous datasets and encourages further investigation of possible use of temporal signals.

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<p>Hydrological predictions for ungauged basins at catchment and regional scales are still challenged by the lack of available data. Under the assumption that the perennial stream network is mostly fed by groundwaters, its spatial extent is controlled by the magnitude of the subsurface hydraulic conductivity (<em>K</em>) with respect to the actual recharge rate (<em>R</em>). In addition, the residence time of groundwater is directly controlled by the storage capacity of the aquifer system, i.e. the porosity (<em>Ө</em>). Here we propose a new inversion approach that jointly considers the spatial organization of observed hydrographic network and the residence times of groundwater measured at springs to infer the geometry of the aquifer system and its hydraulic properties.</p> <p>We used a dataset gathered in an alpine catchment observatory (Natural conservation area of the Massif of Saint-Barthélemy, Pyrenees, France). The extent of the stream network has been mapped using field observation. Residence times have been obtained from concentrations of dissolved CFCs and SF<sub>6</sub> gases measured at 6 spring locations distributed over the catchment. The average transit time is about 30 years for perennial springs with a significant variability across the watershed. The relatively high residence time is also confirmed by high Helium concentrations.</p> <p>In our inversion scheme, we evaluate the accuracy of an ensemble of 3D hydrogeological models with different aquifer geometries and hydraulic properties. We found that topography and aquifer compartmentalization, through the decreasing trend in hydraulic conductivity, are key parameters in setting the spatial pattern of seepage areas and the distribution of transit times across the catchment. In addition, by running transient simulations of the model ensemble we further explore the accuracy of the models by comparing results with measurements of stream discharge and the intermittency of the hydrographic network. We found that intermittence seems to be connected to high transmissive shallow flow structures with low storage capacities (mostly organized within shallow soils and rockslides). However, perennial springs are sustained by deep groundwater flow paths within the bedrock. In perspective, we discuss the potential evolution of the extent, discharge magnitude and the transit time of seeping groundwater under changing recharge scenarios.</p>

<p>Like in other relatively flat coastal areas, flooding by aquifer overflow is a recurring problem on the western coast of Normandy (France). Threats are expected to be enhanced by the rise of the sea level and to have critical consequences on the future development and management of the territory. The delineation of the increased saturation areas is a required step to assess the impact of climate change locally. Preliminary models showed that vulnerability does not result only from the sea side but also from the continental side through the modifications of the hydrological regime.</p><p>We investigate the processes controlling these coastal flooding phenomena by using hydrogeological models calibrated at large scale with an innovative method reproducing the hydrographic network. Reference study sites selected for their proven sensitivity to flooding have been used to validate the methodology and determine the influence of the different geomorphological configurations frequently encountered along the coastal line.</p><p>Hydrogeological models show that the rise of the sea level induces an irregular increase in coastal aquifer saturations extending up to several kilometers inland. Back-littoral channels traditionally used as a large-scale drainage system against high tides limits the propagation of aquifer saturation upstream, provided that channels are not dominantly under maritime influence. High seepage fed by increased recharge occurring in climatic extremes may extend the vulnerable areas and further limit the effectiveness of the drainage system. Local configurations are investigated to categorize the influence of the local geological and geomorphological structures and upscale it at the regional scale.</p>