Sensitivity of Intrinsic Permeability to Electrokinetic Coupling in Shaly and Clayey Porous Media

Classical Darcy's law assumes that the intrinsic permeability of porous media is only dependent on the micro-geometrical and structural properties of the inner geometry of the medium. There are, however, numerous experimental evidences that intrinsic permeability of shaly and clayey porous material is a function of the fluid phase used in the experiments. Several pore-scale processes have been proposed to explain the observed behavior. In this study, we conduct a detailed investigation of one such mechanism, namely the electrokinetic coupling. We have developed a numerical model to simulate this process at the pore-scale, incorporating a refined model of the electrical double layer. The model is used to conduct a detailed sensitivity analysis to elucidate the relative importance of several chemical-physical parameters on the intensity of the electrokinetic coupling. We found that permeability reduction due to this mechanism is likely to occur only if the effective pore-radius is smaller than 10−6m. We also observed that electrokinetic coupling is strongly sensitive to electrophoretic mobility, which is normally reduced in clays compared to free-water conditions. Based on these findings, we set up a suite of stochastic pore-network simulations to quantify the extent of permeability reduction. We found that only if the effective pore-radius is ranging from 5× 10−7m to 5× 10−8, electrokinetic coupling can be responsible for a 5-20% reduction of the intrinsic permeability, and, therefore, this mechanism has a minor impact on situations of practical environmental or mining interes

Fully inorganic oxide-in-oxide ultraviolet nanocrystal light emitting devices

The development of integrated photonics and lab-on-a-chip platforms for environmental and biomedical diagnostics demands ultraviolet electroluminescent materials with high mechanical, chemical and environmental stability and almost complete compatibility with existing silicon technology. Here we report the realization of fully inorganic ultraviolet light-emitting diodes emitting at 390 nm with a maximum external quantum efficiency of ~0.3%, based on SnO(2) nanoparticles embedded in SiO(2) thin films obtained from a solution-processed method. The fabrication involves a single deposition step onto a silicon wafer followed by a thermal treatment in a controlled atmosphere. The fully inorganic architecture ensures superior mechanical robustness and optimal chemical stability in organic solvents and aqueous solutions. The versatility of the fabrication process broadens the possibility of optimizing this strategy and extending it to other nanostructured systems for designed applications, such as active components of wearable health monitors or biomedical devices

Evaluation of Silicate Minerals for pH Control During Bioremediation: Application to Chlorinated Solvents

Accurate control of groundwater pH is of critical importance for in situ biological treatment of chlorinated solvents. This study evaluated a novel approach for buffering subsurface pH that relies on the use of silicate minerals as a long-term source of alkalinity. A screening methodology based on thermodynamic considerations and numerical simulations was developed to rank silicate minerals according to their buffering efficiency. A geochemical model including the main microbial processes driving groundwater acidification and silicate mineral dissolution was developed. Kinetic and thermodynamic data for silicate minerals dissolution were compiled. Results indicated that eight minerals (nepheline, fayalite, glaucophane, lizardite, grossular, almandine, cordierite, and andradite) could potentially be used as buffering agents for the case considered. A sensitivity analysis was conducted to identify the dominant model parameters and processes. This showed that accurate characterization of mineral kinetic rate constants and solubility are crucial for reliable prediction of the acid-neutralizing capacity. In addition, the model can be used as a design tool to estimate the amount of mineral (total mass and specific surface area) required in field application

Numerical experiments on interactions between wave motion and variable-density coastal aquifers

A comprehensive two-dimensional (cross-shore) process-based numerical model of nearshore hydrodynamics (based on the Navier-Stokes equations, k-ε turbulence closure and the Volume-Of-Fluid method), beach morphology, and variable-density groundwater flow (SEAWAT-2000) was developed. This model, which was applied at the field scale, relaxes simplifications in existing models that do not include such detailed mechanistic descriptions. Numerical experiments were conducted to investigate the effects of varying aquifer, beach and wave characteristics (e.g., inland groundwater head, sand grain size, different wave heights and periods) on the coupled system. Spilling and plunging breakers on dissipative and intermediate beaches were simulated. For a given set of boundary conditions, the model was run for 1 y without the hydrodynamic sub-model to achieve a realistic salt-/freshwater interface. Then, the hydrodynamic component was run for 15 min and the model results analyzed. The main features considered were groundwater circulation, saltwater wedge position, in/exfiltration across the beach face, and beach morphology. The predictions of the numerical model agree well with existing understanding and experimental measurements. For an inland watertable that is lower than the still water level (SWL), such that the groundwater flow is mainly landward, on both coarse and fine sand beaches the addition of wave motion moves the saltwater wedge further landward. For an inland watertable that is higher than the SWL, the opposite behavior occurred. The numerical experiments showed that more sediment transport takes place on intermediate beaches than on dissipative beaches. In addition, beach profile variations are greater under plunging breakers, while coarse sand beaches are steeper than fine sand beaches for the same wave conditions. There is a strong correlation between in/exfiltration and beach face deposition/erosion for the coarse beaches, while in/exfiltration has a slight effect on sediment transport for fine beaches. The model is capable of simulating the short-term evolution of foreshore profile changes, and beach watertable and saltwater wedge movement due to interactions between wave motion and coastal groundwater

Analysis of carbon and nitrogen dynamics in riparian soils: Model development

The quality of riparian soils and their ability to buffer contaminant releases to aquifers and streams are connected intimately to moisture content and nutrient dynamics, in particular of carbon (C) and nitrogen (N). A multi-compartment model – named the Riparian Soil Model (RSM) – was developed to help investigate the influence and importance of environmental parameters, climatic factors and management practices on soil ecosystem functioning in riparian areas. The model includes numerous improvements compared to many similar tools, in particular regarding the capability to simulate a wide range of temporal scales, from daily to centuries, along with the ability to predict the concentration and vertical distribution of dissolved organic matter (DOM). The ecological importance of DOM has been highlighted on numerous occasions, and it was found that its concentration controls the amount of soil organic matter (SOM) stored in the soil as well as the respiration rate. The moisture content was computed using a detailed water budget approach, assuming that within each time step all the water above field capacity drains to the layer underneath, until it becomes fully saturated. A mass balance approach was also used for nutrient transport, whereas the biogeochemical reaction network was developed as an extension of an existing C and N turnover model. Temperature changes across the soil profile were simulated using an existing analytical solution of the heat transport equation, assuming periodic temperature changes in the topsoil. To verify the consistency of model predictions and illustrate its capabilities, a synthetic but realistic soil profile in a deciduous forest was simulated. Model parameters were taken from the literature, and model predictions were consistent with experimental observations for a similar scenario. Modelling results stressed the importance of environmental conditions on SOM cycling in soils. The mineral and organic C and N stocks fluctuate at different time scales in response to oscillations in climatic conditions and vegetation inputs/uptake. Low frequency fluctuations with a period larger than 10 y were observed also, which were not connected to any single environmental process

Carbon and nitrogen dynamics in a soil profile: Model development

In order to meet demands for crops, pasture and firewood, the rate of land use change from forested to agricultural uses has steadily increased over several decades, resulting in an increased release of nutrients towards groundwater and surface water bodies. In parallel, the degradation of riparian zones has diminished their capacity to provide critical ecosystem functions, such as the ability to control and buffer nutrient cycles. In recent years, however, the key environmental importance of natural, healthy ecosystems has been progressively recognized and restoration of degraded lands towards their former natural state has become an area of active research worldwide. Land use changes and restoration practices are known to affect both soil nutrient dynamics and their transport to neighbouring areas. To this end, in order to interpret field experiments and elucidate the different mechanisms taking place, numerical tools are beneficial. Microbiological transformations of the soil organic matter, including decomposition and nutrient turnover are controlled to a large extent by soil water content, influenced in turn by climatic and environmental conditions such as precipitation and evapotranspiration. The work presented here is part of the Swiss RECORD project (http://www.cces.ethz.ch/projects/nature/Record), a large collaborative research effort undertaken to monitor the changes in ecosystem functioning in riparian areas undergoing restoration. In this context we have developed a numerical model to simulate carbon and nitrogen transport and turnover in a one-dimensional variably saturated soil profile. The model is based on the zero-dimensional mechanistic batch model of Porporato et al. (Adv. Water Res., 26: 45-58, 2003), but extends its capabilities to simulate (i) the transport of the mobile components towards deeper horizons, and (ii) the vertical evolution of the profile and the subsequent distribution of the organic matter. The soil is divided in four compartments, each representing a different “functional unit”, having different thickness. The three shallower compartments, each variably saturated, correspond to the top soil, the root zone and an intermediate soil layer between the root zone and the aquifer. The deeper compartment represents the unconfined aquifer that receives nutrients infiltrating through the soil profile and always remains water-saturated. Carbon and nitrogen infiltration in the soil profile and their cycling are described by a set of coupled non-linear ordinary differential equations that are numerically integrated. To show the model capabilities in simulating soil nutrients transformations and transport and to illustrate how the model can be used to predict the changes in soil functioning as a result of land use changes, several realistic scenarios, with different soil and vegetation types, were modelled using a stochastically generated precipitation time series

Modelling migration and dissolution of mineral particles in saturated porous media

Understanding and predicting the fate in soils and other porous media of solid mineral particles with grain diameters in the micrometer range is important in a number of environmental and civil engineering applications, including subsurface hydrology, wastewater treatment and oil/gas production. In this context, deep-bed filtration theory is commonly applied to model particle detachment and deposition. Most existing models however neglect some processes that can modify groundwater flow patterns, particle concentration and attachment/detachment coefficients. The aim of this work was to develop a mechanistic model to study the transport and mobilization/immobilization of mineral particles in saturated porous media. The model accounts for particle advection and dispersion, deep-bed filtration, porosity and hydraulic conductivity changes associated with deposition and mobilization, and for particle dissolution. In addition, the deep-bed filtration coefficients vary with the characteristics and composition of the pore-solution, ionic strength and pH in particular. The groundwater flow and reactive transport simulator PHAST was used to implement the model. Measurements from a variety of deep-bed filtration and mineral dissolution experiments were used to calibrate and validate the model. A satisfactory comparison was found in most situations. A sensitivity analysis was subsequently performed to identify the conditions in which some of the processes (such as hydraulic conductivity changes and particle dissolution) can be neglected and therefore less sophisticated numerical tools can be used

Evaluation of silicate minerals for pH control during bioremediation: Application to chlorinated solvents

Accurate control of groundwater pH is of critical importance for in situ biological treatment of chlorinated solvents. This study evaluated a novel approach for buffering subsurface pH that relies on the use of silicate minerals as a long-term source of alkalinity. A screening methodology based on thermodynamic considerations and numerical simulations was developed to rank silicate minerals according to their buffering efficiency. A geochemical model including the main microbial processes driving groundwater acidification and silicate mineral dissolution was developed. Kinetic and thermodynamic data for silicate minerals dissolution were compiled. Results indicated that eight minerals (nepheline, fayalite, glaucophane, lizardite, grossular, almandine, cordierite, and andradite) could potentially be used as buffering agents for the case considered. A sensitivity analysis was conducted to identify the dominant model parameters and processes. This showed that accurate characterization of mineral kinetic rate constants and solubility are crucial for reliable prediction of the acid-neutralizing capacity. In addition, the model can be used as a design tool to estimate the amount of mineral (totalmass and specific surface area) required in field application