Journal of Environmental Engineering

In order to analyze the fate and estimate the transport rates of contaminants through a barrier system, textural parameters such as the specific surface, density, permeability, diffusion coefficient, and flow path tortuosity are usually measured or estimated. The magnitudes of transport parameters of barrier systems are expected to change in response to physicochemical reactions and other environmental stresses, the intensities of which may grow or wane over time. In essence, when discrete catastrophic events (for example, earthquakes) are discounted, the flaws that develop are macroscopic manifestations of microlevel processes. Processes such as crystallization and precipitation add solid material to pore spaces in barriers and can improve barrier performance. Conversely, processes that cause changes in state from solid to liquid (for example, material dissolution) degrade barriers through the creation of larger flow channels. An appreciation of the thermodynamics of contaminant/barrier interactions under various environmental (temperature, pressure, and moisture) conditions is a prerequisite for establishing the bounds for textural changes and estimating contaminant release rates from containment systems. Then, process kinetics can be used to estimate the rate at which such texture-controlling processes may occur. The alternative approach is to conduct numerous “test-and-see” factorial experiments of limited utility, in which one parameter is changed at a time. The latter approach consumes resources excessively, relative to an approach that involves the use of thermodynamics to minimize the number of tests. In this paper, long-term deterioration mechanisms are analyzed, and a framework for their assessment within the context of barrier system performance modeling is presented.

The soil–water characteristic curve (SWCC) of fine-grained soils is usually determined experimentally. In the design of mine waste covers and landfill liners, the unsaturated hydraulic conductivity function, $k\left(h\right)\text{,}$ is often derived theoretically from the measured SWCC. Implicit in these derivations is the transformation of the SWCC to a pore-size distribution (PSD), typically assumed to be constant and monomodal. However, PSD measurements of a clayey till compacted at various water contents after compaction, after flexible-wall permeability testing and before and after SWCC tests show that the PSD of the same material varies significantly under the stated physical conditions. Predictions of the SWCCs using PSDs measured both before and after the SWCC tests significantly underpredicted the values measured. By applying a simple transformation to the PSD to account for the scaling effect from the porosimetry samples (approximately 1 g dry weight) to the SWCC test samples (approximately 200 g dry weight), the predicted SWCCs were found to envelop the measured values. A simple model that simulates the change in PSD during the SWCC test predicted water contents close (1% root mean square error) to the measured SWCCs.

The design of repository seals for deeply buried high-level radioactive wastes incorporates densely compacted clayey barriers around metallic waste canisters. In this paper, a mathematical model that is based on conservation of thermal energy and mass is developed to describe the locations of moisture and temperature fronts within a barrier, around a cylindrical waste canister of 1 m radius, containing radionuclides with half-lives that range from 100 to 10,000 years. The solution developed is axisymmetric: the moisture fraction w and temperature T vary only with time t, and radial distance r from the axis of the cylindrical waste canister. The model produces parabolic partial differential equations. The spatial domain is discretized such that ordinary differential equations that result are solved. Computations using a uniform mesh of 0.1 m and a cooling coefficient of $6.7\times {10}^{-2}$ with assumed but typical data on material properties, indicate that coupling of transport processes would be negligible in the case of radionuclides with long half-lives. Also, a quasisteady vaporization front can form and propagate outward over the course of several decades after waste emplacement. The evolution of the front is somewhat insensitive to the half-life used and the permeability of the clayey barrier material.

Permeable reactive walls have proven to be successful in laboratory and pilot-scale field applications. However, the long-term efficacy of reactive permeable walls has not been established due to the novelty of the technology. Also, the impact of common groundwater ions such as calcium and magnesium (i.e., hardness) on permeable reactive walls is unknown. In theory, the ions may react competitively with chromium in solution and/or other materials on the surface of the zero-valent iron. The ions may also form precipitates that could clog the reactive zone over time, resulting in decreased contaminant removal and a shorter wall lifetime. The purpose of this research was to determine the effects of common groundwater ions on permeable reactive walls. A range of calcium and magnesium concentrations was tested in laboratory columns to determine the effect of these ions on removal of a constant chromium concentration (100 mg/L). Results from the laboratory tests indicated that calcium and magnesium had a significant impact on chromium removal. The most dramatic effects were witnessed at hardness levels up to 140 mg/L as ${\mathrm{CaCO}}_3$ where zero-valent iron capacity was reduced by 45%.

Specific surface area (SSA) values for Fe–smectite alone and mixtures comprising varying proportions of glacial till were determined by nitrogen-adsorption Brunauer–Emmett–Teller analysis under different outgassing temperatures. Additionally, the extent to which the SSA of a mixture was related to the SSA of its components was investigated. A theoretical value for mixture SSA can be computed by summing the products of component SSA and the corresponding weight fraction. This theoretical estimate may not always yield the same value as measured because of a variety of physical and chemical interactions that can lead to either aggregation or dispersion of the constituent matrix. The results are explained in terms of these interactions and within the context of SSA relevance to barrier performance. For mixtures comprising 90% glacial till and 10% Fe–smectite, measured specific surface areas were consistently higher than predicted values for outgassing temperatures beyond 100°C, with a maximum departure of 4 m²/g. An increase in the net repulsive force between particles is likely responsible for the observed change. A striking feature noted in the microporosity analysis is the collapse of the Fe–smectite interlayer at an outgassing temperature of 200°C, possibly as a result of iron reduction.

Paper clay has several properties that make its use in vertical barriers very promising. In this paper the characteristics of two paper sludges (which will also be referred to as paper clays) one from the Erving Paper Company and one from the International Paper Company are considered. To determine the feasibility of paper clay for heavy metal attenuation, batch tests, column tests, hydraulic conductivity tests, and pressure filtration tests were performed. Batch testing showed that the metal, the distribution coefficient $\left(K_d\right)$ ranged from as low as 0.002 L/g for manganese to as high as 6.7 L/g for lead in International paper clay. In the Erving paper clay the $K_d$ ranged from 0.007 L/g for manganese to 1.9 L/g for cadmium, while the International paper clay $K_d$ ranged from 0.002 L/g for manganese to 6.7 L/g for lead. Chemisorption was established as the predominant metal binding mechanism in both clays with the only exceptions being manganese adsorption on International paper clay that indicated a constant partitioning, and copper and lead on Erving paper clay that indicated constant partitioning and cooperative adsorption, respectively. Both hydraulic conductivity and pressure filtration tests revealed permeabilities ranging from ${10}^{-5}$ to ${10}^{-7} \mathrm{cm}/s\text{.}$ Chloride testing showed that the coefficient of hydrodynamic dispersion (D) within paper clay is on the order of ${10}^{-7} {\mathrm{cm}}^2/s\text{.}$

An investigation of the effect of hydrated lime on the permeability and compressibility of two Brazilian soils: a brown saprolitic soil, Soil 1, and a red lateritic soil, Soil 2, is presented. Details of the test methods and materials are given. The results obtained show that the coefficient of permeability of Soil 1 increased about fivefold when 2% lime was added and then decreased on further addition of lime. This is attributed to the creation of chemical bonds and aggregation. As for Soil 2, the coefficient of permeability decreased as lime was added. This is also attributed to the same mechanism except that the bonds are weaker than those developed in Soil 1. The resistance of the soil to compression improved substantially, on the addition of 4% lime. The soils exhibit less improvement on further addition of lime. Lime was also found to be effective in decreasing the potential for collapse of the two soils when compacted to densities lower than their maximum dry densities.

A composite liner composed of a soil/clay liner and a flexible membrane is widely used for waste containment facilities. In this research, an organically modified clay (organoclay BB-40) liner and a high-density polyethylene (HDPE) membrane were studied for preventing the leakage and migration of gasoline from underground storage tanks into the surrounding environment. The equivalent hydraulic conductivity of intact HDPE to gasoline was determined using a specially built system, and the conventional hydraulic conductivity testing method was employed to determine the hydraulic conductivity of compacted organoclays and the permeation rate of gasoline through composite liners. The equivalent hydraulic conductivity of intact HDPE to gasoline was about 10–13 cm/s, and the hydraulic conductivity of the organoclay liner was approximately $6.0\times {10}^{-9}\mathrm{cm}/s\text{,}$ which is nearly 4 orders of magnitude lower than that obtained for unmodified clay. These results show that both organoclay and HDPE are effective in reducing the release of gasoline by advective flow, especially the intact HDPE. The flow of gasoline through the composite liners under the worst condition, was of the same magnitude as that through a single organoclay liner, independent of the flow shape. It can be anticipated that under good contact conditions, the defective HDPE would still be beneficial in reducing the permeation of gasoline due to the decrease of the wetted area of the underlying layer exposed to gasoline leakage.

The hydraulic properties of compacted clay liners can be adversely affected by desiccation cracking. Previous studies evaluated the use of soil additives (such as lime, cement, and sand) for crack reduction. Initial results indicated that soil shrinkage was reduced. However, in many cases, the additives resulted in an increased hydraulic conductivity and decrease in soil plasticity. As a result, there is an increasing interest in the use of fiber reinforcement, which has shown successful results in concrete and other material applications. The present investigation focused on the impact of fiber reinforcement on the development of desiccation cracks in compacted clay samples, as well as the impact of the fiber additives on soil workability, compaction characteristics and hydraulic conductivity. The results of this study indicate that, for the soils of this investigation, the optimum fiber content necessary to achieve maximum crack reduction and maximum dry density, while maintaining acceptable hydraulic conductivity, is between 0.4 and 0.5%. The observed crack reduction for this range of fiber content was approximately 50%, as compared to the unamended soil sample. The maximum observed crack reduction was approximately 90%, for a fiber content of 0.8%. Although the crack reduction could be increased further by increasing the fiber content, the sample hydraulic conductivity increased significantly and the practical limits of mixture workability were exceeded.

Desiccation of exposed clayey materials in cover layers of waste containment systems can ultimately result in the development of cracks that can promote the infiltration of rainwater and/or snowmelt, and escape of gases from buried wastes. Clayey barriers can be stabilized against desiccation by introducing binders such as concentrated polymer liquids or aqueous solutions. In this research, a theoretical analysis of the morphological configuration of molecules of polyethylene oxide (PEO) on montmorillonite (a common barrier mineral) was performed. A methodology for estimating polymer train and loop densities using batch sorption measurements was developed and demonstrated using PEO sorption test data for sodium montmorillonite (specific surface area: 31.82±0.22 m²/g) at initial aqueous PEO (molecular weight 8,000,000) concentrations of 0, 0.5, 1.0, 2.0, and 4.0 g/L. From test data, the distribution coefficient of PEO was determined to be 0.1599 mL/g. The analyses indicate that the distribution of sorbed PEO molecules into trains and loops is very sensitive to PEO chain flexibility. For example, for a function of partition function (λ) value of 0.1, the numbers of PEO molecular trains on 1 g of sodium montmorillonite are estimated to be $3.4\times {10}^{20}$ and $4.7\times {10}^{20}$ for chain flexibilities of 0.2 and 0.8, respectively. The greater the chain flexibility, the greater the opportunity for polymer molecules to lie flat on clay particles. Within an initial PEO concentration range of 0–4.0 g/L, no significant dependence of sorbed polymer molecular configuration on initial concentration was found. Possibly, extension of sorbed polymer molecular loops into barrier soil pores narrows fluid flow channels and hence the desiccation rate. This research enhances understanding of the physico-chemical processes that underlie clay stabilization by aqueous polymers.

Heavy metal migration through compacted, saturated sand and bentonite/soil admixtures were investigated using kinetic, batch sorption tests, and column tests. Sorption of Pb, Zn, and Cd at pH 5 by bentonite is found to be 411.2, 163.4, and 71.8 mL/g, respectively. The permeability of the compacted sand $(8\times {10}^{-4} \mathrm{cm}/s)$ is found to be 6 orders of magniture higher than that of bentonite/soil admixture (about $8\times {10}^{-10} \mathrm{cm}/s)$ when permeated with metal solutions under an effective stress of 34.5 kPa (5 psi). The permeation of metal solutions into bentonite/soil admixture columns does not result in a significant increase in permeability. Experimental results of batch sorption and hydraulic conductivity tests were also analyzed with a computer-based simulation model, POLLUTE, to determine the transport parameters (effective porosity, dispersion coefficient, retardation) of chloride ions and heavy metals in a homogeneous compacted sand and bentonite/soil admixture using a curve fitting technique.

The performance of a soil–bentonite barrier material as a sorbent for heavy metals was investigated in the laboratory using an influent containing 20 mg/L of ${\mathrm{Pb}}^{2+}$ at a pH of 5. The target parameters were the hydraulic conductivity of the soil–bentonite mix and the difference between ${\mathrm{Pb}}^{+2}$ concentrations in the influent and effluent. A hydraulic conductivity of $1.0\times {10}^{-8}\mathrm{cm}/s$ was achieved, the mixture was found to meet common regulatory specifications for hazardous waste containment. After four pore volumes of flow through specimens placed in a column, no ${\mathrm{Pb}}^{2+}$ was detected in the effluent. Sorption was verified through acid extraction and identification of ${\mathrm{Pb}}^{+2}$ on barrier sample cross sections using microanalysis of specimen slices with a scanning electron microscope and the associated energy dispersive x-ray spectrometry. Energy dispersive x-ray spectrometry spectra indicated that the ${\mathrm{Pb}}^{2+}$ partitioned selectively to the bentonite fraction of the mix. The results confirm the ability of this mixture of soil–bentonite to function as an effective barrier for aqueous ${\mathrm{Pb}}^{2+}$ solution. This method of microanalysis appears to have promise as an effective tool for assessing relative affinity of contaminants for specific mineralogical constituents of a barrier mixture and may have applications in sorption performance assessments of other multicomponent barrier systems. If each barrier material is tested separately, the effects of the texture of the mix on sorption and hydraulic characteristics of the mix cannot be effectively assessed.

Clays such as kaolinite and bentonite are widely used in various industries as sorbents. The sorptive characteristics of clays are exploited when they are used in contaminant barrier systems. To use clays effectively, their surface characteristics need to be known; especially, when they are used for contaminant sorption. Available surface area of clay minerals and the characteristics that depend on it are very sensitive to environmental changes such as those that can be induced by changes in the composition of pore fluid. Flow microcalorimetry with a down-stream concentration was used to determine the heats and amounts of adsorption of acids and bases on the clays. Test results presented herein revealed that both kaolinite and bentonite exhibit significantly different adsorption isotherms and heat of wetting under high pH and low pH conditions. Kaolinite has the capacity to adsorb both acidic and basic molecules almost equally. However, it has a tendency to adsorb more base than acid because of its stronger complexation capacity with acids than with bases. On the other hand, bentonite has a tendency to adsorb more acidic than basic molecules per gram. These results also indicate that both kaolinite and bentonite have different heats of wetting characteristics. As the concentrations of the acids and bases increase, the heat of wetting of kaolinite decreases while that of bentonite increases.