Prior uncertainty investigation of density-viscosity dependent joint transport of heat and solute in alluvial sediments

Joint heat and solute tracer tests allow to add diffusion and conduction information to the solute advection-dispersion and help imaging preferential pathways in heterogeneous aquifers. We perform a joint interpretation of heat and solute tracer tests combining deterministic modeling and Bayesian Evidential Learning. The results show a strong influence of the water viscosity. The stochastic simulations highlight the influence of spatial and parameter uncertainty on the resulting breakthrough curves, stressing the need for realistic uncertainty quantification.Joint heat and solute tracer test inversion for imaging preferential pathway

Report on the tracer tests (experimental setup, results and interpretation)

Tracer experiments were performed in the Brévilles test site in order to highlight vertical variations in groundwater fluxes related to vertical variations hydraulic conductivity, to estimate contaminant travel time from several locations in the catchment to the springs and to identify transport processes affecting the fate of solutes in the saturated part of the aquifer. Following a first tracer experiment with uranine and sulforhodamine G in 2003, four tracer injections were performed in November 2005 in different piezometers, using uranine, sulforhodamine B, iodide and lithium. Tracer concentrations were monitored in the injection wells and at the basin outlet (spring and gauging station). Using the FVPDM method, concentration evolutions monitored in the injection wells allowed one to estimate local Darcy fluxes. At the basin outlet, only two tracers were recovered. Analyses of breakthrough curves confirm the stratification of the aquifer with more permeable levels in the lower part. They also suggest the probable occurrence of vertical interactions within the aquifer.AquaTerra - Integrated Modelling of the river-sediment-soil-groundwater system; advanced tools for the management of catchment areas and river basins in the context of global chang

Assessment of nitrate trend in groundwater using a regional scale approach

editorial reviewed6. Clean water and sanitatio

Investigating the respective impacts of groundwater exploitation and climate change on wetland extension over 150 years

International audiencePeatlands are complex ecosystems driven by many physical, chemical, and biological processes. Peat soils have a significant impact on water quality, ecosystem productivity and greenhouse gas emissions. However, the extent of peatlands is decreasing across the world, mainly because of anthropogenic activities such as drainage for agriculture or groundwater abstractions in underlying aquifers. Potential changes in precipitation and temperature in the future are likely to apply additional pressure to wetland. In this context, a methodology for assessing and comparing the respective impacts of groundwater abstraction and climate change on a groundwater-fed wetland (135 km2) located in Northwest France, is presented. A groundwater model was developed, using flexible boundary conditions to represent surface-subsurface interactions which allowed examination of the extent of the wetland areas. This variable parameter is highly important for land management and is usually not considered in impact studies. The model was coupled with recharge estimation, groundwater abstraction scenarios, and climate change scenarios downscaled from 14 GCMs corresponding to the A1B greenhouse gas (GHG) scenario over the periods 1961-2000 and 2081-2100. Results show that climate change is expected to have an important impact and reduce the surface of wetlands by 5.3-13.6%. In comparison, the impact of groundwater abstraction (100% increase in the expected scenarios) would lead to a maximum decrease of 3.7%. Results also show that the impacts of climate change and groundwater abstraction could be partially mitigated by decreasing or stopping land drainage in specific parts of the area. Water management will require an appropriate compromise which encompasses ecosystem preservation, economic and public domain activities

The Finite Volume Point Dilution Method: A tracer technique for monitoring transient Darcy fluxes

Quantification of pollutant mass fluxes is essential for assessing the impact of contaminated sites on their surrounding environment, particularly on adjacent surface water bodies. In this context, it is essential to quantify but also to be able to monitor the variations with time of Darcy fluxes in relation with changes in hydrogeological conditions and groundwater – surface water interactions. The Finite Volume Point Dilution Method (FVPDM) is a new tracer technique that generalizes the single-well point dilution method to the case of finite volumes of tracer fluid and water flush. It is based on an analytical solution derived from a mathematical model proposed recently to accurately model tracer injection into a well. After a brief description of the underlying concepts and mathematical model, an analytical solution is derived for calculating straightforwardly the evolution of concentration of the tracer in the injection well during and after injection operations. It is shown that this new tracer technique is easier to implement in the field than the classical point dilution method while it further allows monitoring changes with time of the magnitude of estimated Darcy fluxes, which is not the case for the former technique. In the scope of the EU FP6 AQUATERRA project, the FVPDM was applied in two experimental sites with contrasted objectives, geological and hydrogeological conditions, and field equipment facilities. In site A, the objective was to estimate contaminant travel times in groundwater to a spring while assessing vertical variations in groundwater fluxes, using a combined FVPDM – classical tracer test, with “non-ideal” experimental conditions. In site B, the purpose was to estimate, in very well controlled experimental conditions, groundwater fluxes flowing out from a contaminated site to a neighbouring river. In both cases, field tracer concentrations monitored in the injection wells were used to fit the calculated modelled concentrations by adjusting the apparent Darcy flux crossing the well screens. Modelling results are very satisfactory and indicate that the methodology is efficient and accurate, with a wide range of potential applications in different environments and experimental conditions, including the monitoring with time of changes in Darcy fluxes

Modeling the diffusive behavior of helium and uranine in a porous/fractured chalk aquifer

Informative data is needed to understand transport processes in the heterogeneous subsurface. For example, realistic modeling of transport processes in fractured rocks requires information about the fracture network and possible fracture-matrix exchanges. Field experiments involving the use of tracers with different values for the molecular diffusion coefficient are promising for imaging possible fracture-matrix exchanges more accurately and on different time and spatial scales. This can bring complementary data for modeling and support the reconstruction of the porous/fractured medium. In this context, dissolved gases (helium, argon, and xenon) and uranine were jointly injected into a saturated and porous/fractured chalk aquifer and recovered at a distance of 7.55 m (convergent test) or in the injection well after a specific ‘resting time’ (push-pull). For both tests, a sub-horizontal orientated fracture was isolated for injection using an inflatable double packer system. Uranine was measured with a field fluorimeter, and concentrations of the recovered dissolved gases were accurately measured on site with a mobile mass spectrometer. The diffusion coefficient of the tested tracers varies by one order of magnitude, resulting in significantly different breakthrough curves of uranine and helium during the convergent test. Analytical solutions involving multi-fracture and multi-channel conceptualization were used to simulate the experimental observations and to account for diffusion in the rock matrix. Dispersivity, fractures aperture and number, channels radius and number were manually adjusted using the experimental uranine and helium breakthrough curves in residence time distribution (RTD). The difference between observations and simulations was minimized by giving equal weight to the peak value, peak time, and slope in the RTD. For the convergent test, the uranine behavior was realistically simulated with the multi-fracture model, while for helium the multi-channel model was required. In contrast, all push-pull results could be simulated with a multi-fracture model as a smaller volume of porous medium was investigated. In addition, the experimental uranine and helium breakthrough curves of the convergent test were numerically simulated using a 3-dimensional model developed with HydroGeoSphere. Multiple discrete fractures are conceptualized, 3-dimensional diffusion is considered, and parameters are manually calibrated. The experimental data are simulated realistically by considering the contrasted diffusion coefficients of the tracers. The study demonstrates the potential of higher diffusive tracers and that such informative field data clearly support further modeling of dual media, including the application of innovative predictive approaches with the goal of more robust simulations and predictions

Study of historical groundwater level changes in two Belgian chalk aquifers in the context of climate change impacts

peer reviewedIn southern Belgium, 23% of abstracted groundwater volumes are from chalk aquifers, representing strategic resources for the region. Due to their specific nature, these chalk aquifers often exhibit singular behaviour and require specific analysis. The quantitative evolution of these groundwater resources is analysed for the Mons Basin and Hesbaye chalk aquifers as a function of past evolution, in the short and long term. Groundwater level time series exhibit decreases when analysed over different periods. This is particularly visible for the Hesbaye chalk aquifer when comparing the 1960–90 and 1990–2020 periods. Such decreases are associated with observed temperature increases and precipitation decreases, inducing a decrease of aquifer recharge, and a probable increase of groundwater abstraction in the adjacent catchment. Past evolution is also discussed considering recent winter and summer drought events. The aquifers exhibit long delays in response to recharge events, particularly where the thickness of the partially saturated zone plays a crucial role in observed delays. Regarding future evolution, simulations of the impact of climate changes using medium–high emission scenarios indicate a probable decrease of the groundwater levels over the Hesbaye chalk aquifer

Impact of climate change on groundwater resources in Wallonia: from past predictions to present observations

editorial reviewedDepuis la fin du 20ème siècle, une prise de conscience de plus en plus généralisée a émergé concernant l’impact du changement climatique sur les ressources en eau en général et en particulier sur les eaux souterraines. En Wallonie, une série de projets et travaux de recherches menés au début des années 2000, en particulier des travaux de modélisation, avaient permis de tester toute une série de scénarios qui, en majorité, avaient conduit à conclure qu’il fallait effectivement redouter un impact négatif du changement climatique sur les ressources en eau souterraine avec notamment des baisses progressives des niveaux de nappes et des débits de base. Certains de ces travaux avaient toutefois également montré qu’il était difficile de déterminer l’ampleur de ces modifications. Partant d’une description de ces travaux antérieurs, une série d’observations récentes sur plusieurs importants systèmes aquifères de Wallonie seront présentées qui viennent étayer ces prédictions et montrer que l’ampleur des changements observés tend plutôt vers les prédictions les plus pessimistes. Sur cette base, quelques pistes en cours de réflexion seront esquissées en vue de s’adapter aux conséquences du changement climatique sur les eaux souterraines et la manière de maintenir un état quantitatif suffisant de ces ressources essentielles pour la Wallonie.Since the end of the 20th century, there has been an increasing awareness of the impact of climate change on water resources in general and on groundwater in particular. In Wallonia, a series of projects and research work carried out in the early 2000s, in particular modelling work, made it possible to test a whole series of scenarios which, in the majority of cases, led to the conclusion that a negative impact of climate change on groundwater resources was indeed to be expected, with in particular progressive decreases in groundwater levels and base flows. However, some of this work also showed that it was difficult to determine the extent of these changes. Starting with a description of these earlier works, a series of recent observations on several major aquifer systems in Wallonia are presented, which support these predictions and show that the extent of the observed changes tends towards the most pessimistic predictions. On this basis, a number of avenues of reflection will be outlined with a view to adapting to the consequences of climate change on groundwater and the way to maintain a sufficient quantitative state of these essential resources for Wallonia