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Risk Assessment for Sustainable Waste Disposal to Landfills
No full textThe notion that current engineering practices and works should be designed and carried out with consideration for posterity is a basic assumption of environmental ethics. In this regard, a sustainable approach to waste disposal should limit greenhouse gas emissions and, at the same time, avoid a detrimental impact on the quality of the groundwater in the long term. Reaching these goals requires adopting a risk assessment procedure that guarantees the protection of the groundwater resource, avoiding recourse to oversized lining systems, with relevant environmental and economic savings. This paper describes the procedure that has been recently introduced in the Italian regulations for landfills and suggests more advanced modelling approaches, which take into account the diffusive transport through the lining system and the contaminant distribution in the underlying aquifer. A numerical example is developed to show the application of the proposed analytical and numerical solutions to a hypothetical landfill for non-hazardous waste. The acceptable concentration of organic and inorganic contaminants contained in the landfill is determined for post-closure conditions both within the service life of the geomembrane and after a complete degradation of the geomembrane. A critical discussion about applying the precautionary principle and adopting excessively low concentration threshold values in the groundwater is presented
Steady-state analysis of pollutant transport to assess landfill liner performance
Full text linkNew analytical solutions are presented for the assessment of the impact of contaminant transport through landfill liners on groundwater quality under steady-state conditions. These solutions can be applied to evaluate the equivalency and the effectiveness of landfill liners, even those that include a geomembrane, taking into account the presence of a natural attenuation layer interposed between the engineered barrier and the underlying aquifer. The impact of a contaminant on groundwater quality is quantified through the determination of the contaminant concentration along the horizontal direction of the groundwater flow for the case of thin aquifers that are only a few meters thick. For the case of thick aquifers, both a closed-form analytical solution and a step-by-step numerical solution are provided for the calculation of the variations in the contaminant concentration in horizontal and vertical directions within the aquifer
The role of diffusion induced electro-osmosis in the coupling between hydraulic and ionic fluxes through semipermeable clay soils
Full text linkMost of the experimental research conducted to date has provided evidence on the semipermeable membrane behaviour of smectite-rich clay soils, the extent of which is typically quantified through the reflection coefficient, when the permeant (electrolyte) solution contains a single monovalent or divalent salt. Under such conditions, the osmotic flow of solution is controlled to a great extent by the different accessibility of ions and water molecules to the soil porosity, which is referred to as the chemico-osmotic effect. However, theoretical simulations of coupled solute and solvent transport suggest that, when two or more cations that diffuse in water at different rates are present simultaneously in the permeant solution, the electro-osmotic effect, which stems from the condition of null electric current density through the porous medium, can be enhanced compared to the case of a single salt to such an extent that it becomes comparable to or even greater than the chemico-osmotic effect. An original closed-form analytical solution to the problem of calculating the diffusion potential, which in turn controls the magnitude of the electro-osmotic effect, is here illustrated, and the relative importance of the aforementioned contributions to multi-electrolyte systems is examined through the interpretation of laboratory test results from the literature pertaining to a bentonite amended clay soil in equilibrium with aqueous mixtures of potassium chloride (KCl) and hydrochloric acid (HCl). The proposed mechanistic model is shown to be able to quantitatively capture the impact of both chemico-osmosis and electro-osmosis on the measured reflection coefficient of smectite clays, thereby breaking new ground in the experimental and theoretical research on the osmotic properties of engineered clay barriers in contact with mixed aqueous electrolyte solutions
From the design of bottom landfill liner systems to the impact assessment of contaminants on underlying aquifers
Full text linkThe most recent advancements of the research activity that has been carried out at the Polytechnic University of Turin since the 1990s are presented, with a focus on the design approaches which can be adopted for the optimisation of the engineered clay barriers that are used as a part of the composite liners of solid waste landfills. A particular attention is devoted to the characterisation of the geosynthetic clay liners (GCLs) in terms of their microstructural features and semipermeable properties, which affect both the liquid and contaminant transport and the swelling–shrinking behaviour upon a variation in the chemical and mechanical boundary conditions. In the first part of the paper, novel analytical solutions are derived in order to account for the influence of the chemico-osmotic counter-flow on the leakage rate through a lining system that consists of a geomembrane (GM) overlying a GCL, as well as for the effect of a variation in the GCL swelling pressure on the hydraulic transmissivity of the GM–GCL interface. In the second part of the paper, a steady-state analysis approach is proposed with the aim to include all the aforementioned phenomena in the assessment of the impact of contaminant migration through the landfill bottom liners on the groundwater quality, taking into account the presence of a natural attenuation layer between the GCL and the underlying aquifer
Phenomenological analysis and physical interpretation of the reflection coefficient of clays
No full textThe reflection coefficient, also known as the chemico-osmotic efficiency co-efficient, is commonly measured in order to quantify the ability of clays to act as semipermeable porous media. However, it should be pointed out that this parameter is not a soil property, but it rather represents the soil response to hydraulic and chemical stresses, so that it depends on which test is used and may vary significantly when the clay void ratio, the specimen preparation method and/or the chemical composition of the pore solution are changed. A physically sound model is used to assess the flow mechanisms that govern the experimental determination of this parameter, considering also the effects of clay fabric
The role of chemico-osmosis in the performance assessment of bentonite-based contaminant barriers
Full text linkAlthough conservative estimates of the containment performance of bentonite-based barriers can be obtained from the classical advective-diffusive transport theory, the semipermeable properties of the bentonite component can determine a significant reduction in the migration rate of contaminants at low salt concentrations (< 100 mM). After introducing the transport equations for a semipermeable porous medium, the paper presents the laboratory testing apparatuses and procedures that allow the chemico-osmotic efficiency coefficient, ω, and the osmotic effective diffusion coefficient, D ∗ω, of bentonites to be experimentally assessed. Finally, the advantages that derive from the ability to model the semipermeable behaviour of bentonites in the design of geoenvironmental barriers are discussed with the aid of a calculation example, which considers the use of geosynthetic clay liners for the bottom lining of waste disposal facilities
Relevance of Chemico-Osmotic and Electro-Osmotic Phenomena in Bentonite-Based Barriers
Full text linkOsmosis is known to play a key role in reducing the transport rate of contaminants through the natural and engineered clay barriers that are used for a number of geoenvironmental applications, such as the lining of landfills and the deep geological disposal of radioactive wastes. Although a significant body of experimental research has focused on the quantification of osmotic phenomena in smectite clays permeated with single-electrolyte solutions, no evidence has been provided about the membrane behaviour of clays in solute mixtures and, specifically, about the so-called osmotic anomalies (i.e. membrane efficiency coefficient outside the 0 to 1 range) that have been documented in the biological and chemical literature for fine-porous charged diaphragms in the presence of two or more electrolytes. In view of the similarities between such fine-porous media and smectite clays, the aim of the paper is to discuss the conditions under which bentonite-based barriers are expected to exhibit the aforementioned osmotic anomalies, which are shown to be caused by the different diffusivities and electrochemical valences of the migrating cations
LA PROGETTAZIONE GEOTECNICA SECONDO L'EC7: I POSSIBILI APPROCCI ED IL CONFRONTO CON L'ATTUALE NORMATIVA GEOTECNICA ITALIANA-IL PROGETTO DEL PALO SINGOLO
No full textCritical issues in the determination of the bentonite cation exchange capacity
Full text linkThe swelling pressure and transport properties of bentonites are controlled by the electric charge density of solid particles, which is commonly estimated from the laboratory measurement of the cation exchange capacity (CEC). However, the standard ammonium displacement method for CEC determination does not take into account the fabric changes that occur in bentonites under exposure to high salt concentration solutions. A series of laboratory tests was conducted to assess the relevance of such a critical issue, by varying the concentration of the extracting KCl solution with respect to that of the standard test. The obtained results show that the release of the adsorbed ammonium cations depends on the bentonite fabric, which is controlled by the KCl concentration. As a consequence, the ammonium displacement method may provide an unrepresentative estimate of the CEC of bentonites. The methylene blue titration method, despite its apparently more limited accuracy, instead seems to provide a more reliable estimation of the CEC, as the bentonite fabric is maintained dispersed during the test
Chemico-osmotic transport properties and mechanical behaviour of a natural sodium bentonite
No full textThe relevance of the semipermeable properties of bentonites, which affect both their transport processes and mechanical behaviour, was assessed through the determination of two parameters: the reflection coefficient, w, and the swell coefficient, w1. Laboratory tests were conducted, using a new apparatus, with the aim of measuring w and w1 of a natural sodium bentonite, while varying both the bentonite void ratio and the solute concentration of the sodium chloride (NaCl) solutions in contact with the bentonite specimens. The obtained results show that both parameters can be interpreted through a mechanistic model, in which the electric charge of clay particles is taken into account via a single parameter, csk0, that is referred to as the "solid skeleton electric charge concentration". A constant value of csk,0 = 110 mM was found to provide an accurate interpretation of all the experimental data, at least within the investigated range of bentonite void ratios (e varying from 3.33 to 4.18) and NaCl concentrations of the external bulk solutions (cs varying from 5 mM to 90 mM). The obtained results support the hypothesis that both chemical osmosis and swelling pressure are macroscopic manifestations of the same interactions, which occur at the microscopic scale, between the clay particles and the ions contained in the pore solution, and that both of them can be modelled through a single theoretical framework
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