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The freeze-thaw cycles in periglacial areas during the Quaternary glacials increased frost weathering, leading to a disintegration of rock formations. Transported downslope, clasts allowed in some areas the formation of stratified slope deposits known as “grèzes litées”. This study reviews the existing theories and investigates the grèzes litées deposits of Enscherange and Rodershausen in Luxembourg. This process was reinforced by the lithostructural control of the parent material expressed by the dip of schistosity (66°) and its orientation parallel to the main slopes in the area. This gave opportunities to activate the frost-weathering process on top of the ridge where the parent material outcropped. As the stratified slope deposits have a dip of 23° and as there is no significant lateral variation in rock fragment size, slope processes that involve only gravity are excluded and transportation in solifluction lobes with significant slopewash and sorting processes is hypothesized. The Enscherange formation, the biggest known outcrop of grèzes litées in north-western Europe, shows evidence of clear layering over the whole profile depth. A palaeolandscape reconstruction shows that ridges must have been tens of metres higher than presently. The investigation of the matrix composition shows Laacher See tephra in the overlying periglacial cover bed with infiltrations of the minerals in the reworked upper layer of the grèzes litées deposit. Chronostratigraphic approaches using the underlying cryoturbation zone and Laacher See heavy minerals in the overlying topsoil place the formation of grèzes litées deposits in the Late Pleistocene.

Within the Critical Zone, regolith plays a key role in the fundamental hydrological functions of water collection, storage, mixing and release. Electrical Resistivity Tomography (ERT) is recognized as a remarkable tool for characterizing the geometry and properties of the regolith, overcoming limitations inherent to conventional borehole-based investigations. For exploring shallow layers, a small electrode spacing (ES) will provide a denser set of apparent resistivity measurements of the subsurface. As this option is cumbersome and time-consuming, smaller ES - albeit offering poorer shallow apparent resistivity data - are often preferred for large horizontal ERT surveys. To investigate the negative trade-off between larger ES and reduced accuracy of the inverted ERT images for shallow layers, we use a set of synthetic conductive/resistive/conductive three-layered soil-saprock/saprolite-bedrock models in combination with a reference field dataset. Our results suggest that an increase in ES causes a deterioration of the accuracy of the inverted ERT images in terms of both resistivity distribution and interface delineation and, most importantly, that this degradation increases sharply when the ES exceeds the thickness of the top subsurface layer. This finding, which is obvious for the characterization of shallow layers, is also relevant even when solely aiming for the characterization of deeper layers. We show that an oversized ES leads to overestimations of depth to bedrock and that this overestimation is even more important for subsurface structures with high resistivity contrast. To overcome this limitation, we propose adding interpolated levels of surficial apparent resistivity relying on a limited number of ERT profiles with a smaller ES. We demonstrate that our protocol significantly improves the accuracy of ERT profiles when using large ES, provided that the top layer has a rather constant thickness and resistivity. For the specific case of large-scale ERT surveys the proposed upgrading procedure is cost-effective in comparison to protocols based on small ES. [ABSTRACT FROM AUTHOR]

Within the Critical Zone, regolith plays a key role in the fundamental hydrological function of water collection, storage, mixing and release. Electrical Resistivity Tomography (ERT) is recognized as a remarkable tool for characterizing the geometry and properties of the regolith, overcoming limitations inherent to conventional borehole-based investigations. However, ERT measurements with a high vertical resolution remain restricted to shallow depths, essentially due to the requirement of small electrode spacing increments (ESI). Under these circumstances, the use of ERT measurements for large horizontal surveys remains cumbersome and time-consuming. Here we focus on the need to optimize the ESI parameter in order to adequately characterize the subsurface fabric. We use a set of synthetic three-layered soil-saprock/saprolite-bedrock models in combination with a field dataset. We demonstrate that oversized ESI can significantly affect our perception of shallow subsurface structures by missing important layers and increasing the ill-posed inverse problem effects. More precisely, we document how a thin surficial layer can influence inverted ERT results and cause a resistivity bias, both at the surface and at deeper horizons. To overcome this limitation, we propose adding interpolated levels of surficial apparent resistivity based on a limited number of ERT profiles with small ESI. We demonstrate that our protocol significantly improves the accuracy of ERT profiles based on large ESI. Our protocol is time and cost efficient - especially in the case of large-scale ERT surveys. [ABSTRACT FROM AUTHOR]

Diatoms are unicellular eukaryotic algae that are widely distributed in aquatic environments. In particular, their presence is abundant in stream flows, whereas the instance and transport of cells through subsurface flow is controversial. In this work, we study the transport of diatoms through soils by designing laboratory percolation experiments on undisturbed loamy and sandy soil cores. To label diatom tracers and enhance their detectability, we develop protocols to functionalize frustules of purified diatomite and Conticribra weissflogii cultures with rhodamine 123 and 2-(4-pyridyl)-5[4-dimethylaminoethyl-aminocarbamoyl)-methoxy]phenyloxazole, respectively. Labelled diatoms demonstrate good detectability and resilience under weathering agents. We execute percolation experiments for maximum time domains of approximately 600 min where volumes of water up to 3600 ml are deployed on the soil core surface. Under such experimental conditions, fluorescent diatoms are not recovered in samples of percolated material as per microscopy and spectrofluorometry. While further field studies are needed to fully investigate diatom transport, our findings shed new light on the potential of using biotic tracers for surface hydrological processes. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The synthesis of experimental understanding of catchment behaviour and its translation into qualitative perceptual models is an important objective of hydrological sciences. We explore this challenge by examining the cumulative understanding of the hydrology of three experimental catchments and how it evolves through the application of different investigation techniques. The case study considers the Huewelerbach, Weierbach and Wollefsbach headwater catchments of the Attert basin in Luxembourg. Subsurface investigations including bore holes and pits, analysis of soil samples and Electrical Resistivity Tomography measurements are presented and discussed. Streamflow and tracer data are used to gain further insights into the streamflow dynamics of the catchments, using end-member mixing analysis and hydrograph separation based on dissolved silica and electrical conductivity. We show that the streamflow generating processes in all three catchments are controlled primarily by the subsolum and underlying bedrock. In the Huewelerbach, the permeable sandstone formation supports a stable groundwater component with little seasonality, which reaches the stream through a series of sources at the contact zone with the impermeable marls formation. In the Weierbach, the schist formation is relatively impermeable and supports a 'fill and spill'-type of flow mechanism; during wet conditions, it produces a delayed response dominated by pre-event water. In the Wollefsbach, the impermeable marls formation is responsible for a saturation-excess runoff generating process, producing a fast and highly seasonal response dominated by event water. The distinct streamflow generating processes of the three catchments are represented qualitatively using perceptual models. The perceptual models are in turn translated into quantitative conceptual models, which simulate the hydrological processes using networks of connected reservoirs and transfer functions. More generally, the paper illustrates the evolution of perceptual models based on experimental fieldwork data, the translation of perceptual models into conceptual models and the value of different types of data for processes understanding and model representation. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The geological map is the basic and necessary information to perform a soil survey. Nevertheless, the scale of the geological map does not always correspond to the requirements of some hydropedological studies, especially when they are performed at the headwater catchment scale. Furthermore, in temperate climate regions, where the outcrops are not always available, the soil survey can contribute to give relevant information on the surface geology. In this paper, we show the importance of the soil C-horizon's description regarding the identification of the underlying bedrock. Performed in a hydrological experimental catchment in the Grand Duchy of Luxembourg, this approach allows to better determine the limits of the geological strata at a larger scale and delivers more relevant information to the hydrologists. We highlight the presence of the "Formation de Mortinsart" (ko) from Rhetian age, which was not mapped at the geological scale and which plays a key role in the water pathway and residence time in this little catchment. [ABSTRACT FROM AUTHOR]