Water sampling in low productive boreholes: how to ensure of the representativeness of sampling?

International audienceGood practices of wellbore purging advice to draw three to five times the volume of the water column prior to sample. An extensive literature in the past 20 years has established the biases that may be linked to this procedure with emphasis on contaminant sampling and the benefit low-flow sampling may have to avoid redistribution of contaminants in boreholes because of the flow-weighted average character of pumping (Einarson, 2006, Handbook Environmental Site Characterization and GroundWater Monitoring). Low-flow sampling may produce flow-biased samples if operated in long-screened boreholes where vertical gradients exist (McMillan et al., 2014, J. Contam. Hydrol. 1699, 50-61) and is better suited for high permeability boreholes (ISO Standards). Water sampling in low permeability aquifers remains challenging especially for boreholes drilled for water table level monitoring (long-screened boreholes; pumps may not be lowered down in the screened interval). Fluid Electrical Conductivity logging prior to pumping and sampling may help in locating the productive levels albeit the information obtained under ambient flow conditions may be of less relevance than the data collected using salt injection (e.g. Lasher and Nel, 2013, Groundwater Division Conference, Durban). Fiber optic Distributed Temperature Sensing may also resolve hydraulic (McMillan, 2015, PhD dissertation) but is not of common use. We refer to investigations performed in two boreholes (Labruguière and Valdurenque) of low permeabilities (10 −6 to 10 −7 m.s −1), located in detritic formations in SW France. These two boreholes, of 170 m and 123 m depth respectively, have long screened sections (128-170 m and 75-123 m respectively). With such geometries, the sub-mersible pump cannot be placed in the screened interval to perform volume purge. This raises the question of how long the pumping has to be done to get water representative of the downhole chemistry. The purge of three times the volume of the water column is unrealistic. For Labruguière borehole, it would take 12 hours cumulated at ≤1 m 3 .h −1 pumping rate, each session cannot last more than 2 hours (dewatering) and 12 hours are needed to recover the water table level. We thus refer to deep sampling to assess the usefulness of such a method. Several levels were determined on the basis of ambient logging (temperature, conductivity, pH, dissolved oxygen, redox potential). In parallel we use pumping to assess the purge process inside the borehole and determine the minimum amount of drawn water needed to get water from the screens. This also highlights that alternative method to judge of representativeness, such as stabilization of physico-chemical parameters, may lead to false positives, i.e. the parameters were stabilized but the water chemistry was not that of the screened section. Three cycles of logging – deep sampling – pumping were done in each borehole. Based on field data and laboratory analyses, it appears that a protocol for deep boreholes characterization may refer to 1) borehole logging (information on ambient structure of the water column), 2) slight solicitation of the borehole by pumping (renewal of water at productive levels), and 3) deep sampling at the depth(s) suggested by borehole logs

L'instabilité elliptique en géophysique

L'étude de l'instabilité elliptique a été motivée par des problèmes rencontrés en aérodynamique (instabiltés secondaires diverses, vortex de bout d'ailes, turbulence), mais un autre intérêt suscité par cette instabilité elliptique relève de la géophysique. En effet, lorsqu'une planète tourne autour d'un soleil, et (ou) qu'une lune tourne autour de la planète, le noyau liquide de celle-ci subit une déformation elliptique (une marée) causée par le champ de gravitation. La rotation de la planète sur son axe pourrait alors faire résonner des ondes de Kelvin dans le noyau liquide ce qui résulterait en l'apparition de l'instabilité elliptique. Nos résultats expérimentaux et théoriques montrent qu'en effet, le mode dit de "spin-over" apparaît au seuil de l' instabilité dans la géométrie sphérique et qu'une dynamique intermittente existe à plus haut nombre de Reynolds. Finalement, une expérience sous champ magnétique et utilisant un métal liquide, met en évidence la génération d'un champ magnétique directement induit par l'instabilité

Elliptical instability of a flow in a rotating shell

A theoretical and experimental study of the spin-over mode induced by the elliptical instability of a flow contained in a slightly deformed rotating spherical shell is presented. This geometrical configuration mimics the liquid rotating cores of planets when deformed by tides coming from neighboring gravitational bodies. Theoretical estimations for the growth rates and for the non linear amplitude saturations of the unstable mode are obtained and compared to experimental data obtained from Laser D\"{o}ppler anemometry measurements. Visualizations and descriptions of the various characteristics of the instability are given as functions of the flow parameter

CO 2 Migration Monitoring Methodology in the Shallow Subsurface: Lessons Learned From the CO 2 FIELDLAB Project

International audienceA CO 2 migration field laboratory for testing of monitoring methods has been established in the glaciofluvial-glaciomarine Holocene deposits of the Svelvik ridge, near Oslo. A shallow CO 2 injection experiment was conducted in September 2011 in which approximately 1700 kg of CO 2 was injected at 18 m depth below surface. The objectives of this experiment were to (i) detect and, where possible, quantify migrated CO 2 concentrations, (ii) evaluate the sensitivity of the monitoring tools and (iii) study the impact of the vadose zone on measurements. This paper describes the injection, discusses the joint interpretation of the results and suggests some recommendations for further work

An Expert Position Paper from the Special Interest Group on Sensitive Skin of the International Forum for the Study of Itch

Sensitive skin is a frequent complaint in the general population, in patients, and among subjects suffering from itch. The International Forum for the Study of Itch (IFSI) decided to initiate a special interest group (SIG) on sensitive skin. Using the Delphi method, sensitive skin was defined as “A syndrome defined by the occurrence of unpleasant sensations (stinging, burning, pain, pruritus, and tingling sensations) in response to stimuli that normally should not provoke such sensations. These unpleasant sensations cannot be explained by lesions attributable to any skin disease. The skin can appear normal or be accompanied by erythema. Sensitive skin can affect all body locations, especially the face”. This paper summarizes the background, unresolved aspects of sensitive skin and the process of developing this definition

Zebra : Building Efficient Network Message Parsers for Embedded Systems

4 pagesInternational audienceSupporting standard text-based protocols in embedded systems is challenging because of the often limited computational resources that embedded systems provide. To overcome this issue, a promising approach is to build parsers directly in hardware. Unfortunately, developing such parsers is a daunting task for most developers as it is at the crossroads of several areas of expertise, such as low-level network programming, or hardware design. In this paper, we propose Zebra, a generative approach to drastically ease the development of hardware parsers and their use in network applications. To validate our approach, we have used Zebra to generate hardware parsers for widely used protocols, namely HTTP, SMTP, SIP, and RTSP. Our experiments show that Zebra-based parsers are up to 11 times faster than software-based parsers

Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review

In the West African Sahel, two paradoxical hydrological behaviors have occurred during the last five decades. The first paradox was observed during the 1968–1990s ‘Great Drought’ period, during which runoff significantly increased. The second paradox appeared during the subsequent period of rainfall recovery (i.e., since the 1990s), during which the runoff coefficient continued to increase despite the general re-greening of the Sahel. This paper reviews and synthesizes the literature on the drivers of these paradoxical behaviors, focusing on recent works in the West African Sahelo/Sudanian strip, and upscaling the hydrological processes through an analysis of recent data from two representative areas of this region. This paper helps better determine the respective roles played by Land Use/Land Cover Changes (LULCC), the evolution of rainfall intensity and the occurrence of extreme rainfall events in these hydrological paradoxes. Both the literature review and recent data converge in indicating that the first Sahelian hydrological paradox was mostly driven by LULCC, while the second paradox has been caused by both LULCC and climate evolution, mainly the recent increase in rainfall intensity

The 'absolute existence' of phlogiston: the losing party's point of view.

Long after its alleged demise, phlogiston was still presented, discussed and defended by leading chemists. Even some of the leading proponents of the new chemistry admitted its ‘absolute existence’. We demonstrate that what was defended under the title ‘phlogiston’ was no longer a particular hypothesis about combustion and respiration. Rather, it was a set of ontological and epistemological assumptions and the empirical practices associated with them. Lavoisier’s gravimetric reduction, in the eyes of the phlogistians, annihilated the autonomy of chemistry together with its peculiar concepts of chemical substance and quality, chemical process and chemical affinity. The defence of phlogiston was the defence of a distinctly chemical conception of matter and its appearances, a conception which rejected the chemist’s acquaintance with details and particularities of substances, properties and processes and his skills of adducing causal relations from the interplay between their complexity and uniformity

Efficient Methanol Production on the Dark Side of a Prestellar Core

We present Atacama Large Millimeter/submillimeter Array maps of the starless molecular cloud core Ophiuchus/H-MM1 in the lines of deuterated ammonia (ortho-NH2D), methanol (CH3OH), and sulfur monoxide (SO). The dense core is seen in NH2D emission, whereas the CH3OH and SO distributions form a halo surrounding the core. Because methanol is formed on grain surfaces, its emission highlights regions where desorption from grains is particularly efficient. Methanol and sulfur monoxide are most abundant in a narrow zone that follows the eastern side of the core. This side is sheltered from the stronger external radiation field coming from the west. We show that photodissociation on the illuminated side can give rise to an asymmetric methanol distribution but that the stark contrast observed in H-MM1 is hard to explain without assuming enhanced desorption on the shaded side. The region of the brightest emission has a wavy structure that rolls up at one end. This is the signature of Kelvin-Helmholtz instability occurring in sheared flows. We suggest that in this zone, methanol and sulfur are released as a result of grain-grain collisions induced by shear vorticity.Peer reviewe

Multidimensional method-of-lines transport for atmospheric flows over steep terrain using arbitrary meshes

Including terrain in atmospheric models gives rise to mesh distortions near the lower boundary that can degrade accuracy and challenge the stability of transport schemes. Multidimensional transport schemes avoid splitting errors on distorted, arbitrary meshes, and method-of-lines schemes have low computational cost because they perform reconstructions at fixed points. This paper presents a multidimensional method-of-lines finite volume transport scheme, “cubicFit”, which is designed to be numerically stable on arbitrary meshes. Stability conditions derived from a von Neumann analysis are imposed during model initialisation to obtain stability and improve accuracy in distorted regions of the mesh, and near steeply-sloping lower boundaries. Reconstruction calculations depend upon the mesh only, needing just one vector multiply per face per time-stage irrespective of the velocity field. The cubicFit scheme is evaluated using three, idealised numerical tests. The first is a variant of a standard horizontal transport test on severely distorted terrain-following meshes. The second is a new test case that assesses accuracy near the ground by transporting a tracer at the lower boundary over steep terrain on terrain-following meshes, cut-cell meshes, and new, slanted-cell meshes that do not suffer from severe time-step constraints associated with cut cells. The third, standard test deforms a tracer in a vortical flow on hexagonal-icosahedral meshes and cubed-sphere meshes. In all tests, cubicFit is stable and largely insensitive to mesh distortions, and cubicFit results are more accurate than those obtained using a multidimensional linear upwind transport scheme. The cubicFit scheme is second-order convergent regardless of mesh distortions