Modélisation mathématique de l'évolution, à long terme, des teneurs en nitrates dans la nappe aquifère des craies du Crétacé de Hesbaye (Belgique)

La nappe aquifère de Hesbaye, logée dans les craies du Crétacé, est sollicitée à raison de trente millions de mètres cubes par an. Bien que naturellement protégée par une épaisseur de 5 à 20 mètres de limons, de nombreux indices montrent une dégradation de la qualité des eaux souterraines, notamment par les nitrates. Les concentrations en nitrates atteignent 15 à 25 mg.l-1 dans la partie semi-captive de la nappe et sont systématiquement supérieures à 35 mg.l-1 dans la partie libre. Malgré de fortes fluctuations temporelles, les teneurs augmentent en moyenne de 0,35 mg.l-1 à 0,7 mg.l-1 par an selon la situation semi-captive ou libre de la nappe.La détermination des paramètres hydrodynamiques et de transport de la craie par plus de 35 traçages répartis sur 11 sites, a permis de réaliser un modèle local (10 km2) de transport simulant la propagation des nitrates dans la nappe. Le modèle a montré que cette dernière est, malgré une certaine homogénéisation, très sensible aux apports de surface engendrant une très forte variation spatiale des concentrations. La nappe réagit de manière très différente selon que les apports de surface sont d'origine ponctuelle ou diffuse. Pour les pollutions ponctuelles, les concentrations fluctuent rapidement avec des valeurs maximales et minimales observées respectivement en périodes de basses eaux et de hautes eaux. Cette situation est liée à un phénomène de dilution de la pollution par les eaux en provenance de l'amont. En cas de suppression d'une pollution ponctuelle, la qualité de la nappe s'améliore rapidement (délai de 1 à 2 ans). Pour les pollutions diffuses, les concentrations minimales s'observent en période de rabattement de la nappe : le front de nitrates migre plus lentement (environ 1 à 2 m par an) que les vitesses de rabattement de la nappe (jusqu'à 5 m par an) et les intrants restent nuls durant des périodes pouvant aller jusqu'à 3 ans.Différentes simulations mathématiques ont montré que si la quantité d'intrants d'origine diffuse diminue de manière permanente, la nappe mettra une vingtaine d'années pour se rééquilibrer. Ces constatations sont primordiales dans le cadre de la mise en œuvre de mesures de protection puisque, si les résultats de la suppression des pollutions ponctuelles sont rapidement mais localement observés, ceux liés à la diminution des pollutions d'origine diffuse sont observés dans des délais nettement plus longs (une à deux décennies).Ces résultats montrent clairement que toute gestion qualitative des aquifères doit être basée sur des actions à long terme.The Hesbaye area is located in the northeastern part of Belgium. The aquifer formations consist of chalk deposits. Groundwater provides about 80,000 m3 d-1. Despite 5 to 20 meters of superficial loess deposits, the groundwater quality is threatened by increasing nitrate concentrations of 0.35 mg×L-1 per year in the semi-confined part of the aquifer to 0.7 mg×L-1 in the unconfined aquifer. Presently, nitrate concentrations are between 15 and 25 mg×L-1 in the semi-confined part of the aquifer but are more than 35 mg×L-1 (reaching locally 150 mg×L-1) in the unconfined part that covers 95% of the area. Nitrate concentrations have such a high spatial variation that various statistical treatments (such as kriging used to draw iso-concentration maps) have failed. This failure is due to the fact that the concentrations are highly influenced by surface land use (grass land, culture land, villages, point source pollutants, etc.). In addition, nitrate content in the aquifer varies vertically with decreasing values at depth (gradient of 0.7 mg×L-1 ×m-1).Aquifer parameters were determined by 38 pumping and tracer tests conducted in radial convergent or cylindrical flow at 11 sites. Results showed that hydraulic conductivity values ranged from 1 × 10-6 m×s-1 to 4 × 10-2 m×s-1 and effective porosities from 0.5% to 7%, showing that the aquifer was heterogeneous. Dispersivity values were affected by scale effects and varied according to chalk weathering or fracture zones. They ranged from less than 5 m in fractures to more than 60 m in weathered chalk (as in the upper part of the aquifer) and in the chalk matrix. In the chalk, transport processes were influenced by the immobile water effect due to diffusive transfer from the moving to the non-moving fluid. Non-effective porosity filled by non-moving fluid was estimated between 8 to 42%. The transfer constant ranged from 0.98 × 10-7 s-1 to 10 × 10-7 s-1.The determination of the transport parameters allowed simulation of nitrate transport at a regional scale. The SUFT3D (Saturated and Unsaturated Flow and Transport Model), developed by the Hydrogeology Section of the Georesources, Geotechnologies and Building Materials Department of Liege University was used. The modelled groundwater zone was defined as a 2.0 x 4.5 km rectangle of 10 km2. The aquifer was subdivided into 6 layers of 3350 cells (50 x 50 m wide and 3 to 15 m thick). Boundary flow conditions were defined as a prescribed head (Dirichlet conditions) to the north and the south of the area modelled. As the model simulations run for a time period of 30 years, the northern Dirichlet conditions had to be adapted to the regional and seasonal water table fluctuations that were observed during this period. At the south boundary, as the aquifer is drained by the river Geer, the water table is fixed at the river bed altitude. The eastern and western boundaries were, according to the regional piezometry, assumed to be impermeable. For the transport boundary conditions, prescribed flux (Cauchy conditions) was used for the aquifer top. Elsewhere Neumann conditions were usedSimulations were run for the period from 1963 to 1992. Nitrate inputs were averaged yearly and estimated according to actual input conditions. These conditions were calculated by simulation of nitrate flows through the non-saturated part of the aquifer using the EPIC-Model and taking into account the amount of nitrate fertilisers used by farmers (given by the Belgian government Statistical Institute). Initial conditions were calculated according to the 1963 nitrate inputs.Simulations demonstrated that it is important to distinguish the origin of the pollution as either point or non-point (diffuse) sources. For point source pollutants (such as contaminated infiltration basins), aquifer nitrate concentrations increased during low water level periods due to weaker dilution linked with a poor regional water gradient. During high groundwater levels, dilution is more important and the nitrate concentration decreases. If a point source pollutant is suppressed, aquifer quality is improved within one to two years. This demonstrates the importance of protective actions that could be applied within the framework of the protection zones around collecting galleries and pumping fields.For diffuse contamination the mean input over the area (10 m depth below cropped areas) increased from 1.32 × 10-7 mg×m-2 ×s-1 in 1963 to 5.14 × 10-7 mg×m-2 ×s-1 (i.e., a factor of four). According to these values, concentrations ranged from 11 mg×L-1 to 22 mg×L-1 (i.e., increasing by 0.5 mg×L-1 per year) between 1963 and 1992. Predictive simulations, using 1992 input, show that it will take more or less 30 years for the aquifer to be in equilibrium with the 1992 input. At that time the mean concentration value will be around 30 mg×L-1.The main results of the simulations clearly show that if actions are taken to decrease nitrate inputs, even if the aquifer nitrate contents rapidly react to the new input, nitrate levels will decrease slowly and take about 30 years to be in equilibrium with the new inputs. This long delay is due to the immobile water effect that is characteristic of the chalk aquifer. Thus it is important to inform environmentalists who work on action programs (such as the water directive imposed by the European Community in the vulnerable zones) that the effects of their actions must be based on 10 to 20 year scenarios. To this estimation, based on the reaction time of the aquifer to a new input, one must also add the time transfer of the pollutant through the unsaturated part of the aquifer

Détermination de la structure tectonique de l'aquifère crayeux du littoral Nord Pas-de-Calais par prospection géophysique couplée à des observations par forage. Conséquence sur la répartition d'eau salée

Sur le littoral du Nord - Pas-de-Calais (France), la Craie d'âge Crétacé supérieur constitue l'aquifère le plus exploité régionalement pour l'alimentation en eau tant potable qu'industrielle. Sur la frange la plus littorale, l'aquifère crayeux, semi-captif est affecté par de nombreuses failles. Dans certains secteurs, la profondeur de la craie et la localisation des failles, restent aléatoires suite au manque de forages et d'affleurements. Afin de palier à ces lacunes, trois méthodes géophysiques (sondages électriques et sismiques, profilage électromagnétique) ont été appliquées.La synthèse des données obtenues a permis de démontrer que la craie est découpée en une série de compartiments par des accidents tectoniques présentant un affaissement vers le Sud et vers l'Ouest. L'interprétation des données géophysiques a également permis d'obtenir des informations sur la répartition de l'interface eau douce - eau salée au sein de l'aquifère crayeux.Dans la région, le concept classique de biseau salé ne permet pas d'expliquer l'irrégularité spatiale des intrusions. L'étude semble démontrer que la répartition et l'extension des intrusions d'eau salée peuvent être corrélées avec la localisation des accidents tectoniques, les intrusions les plus éloignées du rivage se faisant au droit des zones faillées.Ceci amène à proposer une nouvelle approche concernant l'extension des intrusions d'eau salée : elles seraient directement tributaires de la géométrie et des caractéristiques hydrodynamiques de l'aquifère crayeux, ces dernières étant fortement influencées par la présence des failles.Along the North Pas-de-Calais coast between a point south of Boutonnière du Boulonnais and the Authie River, Cretaceous rocks are limited to the east by a paleocliff that overlooks the Quaternary coastal and dune deposits. The chalk aquifer is connected to the Quaternary aquifers and further west to the sea water. Close to the seashore the top of the chalk plateau falls rapidly from 20 m above sea level to 20 m below sea level, causing the aquifer to grade from being unconfined in the east to confined in the west. This chalky aquifer is cut by several faults. A structural survey (LOUCHE et al., 1997) based on borehole data was carried out to evaluate whether the rate and direction of groundwater flow are controlled by the different faults. Two major fault groups striking N030° and N110° have been revealed by the structural map, with a third minor group (N 160- 170°) to the north of the Canche. The two major groups, created during tectonic activity from the Palaeozoic to the present, are composed respectively of structures with general faulting directions of N 100-110° and N 020-040°, previously described by COLBEAUX et al. (1993). Chalk depth and fault location are unknown in certain areas due to the lack of bore holes and outcrops. With the aim of overcoming these deficiencies, three traditional geophysical techniques (electrical sounding, seismic logging and electromagnetic profiling) were carried out between the Canche and Authie rivers in the coastal area, west of the chalky cliffs.The interpretation of the geophysical results allows us to confirm the presence of previously located faults and identify new ones in the area where the chalk is overlain by Quaternary cover. The chalky aquifer is hence divided by the faults into a series of compartments that are inclined to the south or to the west. In addition, anomalies have been recorded in the fresh water - salt water interface on the Picardy continental shelf (MERCIER & BACROT, 1990). These anomalous zones are aligned along the northern extension of the sub N-S faults proposed by MENNESSIER & BEUN (1980), located a few kilometres to the south. Based on indirect evidence, these latter authors interpreted these faults as being responsible for considerable displacement of a major part of the Quaternary deposits. Given the lack of direct observations on salt-water distribution, geophysical results, and more specifically the electrical soundings, are required to provide supplementary information in order to better understand this phenomenon. Furthermore, the study of the salt water location, using electrical soundings, points out the juxtaposition of deep saline intrusions, demonstrating that the concept of a homogeneous wedge-shaped body parallel to the seashore can no longer be used to describe this chalky aquifer.In the Bas-Champs area, electromagnetic profiling and three electrical soundings provided additional information on the relation between tectonic events and saline water intrusion (Fig. 3). The electromagnetic profiling shows the presence of two faults (Fig. 6). The vertical faults have an apparent low electrical resistivity (2.5 Ω.m) indicating the presence of brackish or salt water. Electrical sounding carried out to the south of the electromagnetic profile (SE9) shows a decrease in the electrical resistivity from 12.5 m depth, also confirming the presence of salt water. The results of an electrical sounding located to the north of the electromagnetic profile (SE10) show that the ground is saturated with fresh water up to at least 27.2 m (resistivity of 90 Ω.m). At 1 km distance, there is a juxtaposition of three sectors separated by faults filled up with salt water. However, an electrical sounding (SE2, Fig. 2) located near the sea showed that the ground is saturated with fresh water up to at least 52.7 m deep.Using geological, hydrogeological and geophysical data correlations, between structural context and saltwater intrusion could be established and a new hypothesis for saltwater intrusion for this study area could be proposed. Intrusion is directly dependent on the geometry and the hydrodynamic characteristics of the chalky aquifer, as reflected by its heterogeneous nature.Salt water seems to follow preferential intrusion directions corresponding to faults rather than the classic model of a salt wedge. These results suggest the hypothesis that the seawater intrusion distribution relates to the tectonic configuration of the aquifer. The chalky series is cut into hydrogeological compartments individualised by tectonic accidents, which are preferential axes for sea intrusion. The presence of a saltwater front located far from the coast, as indicated by electrical soundings, could be explained by the geological history of the maritime plain, where the Pleistocene and Holocene seas covered the maritime plain up to the fossil cliff (LOUCHE et al., 1998)

Dissolution Behaviour of Alkali-activated Fe-rich Non-ferrous Metallurgy Slag in Acetic Acid

The application of Fe-rich non-ferrous metallurgy slag (NFS, within a FeOx-SiO2-Al2O3-CaO system) in alkali-activated materials requires detailed information on the durability performance. The present study investigates the durability of alkali activated NFS (AA-NFS) exposed to acetic acid to simulate the attack of a concrete by organic acids present in animal manure or sewage systems. The dissolution behavior of NFS and alkali-activated NFS (AA-NFS) was assessed by immersing NFS and AA-NFS in a 3 wt. % acetic acid solution at a liquid to solid weight ratio of 1000. The dissolved ions in the acetic solution from NFS and AA-NFS were measured at different time intervals over 7 days. Through the comparison of NFS with AA-NFS, the dissolution behavior of unreacted slag and binder in AA-NFS could be evaluated separately, considering a calculated amount of 47.7 wt.% of unreacted slag was present in the AA-NFS. The results demonstrated that the binder dissolves slightly faster than slag in the first 4 hours. Over half of the dissolution rate of the Ca, Fe and Al in AA-NFS was due to binder. While for Si, 41% total dissolution rate in AA-NFS was from binder and the other 59% was from unreacted slag. After 7 days however, the dissolved fraction of slag was higher than binder. About 90% Ca, 79% Fe and 71% Al in slag was ended up in the acetic acid solution, which is higher than that in binder (74%, 62% and 56%, respectively). A significantly higher difference was found for the total dissolved fraction of Si in slag (86%) compared to that in binder (43%). The highly connected silicate network in the binder remains largely intact as silica gel, while due to the low connectivity of the silicate species in the slag the silicate dissolves after the other elements have left the structure. Overall these results suggest that Ca has the lowest dissolution resistance in binder, followed by Fe, Al and Si

Surface energy budget and thermal inertia at Gale Crater: Calculations from ground‐based measurements

The analysis of the surface energy budget (SEB) yields insights into soil‐atmosphere interactions and local climates, while the analysis of the thermal inertia ( I ) of shallow subsurfaces provides context for evaluating geological features. Mars orbital data have been used to determine thermal inertias at horizontal scales of ~10 4  m 2 to ~10 7  m 2 . Here we use measurements of ground temperature and atmospheric variables by Curiosity to calculate thermal inertias at Gale Crater at horizontal scales of ~10 2  m 2 . We analyze three sols representing distinct environmental conditions and soil properties, sol 82 at Rocknest (RCK), sol 112 at Point Lake (PL), and sol 139 at Yellowknife Bay (YKB). Our results indicate that the largest thermal inertia I  = 452 J m −2  K −1  s −1/2 (SI units used throughout this article) is found at YKB followed by PL with I  = 306 and RCK with I  = 295. These values are consistent with the expected thermal inertias for the types of terrain imaged by Mastcam and with previous satellite estimations at Gale Crater. We also calculate the SEB using data from measurements by Curiosity's Rover Environmental Monitoring Station and dust opacity values derived from measurements by Mastcam. The knowledge of the SEB and thermal inertia has the potential to enhance our understanding of the climate, the geology, and the habitability of Mars. Key Points We calculate the thermal inertia and surface energy budget at Gale Crater We use MSL REMS measurements for our calculationsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108664/1/jgre20287.pd

Imaging groundwater infiltration dynamics in the karst vadose zone with long-term ERT monitoring

Water infiltration and recharge processes in karst systems are complex and difficult to measure with conventional hydrological methods. In particular, temporarily saturated groundwater reservoirs hosted in the vadose zone can play a buffering role in water infiltration. This results from the pronounced porosity and permeability contrasts created by local karstification processes of carbonate rocks. Analyses of time-lapse 2-D geoelectrical imaging over a period of 3 years at the Rochefort Cave Laboratory (RCL) site in south Belgium highlight variable hydrodynamics in a karst vadose zone. This represents the first long-term and permanently installed electrical resistivity tomography (ERT) monitoring in a karst landscape. The collected data were compared to conventional hydrological measurements (drip discharge monitoring, soil moisture and water conductivity data sets) and a detailed structural analysis of the local geological structures providing a thorough understanding of the groundwater infiltration. Seasonal changes affect all the imaged areas leading to increases in resistivity in spring and summer attributed to enhanced evapotranspiration, whereas winter is characterised by a general decrease in resistivity associated with a groundwater recharge of the vadose zone. Three types of hydrological dynamics, corresponding to areas with distinct lithological and structural features, could be identified via changes in resistivity: (D1) upper conductive layers, associated with clay-rich soil and epikarst, showing the highest variability related to weather conditions; (D2) deeper and more resistive limestone areas, characterised by variable degrees of porosity and clay contents, hence showing more diffuse seasonal variations; and (D3) a conductive fractured zone associated with damped seasonal dynamics, while showing a great variability similar to that of the upper layers in response to rainfall events. This study provides detailed images of the sources of drip discharge spots traditionally monitored in caves and aims to support modelling approaches of karst hydrological processes

Retrieving hydrological connectivity from empirical causality in karst systems

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Retrieving hydrological connectivity from empirical causality in karst systems

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Evidence-based planning and costing palliative care services for children : novel multi-method epidemiological and economic exemplar

Background: Children’s palliative care is a relatively new clinical specialty. Its nature is multi-dimensional and its delivery necessarily multi-professional. Numerous diverse public and not-for-profit organisations typically provide services and support. Because services are not centrally coordinated, they are provided in a manner that is inconsistent and incoherent. Since the first children’s hospice opened in 1982, the epidemiology of life-limiting conditions has changed with more children living longer, and many requiring transfer to adult services. Very little is known about the number of children living within any given geographical locality, costs of care, or experiences of children with ongoing palliative care needs and their families. We integrated evidence, and undertook and used novel methodological epidemiological work to develop the first evidence-based and costed commissioning exemplar. Methods: Multi-method epidemiological and economic exemplar from a health and not-for-profit organisation perspective, to estimate numbers of children under 19 years with life-limiting conditions, cost current services, determine child/parent care preferences, and cost choice of end-of-life care at home. Results: The exemplar locality (North Wales) had important gaps in service provision and the clinical network. The estimated annual total cost of current children’s palliative care was about £5.5 million; average annual care cost per child was £22,771 using 2007 prevalence estimates and £2,437- £11,045 using new 2012/13 population-based prevalence estimates. Using population-based prevalence, we estimate 2271 children with a life-limiting condition in the general exemplar population and around 501 children per year with ongoing palliative care needs in contact with hospital services. Around 24 children with a wide range of life-limiting conditions require end-of-life care per year. Choice of end-of-life care at home was requested, which is not currently universally available. We estimated a minimum (based on 1 week of end-of-life care) additional cost of £336,000 per year to provide end-of-life support at home. Were end-of-life care to span 4 weeks, the total annual additional costs increases to £536,500 (2010/11 prices). Conclusions: Findings make a significant contribution to population-based needs assessment and commissioning methodology in children’s palliative care. Further work is needed to determine with greater precision which children in the total population require access to services and when. Half of children who died 2002-7 did not have conditions that met the globally used children's palliative care condition categories, which need revision in light of findings

Diagnosing Clostridioides difficile infections with molecular diagnostics: multicenter evaluation of revogene C. difficile assay

Clostridioides difficile infections are a significant threat to our healthcare system, and rapid and accurate diagnostics are crucial to implement the necessary infection prevention and control measurements. Nucleic acid amplification tests are such reliable diagnostic tools for the detection of toxigenic Clostridioides difficile strains directly from stool specimens. In this multicenter evaluation, we determined the performance of the revogene C. difficile assay. The analysis was conducted on prospective stool specimens collected from six different sites in Europe. The performance of the revogene C. difficile assay was compared to the different routine diagnostic methods and, for a subset of the specimens, against toxigenic culture. In total, 2621 valid stool specimens were tested, and the revogene C. difficile assay displayed a sensitivity/specificity of 97.1% [93.3-99.0] and 98.9% [98.5-99.3] for identification of Clostridioides difficile infection. Discrepancy analysis using additional methods improved this performance to 98.8% [95.8-99.9] and 99.6% [99.2-99.8], respectively. In comparison to toxigenic culture, the revogene C. difficile assay displayed a sensitivity/specificity of 93.0% [86.1-97.1] and 99.5% [98.7-99.9], respectively. These results indicate that the revogene C. difficile assay is a robust and reliable aid in the diagnosis of Clostridioides difficile infections.This article is freely available via Open Access. Click on the Publisher URL to access it via the publisher's site.This study was supported by grants from GenePOC, now part of Meridian Biosciences.published version, accepted versio