Integrated Surface and Subsurface Hydrological Modeling with Snowmelt and Pore Water Freeze-Thaw.

For the simulation of winter hydrological processes a gap in the availability of flow models existed: one either had the choice between (1) physically-based and fully-integrated, but computationally very intensive, or (2) simplified and compartamentalized, but computationally less expensive, simulators. To bridge this gap, we here present the integration of a computationally efficient representation of winter hydrological processes (snowfall, snow accumulation, snowmelt, pore water freeze-thaw) in a fully-integrated surface water-groundwater flow model. This allows the efficient simulation of catchment-scale hydrological processes in locations significantly influenced by winter processes. Snow accumulation and snowmelt are based on the degree-day method and pore water freeze-thaw is calculated with a vertical heat conduction approach. This representation of winter hydrological processes is integrated into the fully-coupled surface water-groundwater flow model HydroGeoSphere. A benchmark for pore water freeze-thaw as well as two illustrative examples are provided.

Integrated Hydrological Modeling of Climate Change Impacts in a Snow-Influenced Catchment.

The potential impact of climate change on water resources has been intensively studied for different regions and climates across the world. In regions where winter processes such as snowfall and melting play a significant role, anticipated changes in temperature might significantly affect hydrological systems. To address this impact, modifications have been made to the fully integrated surface-subsurface flow model HydroGeoSphere (HGS) to allow the simulation of snow accumulation and melting. The modified HGS model was used to assess the potential impact of climate change on surface and subsurface flow in the Saint-Charles River catchment, Quebec (Canada) for the period 2070 to 2100. The model was first developed and calibrated to reproduce observed streamflow and hydraulic heads for current climate conditions. The calibrated model was then used with three different climate scenarios to simulate surface flow and groundwater dynamics for the 2070 to 2100 period. Winter stream discharges are predicted to increase by about 80, 120, and 150% for the three scenarios due to warmer winters, leading to more liquid precipitation and more snowmelt. Conversely, the summer stream discharges are predicted to fall by about 10, 15, and 20% due to an increase in evapotranspiration. However, the annual mean stream discharge should remain stable (±0.1 m3 /s). The predicted increase in hydraulic heads in winter may reach 15 m and the maximum decrease in summer may reach 3 m. Simulations show that winter processes play a key role in the seasonal modifications anticipated for surface and subsurface flow dynamics.

Atmospheric Carbon Mineralization in an Industrial-Scale Chrysotile Mining Waste Pile.

Magnesium-rich minerals that are abundant in ultramafic mining waste have the potential to be used as a safe and permanent sequestration solution for carbon dioxide (CO2). Our understanding of thermo-hydro-chemical regimes that govern this reaction at an industrial scale, however, has remained an important challenge to its widespread implementation. Through a year-long monitoring experiment performed at a 110 Mt chrysotile waste pile, we have documented the existence of two distinct thermo-hydro-chemical regimes that control the ingress of CO2 and the subsequent mineral carbonation of the waste. The experimental results are supported by a coupled free-air/porous media numerical flow and transport model that provides insights into optimization strategies to increase the efficiency of mineral sequestration at an industrial scale. Although functioning passively under less-than-optimal conditions compared to laboratory-scale experiments, the 110 Mt Thetford Mines pile is nevertheless estimated to be sequestering up to 100 tonnes of CO2 per year, with a potential total carbon capture capacity under optimal conditions of 3 Mt. Annually, more than 100 Mt of ultramafic mine waste suitable for mineral carbonation is generated by the global mining industry. Our results show that this waste material could become a safe and permanent carbon sink for diffuse sources of CO2.

Channel representation in physically based models coupling groundwater and surface water: pitfalls and how to avoid them.

Recent models that couple three-dimensional subsurface flow with two-dimensional overland flow are valuable tools for quantifying complex groundwater/stream interactions and for evaluating their influence on watershed processes. For the modeler who is used to defining streams as a boundary condition, the representation of channels in integrated models raises a number of conceptual and technical issues. These models are far more sensitive to channel topography than conventional groundwater models. On all spatial scales, both the topography of a channel and its connection with the floodplain are important. For example, the geometry of river banks influences bank storage and overbank flooding; the slope of the river is a primary control on the behavior of a catchment; and at the finer scale bedform characteristics affect hyporheic exchange. Accurate data on streambed topography, however, are seldom available, and the spatial resolution of digital elevation models is typically too coarse in river environments, resulting in unrealistic or undulating streambeds. Modelers therefore perform some kind of manual yet often cumbersome correction to the available topography. In this context, the paper identifies some common pitfalls, and provides guidance to overcome these. Both aspects of topographic representation and mesh discretization are addressed. Additionally, two tutorials are provided to illustrate: (1) the interpolation of channel cross-sectional data and (2) the refinement of a mesh along a stream in areas of high topographic variability.

Physically based groundwater vulnerability assessment using sensitivity analysis methods.

A general physically based method is presented to assess the vulnerability of groundwater to external pressures by numerical simulation of groundwater flow. The concept of groundwater vulnerability assessment considered here is based on the calculation of sensitivity coefficients for a user-defined groundwater state for which we propose several physically based indicators. Two sensitivity analysis methods are presented: the sensitivity equation method and the adjoint operator method. We show how careful selection of a method can significantly minimize the computational effort. An illustration of the general methodology is presented for the Herten aquifer analog (Germany). This application to a simple, yet insightful, case demonstrates the potential use of this general and physically based vulnerability assessment method to complex aquifers.

Nitrate reduction in geologically heterogeneous catchments--a framework for assessing the scale of predictive capability of hydrological models.

In order to fulfil the requirements of the EU Water Framework Directive nitrate load from agricultural areas to surface water in Denmark needs to be reduced by about 40%. The regulations imposed until now have been uniform, i.e. the same restrictions for all areas independent of the subsurface conditions. Studies have shown that on a national basis about 2/3 of the nitrate leaching from the root zone is reduced naturally, through denitrification, in the subsurface before reaching the streams. Therefore, it is more cost-effective to identify robust areas, where nitrate leaching through the root zone is reduced in the saturated zone before reaching the streams, and vulnerable areas, where no subsurface reduction takes place, and then only impose regulations/restrictions on the vulnerable areas. Distributed hydrological models can make predictions at grid scale, i.e. at much smaller scale than the entire catchment. However, as distributed models often do not include local scale hydrogeological heterogeneities, they are typically not able to make accurate predictions at scales smaller than they are calibrated. We present a framework for assessing nitrate reduction in the subsurface and for assessing at which spatial scales modelling tools have predictive capabilities. A new instrument has been developed for airborne geophysical measurements, Mini-SkyTEM, dedicated to identifying geological structures and heterogeneities with horizontal and lateral resolutions of 30-50 m and 2m, respectively, in the upper 30 m. The geological heterogeneity and uncertainty are further analysed by use of the geostatistical software TProGS by generating stochastic geological realisations that are soft conditioned against the geophysical data. Finally, the flow paths within the catchment are simulated by use of the MIKE SHE hydrological modelling system for each of the geological models generated by TProGS and the prediction uncertainty is characterised by the variance between the predictions of the different models.

Comparing discrete fracture and continuum models to predict contaminant transport in fractured porous media.

We used the FRAC3Dvs numerical model (Therrien and Sudicky 1996) to compare the dual-porosity (DP), equivalent porous medium (EPM), and discrete fracture matrix diffusion (DFMD) conceptual models to predict field-scale contaminant transport in a fractured clayey till aquitard. The simulations show that the DP, EPM, and DFMD models could be equally well calibrated to reproduce contaminant breakthrough in the till aquitard for a base case. In contrast, when groundwater velocity and degradation rates are modified with respect to the base case, the DP method simulated contaminant concentrations up to three orders of magnitude different from those calculated by the DFMD model. In previous simulations of well-characterized column experiments, the DFMD method reproduced observed changes in solute transport for a range of flow and transport conditions comparable to those of the field-scale simulations, while the DP and EPM models required extensive recalibration to avoid high magnitude errors in predicted mass transport. The lack of robustness with respect to variable flow and transport conditions suggests that DP models and effective porosity EPM models have limitations for predicting cause-effect relationships in environmental planning. The study underlines the importance of obtaining well-characterized experimental data for further studies and evaluation of model key process descriptions and model suitability.

Calibration and validation of an integrated nitrate transport model within a well capture zone.

Groundwater contamination by nitrate was investigated in an agricultural area in southern Quebec, Canada, where a municipal well is the local source of drinking water. A network of 38 piezometers was installed within the capture zone of the municipal well to monitor water table levels and nitrate concentrations in the aquifer. Nitrate concentrations were also measured in the municipal well. A Water flow and Nitrate transport Global Model (WNGM) was developed to simulate the impact of agricultural activities on nitrate concentrations in both the aquifer and municipal well. The WNGM first uses the Agriflux model to simulate vertical water and nitrate fluxes below the root zone for each of the seventy agricultural fields located within the capture zone of the municipal well. The WNGM then uses the HydroGeoSphere model to simulate three-dimensional variably-saturated groundwater flow and nitrate transport in the aquifer using water and nitrate fluxes computed with the Agriflux model as the top boundary conditions. The WNGM model was calibrated by reproducing water levels measured from 2005 to 2007 in the network of piezometers and nitrate concentrations measured in the municipal well from 1997 to 2007. The nitrate concentrations measured in the network of piezometers, however, showed greater variability than in the municipal well and could not be reproduced by the calibrated model. After calibration, the model was validated by successfully reproducing the decrease of nitrate concentrations observed in the municipal well in 2006 and 2007. Although it cannot predict nitrate concentrations in individual piezometers, the calibrated and validated WNGM can be used to assess the impact of changes in agricultural practices on global nitrate concentrations in the aquifer and in the municipal well.

A review of thermal response test analysis using pumping test concepts.

The design of ground-coupled heat pump systems requires knowledge of the thermal properties of the subsurface and boreholes. These properties can be measured with in situ thermal response tests (TRT), where a heat transfer fluid flowing in a ground heat exchanger is heated with an electric element and the resulting temperature perturbation is monitored. These tests are analogous to standard pumping tests conducted in hydrogeology, because a system that is initially assumed at equilibrium is perturbed and the response is monitored in time, to assess the system's properties with inverse modeling. Although pumping test analysis is a mature topic in hydrogeology, the current analysis of temperature measurements in the context of TRTs is comparatively a new topic and it could benefit from the application of concepts related to pumping tests. The purpose of this work is to review the methodology of TRTs and improve their analysis using pumping test concepts, such as the well function, the superposition principle, and the radius of influence. The improvements are demonstrated with three TRTs. The first test was conducted in unsaturated waste rock at an active mine and the other two tests aimed at evaluating the performance of thermally enhanced pipe installed in a fully saturated sedimentary rock formation. The concepts borrowed from pumping tests allowed the planning of the duration of the TRTs and the analysis of variable heat injection rate tests accounting for external heat transfer and temperature recovery, which reduces the uncertainty in the estimation of thermal properties.

Modeling surface water-groundwater interaction with MODFLOW: some considerations.

The accuracy with which MODFLOW simulates surface water-groundwater interaction is examined for connected and disconnected losing streams. We compare the effect of different vertical and horizontal discretization within MODFLOW and also compare MODFLOW simulations with those produced by HydroGeoSphere. HydroGeoSphere is able to simulate both saturated and unsaturated flow, as well as surface water, groundwater and the full coupling between them in a physical way, and so is used as a reference code to quantify the influence of some of the simplifying assumptions of MODFLOW. In particular, we show that (1) the inability to simulate negative pressures beneath disconnected streams in MODFLOW results in an underestimation of the infiltration flux; (2) a river in MODFLOW is either fully connected or fully disconnected, while in reality transitional stages between the two flow regimes exist; (3) limitations in the horizontal discretization of the river can cause a mismatch between river width and cell width, resulting in an error in the water table position under the river; and (4) because coarse vertical discretization of the aquifer is often used to avoid the drying out of cells, this may result in an error in simulating the height of the groundwater mound. Conditions under which these errors are significant are investigated.

Simulating the injection of micellar solutions to recover diesel in a sand column.

This paper presents numerical simulations of laboratory experiments where diesel, initially present at 18% residual saturation in a sand column, was recovered by injecting a micellar solution containing the surfactant Hostapur SAS-60 (SAS), and two alcohols, n-butanol (n-BuOH), and n-pentanol (n-PeOH). The micellar solution was developed and optimized for diesel recovery using phase diagrams and soil column experiments. Numerical simulations with the compositional simulator UTCHEM agree with the experimental results and show that the entire residual diesel in the sand column was recovered after the downward injection of 5 pore volumes of the micellar solution. Recovery of diesel occurs by enhanced solubility in the microemulsion phase and by mobilization. An additional series of simulations investigated the effects of phase transfer, alcohol partitioning, and component segregation on diesel recovery. These simulations indicate that diesel can be accurately represented in the model by a single component, but that the pseudo-component approach for active matter and the assumption of local phase equilibrium leads to an underestimation of diesel mobilization.

Numerical analysis of water and solute transport in variably-saturated fractured clayey till.

This study numerically investigates the influence of initial water content and rain intensities on the preferential migration of two fluorescent tracers, Acid Yellow 7 (AY7) and Sulforhodamine B (SB), through variably-saturated fractured clayey till. The simulations are based on the numerical model HydroGeoSphere, which solves 3D variably-saturated flow and solute transport in discretely-fractured porous media. Using detailed knowledge of the matrix, fracture, and biopore properties, the numerical model is calibrated and validated against experimental high-resolution tracer images/data collected under dry and wet soil conditions and for three different rain events. The model could reproduce reasonably well the observed preferential migration of AY7 and SB through the fractured till, although it did not capture the exact depth of migration and the negligible impact of the dead-end biopores in a near-saturated matrix. A sensitivity analysis suggests fast flow mechanisms and dynamic surface coating in the biopores, and the presence of a plough pan in the till.

Yellow fever vaccine induces integrated multilineage and polyfunctional immune responses.

Correlates of immune-mediated protection to most viral and cancer vaccines are still unknown. This impedes the development of novel vaccines to incurable diseases such as HIV and cancer. In this study, we have used functional genomics and polychromatic flow cytometry to define the signature of the immune response to the yellow fever (YF) vaccine 17D (YF17D) in a cohort of 40 volunteers followed for up to 1 yr after vaccination. We show that immunization with YF17D leads to an integrated immune response that includes several effector arms of innate immunity, including complement, the inflammasome, and interferons, as well as adaptive immunity as shown by an early T cell response followed by a brisk and variable B cell response. Development of these responses is preceded, as demonstrated in three independent vaccination trials and in a novel in vitro system of primary immune responses (modular immune in vitro construct [MIMIC] system), by the coordinated up-regulation of transcripts for specific transcription factors, including STAT1, IRF7, and ETS2, which are upstream of the different effector arms of the immune response. These results clearly show that the immune response to a strong vaccine is preceded by coordinated induction of master transcription factors that lead to the development of a broad, polyfunctional, and persistent immune response that integrates all effector cells of the immune system.

Discretizing the fracture-matrix interface to simulate solute transport.

This article examines the required spatial discretization perpendicular to the fracture-matrix interface (FMI) for numerical simulation of solute transport in discretely fractured porous media. The discrete-fracture, finite-element model HydroGeoSphere (Therrien et al. 2005) and a discrete-fracture implementation of MT3DMS (Zheng 1990) were used to model solute transport in a single fracture, and the results were compared to the analytical solution of Tang et al. (1981). To match analytical results on the relatively short timescales simulated in this study, very fine grid spacing perpendicular to the FMI of the scale of the fracture aperture is necessary if advection and/or dispersion in the fracture is high compared to diffusion in the matrix. The requirement of such extremely fine spatial discretization has not been previously reported in the literature. In cases of high matrix diffusion, matching the analytical results is achieved with larger grid spacing at the FMI. Cases where matrix diffusion is lower can employ a larger grid multiplier moving away from the FMI. The very fine spatial discretization identified in this study for cases of low matrix diffusion may limit the applicability of numerical discrete-fracture models in such cases.

Comparative study of methods for WHPA delineation.

Human activities, whether agricultural, industrial, commercial, or domestic, can contribute to ground water quality deterioration. In order to protect the ground water exploited by a production well, it is essential to develop a good knowledge of the flow system and to adequately delineate the area surrounding the well within which potential contamination sources should be managed. Many methods have been developed to delineate such a wellhead protection area (WHPA). The integration of more information on the geologic and hydrogeologic characteristics of the study area increases the precision of any given WHPA delineation method. From a practical point of view, the WHPA delineation methods allowing the simplest and least expensive integration of the available information should be favored. This paper presents a comparative study in which nine different WHPA delineation methods were applied to a well and a spring in an unconfined granular aquifer and to a well in a confined highly fractured rock aquifer. These methods range from simple approaches to complex computer models. Hydrogeological mapping and numerical modeling with MODFLOW-MODPATH were used as reference methods to respectively compare the delineation of the zone of contribution and the zone of travel obtained from the various WHPA methods. Although applied to simple ground water flow systems, these methods provided a relatively wide range of results. To allow a realistic delineation of the WHPA in aquifers of variable geometry, a WHPA delineation method should ensure a water balance and include observed or calculated regional flow characteristics.