Migration of Heavy Metals in Saturated Sand and Bentonite/Soil Admixture

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 (8×10?4?cm/s) is found to be 6 orders of magniture higher than that of bentonite/soil admixture (about 8×10?10?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.     

Volatile Organic Compound (VOC) Transport through Compacted Clay

Movement of volatile organic compounds (VOCs) through compacted clay liners was investigated using laboratory-scale column and tank tests. Hydraulic conductivity of the compacted clay was not significantly impacted by the introduction of VOCs in concentrations up to 20 mg∕L. Soil-water partition coefficients of the seven VOCs tested had a strong logarithmic relationship with the octanol-water partition coefficient. Partition coefficients from batch tests were in good agreement with those measured directly on soil samples at the termination of the column∕tank tests. The VOCs were degraded in the clay, with estimated half-lives ranging from 2 to 116 days. Mechanical dispersion was not significant in the range of the hydraulic conductivities of the test specimens (i.e., <10?7 cm∕s). Effective molecular diffusion coefficients were mostly in 10?6 cm2∕s and generally decreased with increasing aqueous solubility. Mass transport parameters of VOCs in clay liners can be estimated from laboratory batch tests and properly prepared small-scale column tests. However, accounting for degradation of VOCs and minimizing the number of transport parameters that are simultaneously estimated from a single response-time record are important considerations for accurate determination of transport parameters.     

Relating Batch and Column Diffusion Coefficients for Leachable Contaminants in Particulate Waste Materials

For constructed facilities in which waste materials are used as partial substitutes for traditional aggregates, it is usually necessary to perform contaminant leachability tests to assess the long-term emission of contaminants from the facilities during service. Such tests can be performed under batch and column flow-through conditions. It is usually desirable to establish the relationship between leached contaminant concentrations obtained through both tests. Using Al and Cu diffusion coefficients as the target parameter, an analytical model is developed and presented herein with experimental data from acidic solution (pH of 4.5) leach testing of asphalt concrete that was amended with municipal solid waste incinerator bottom ash in weight percentages ranging from 0 to 20 to assess the equivalence of both sets of leaching conditions. The results for Al show that at higher column flowrates indexed by Peclet numbers in excess of 5.5, there is no defined relationship between Al diffusion coefficients obtained through both methods. Fluid flow at lower Peclet numbers approach batch conditions and afford an opportunity for the use of the analytical model presented in this paper provided comparisons are made at equivalent liquid/solid ratios. The values of the batch diffusion coefficients obtained for Al are of the order of 10?10–10?6?cm2/s. For column leaching of Al, the range is 10?8–10?7?cm2/s. No measurable quantity of Cu was obtained under both batch and column leaching conditions.     

Long-Term Tritium Transport through Field-Scale Compacted Soil Liner

A 13-year study of tritium transport through a field-scale earthen liner was conducted by the Illinois State Geological Survey to determine the long-term performance of compacted soil liners in limiting chemical transport. Two field-sampling procedures (pressure-vacuum lysimeter and core sampling) were used to determine the vertical tritium concentration profiles at different times and locations within the liner. Profiles determined by the two methods were similar and consistent. Analyses of the concentration profiles showed that the tritium concentration was relatively uniformly distributed horizontally at each sampling depth within the liner and thus there was no apparent preferential transport. A simple one-dimensional analytical solution to the advective–dispersive solute transport equation was used to model tritium transport through the liner. Modeling results showed that diffusion was the dominant contaminant transport mechanism. The measured tritium concentration profiles were accurately modeled with an effective diffusion coefficient of 6×10?4?mm2/s, which is in the middle of the range of values reported in the literature.     

Evaluation of Multidimensional Transport through a Field-Scale Compacted Soil Liner

A field-scale compacted soil liner was constructed at the University of Illinois at Urbana-Champaign by the U.S. Environmental Protection Agency (USEPA) and Illinois State Geological Survey in 1988 to investigate chemical transport rates through low permeability compacted clay liners (CCLs). Four tracers (bromide and three benzoic acid tracers) were each added to one of four large ring infiltrometers (LRIs) while tritium was added to the pond water (excluding the infiltrometers). Results from the long-term transport of Br? from the localized source zone of LRI are presented in this paper. Core samples were taken radially outward from the center of the Br? LRI and concentration depth profiles were obtained. Transport properties were evaluated using an axially symmetric transport model. Results indicate that (1) transport was diffusion controlled; (2) transport due to advection was negligible and well within the regulatory limits of ksat ? 1×10?7?cm/s; (3) diffusion rates in the horizontal and vertical directions were the same; and (4) small positioning errors due to compression during soil sampling did not affect the best fit advection and diffusion values. The best-fit diffusion coefficient for bromide was equal to the molecular diffusion coefficient multiplied by a tortuosity factor of 0.27, which is within 8% of the tortuosity factor (0.25) found in a related study where tritium transport through the same liner was evaluated. This suggests that the governing mechanisms for the transport of tritium and bromide through the CCL were similar. These results are significant because they address transport through a composite liner from a localized source zone which occurs when defects or punctures in the geomembrane of a composite system are present.     

Centrifuge Permeameter for Unsaturated Soils. II: Measurement of the Hydraulic Characteristics of an Unsaturated Clay

This paper presents the hydraulic characteristics of an unsaturated, compacted clay, including its soil-water retention curve (SWRC) and hydraulic conductivity function (K function), determined using a new centrifuge permeameter developed at the University of Texas at Austin. A companion paper describes the apparatus, its instrumentation layout, and data reduction procedures. Three approaches are evaluated in this study to define the SWRC and K function of the compacted clay under both drying and wetting paths, by varying the inflow rate, the g level, or both. For imposed inflow rates ranging from 20 to 0.1 mL/h and g levels ranging from 10 to 100 g, the measured matric suction ranged from 5 to 70 kPa, the average volumetric water content ranged from 23 to 33%, and the hydraulic conductivity ranged from 2×10?7 to 8×10?11?m/s. The SWRCs and K functions obtained using the three different testing approaches were very consistent, and yielded suitable information for direct determination of the hydraulic characteristics. The approaches differed in the time required to complete a testing stage and in the range of measured hydraulic conductivity values. The g level had a negligible effect on the measured hydraulic characteristics of the compacted clay. The SWRCs and K functions defined using the centrifuge permeameter are consistent with those obtained using pressure chamber and column infiltration tests. The K functions defined using the centrifuge permeameter follow the same shape as those obtained from predictive relationships, although the measured and predicted K functions differ by two orders of magnitude at the lower end of the volumetric water content range.     

Tropical Soils–Acid Mine Drainage Interactions: Breakthrough Curves and Some Transport Parameters

The interactions between acid mine drainage (AMD) and three tropical clayey soils have been studied through hydraulic conductivity and batch sorption/dissolution testing. After 19–24 pore volumes of AMD flow through the soils, the measured final hydraulic conductivity ranged between 1.1×10?11 and 3.3×10?11?m/s. The pH, electrical conductivity, and solute breakthrough curves obtained suggested the soils had low acid-buffering capacities. Also, several chemical species were released from the soils due to AMD attack of soil grains and mineral. The batch tests revealed high dissolution of species such as sodium, cobalt, and sulfate from the soils by AMD. The results also indicated dissolution of metals from soil constituents, desorption of chemical species, and transformation and dissolution of soil minerals due to AMD attack. The results of the study showed that, in general, the soils were not effective in attenuating contaminants present in the AMD. Thus, they may not be used alone to construct liners for acid-generating waste mine containment facilities.     

Optimal Design of a Compacted Soil Liner Containing Sorptive Amendments

The design of a compacted soil liner that includes sorptive amendments is presented and evaluated as a combinatorial optimization problem. An objective function based on the materials costs, opportunity costs, and construction costs of the liner was used to evaluate the effect of incorporating four sorptive materials: benzyltriethylammnonium-bentonite, hexadecyltrimethylammonium-bentonite, shale, and granular activated carbon (GAC) into a compacted clay liner in order to mitigate transport of organic solutes through the liner. The results from this study indicate that the inclusion of sorptive amendments as a component in compacted soil liners can effectively retard the transport of organic contaminants through the liner without violating regulatory hydraulic conductivity requirements. In all cases when aqueous transport was considered as a constraint in the objective function formulation, the resulting liner always contained some amount of sorptive amendment. In general, shale and GAC were selected for use in the sorptive liner design for all organic solutes tested. The modeling framework presented in this study is general and could be used to evaluate other types of sorptive materials or additional constraints, and thus represents a flexible new tool for the design of compacted soil liners.     

Yield, Strength, and Critical State Behavior of a Tropical Saturated Soil

The paper reports laboratory investigations carried out on a tropical soil profile to study its compressibility, strength, critical state and limit state conditions, and their variation with depth. The soil profile comprises a reddish lateritic layer (horizon B) underlain by a saprolitic soil (horizon C) from which a number of block samples were taken. A series of isotropic and anisotropic compression tests, and drained and undrained triaxial tests, were conducted on specimens sampled at depths between 1.0 and 7.0 m, and also in the exposed saprolitic soil. Special triaxial tests, with the pore pressure increased to induce failure, were performed to investigate the failure at low stress levels. On this basis a tensile cutoff on the failure envelope was defined. In order to assess the influence of the natural soil structure, drained and undrained triaxial tests were carried out on compacted samples obtained from depths of 1.0 and 5.0 m. Higher strength parameters were measured for the horizon C soil, which is consistent with its lower clay content. A nonlinearity in the critical state line in q:p′ stress space was identified, but linear regression was used to obtain critical state parameters. The limit state curves for soils from horizon B are centered on the hydrostatic axis, but limit state curves for horizon C suggested anisotropic behavior.     

Transit-Time Design for Diffusion through Composite Liners

Transit-time design methods are presented in this paper for determining the design thickness for composite liners consisting of a geomembrane and a compacted soil liner or geosynthetic clay liner. The design methods are based on a closed-form analytical solution for transient solute diffusion of volatile organic compounds in a composite liner and results from a numerical model. An analytical solution for diffusion in a two-layer soil profile, which is useful for transit-time design of composite liners, is also presented. The analytical solutions are used to develop graphical solution charts that can be used to design composite liners for which the effluent concentration and contaminant flux are less than a specified value. Design examples are included for a composite liner having a compacted soil liner and a composite liner having a geosynthetic clay liner. The method is relatively simple to apply and can be used for preliminary design of composite liners, evaluating experimental results, and verifying more complex numerical models.     

Organoclay with Soil-Bentonite Admixture as Waste Containment Barriers

Organoclays, clays modified by cationic surfactants, for engineering applications have recently drawn great attention because of their high organic removal capacity. In this study, the potential use of organoclays with soil-bentonite admixtures as waste containment barriers is investigated by experimental tests such as batch equilibrium sorption studies, compaction tests, and hydraulic conductivity tests. Sorption isotherms of total organic carbon (TOC), a gross organic term, by five different types of soil admixtures are nonlinear. The soil specimen with more organoclays exhibits higher organic sorption capacity and a larger retardation factor. The specimens with 20% by dry weight of bentonite have higher optimum water content and plasticity. With the addition of bentonite in the soil material consisting of completely decomposed volcanic rock (CDV) (natural soils) and organoclays, the hydraulic conductivity to leachate decreases from about 10?7 to 10?8 cm∕s. This indicates that the presence of bentonite in the admixtures is important in reducing hydraulic conductivity.     

Axial Dispersion and Mass Transfer Controlled Simulation Study of Chromium (VI) Adsorption onto Tree Leaves and Activated Carbon

In this paper, the feasibility and efficacy of chromium (Cr(VI)) removal using three different kinds of tree leaves viz. Emblica officinalis, Azadirachta indica, Eucalyptus agglomerata, and the activated carbon is examined through batch and continuous flow experiments. Pretreatments were given to the selected tree leaf powders to remove the natural pigments and lignin present. Batch and continuous flow experiments have been conducted to study the kinetics of adsorption, effects of pH, adsorbent dose, contact time, bed depth, flow rate, and initial Cr(VI) concentration on Cr(VI) adsorption onto the selected adsorbents. The adsorption capacity is observed higher for Emblica officinalis followed by Eucalyptus agglomerata and Azadirachta indica. The adsorption equilibrium is reached in less than 30 min and the maximum Cr(VI) uptake occurred at pH 3.0 under the test conditions. The results are also compared with the commercially available activated carbon. A mathematical model incorporating diffusion, advection, and mass transfer mechanisms available in the literature has been simplified and is then tested to simulate the laboratory and literature data. A simple method for the determination of saturation Cr(VI) concentration along the length of column has been presented. The study reveals that the model incorporating the molecular diffusion and the mass transfer mechanisms simulates better the Cr(VI) adsorption onto tree leaf powders than the literature model and the advection term plays only a negligible role due to low flow rates applied during the experiments. The model parameters, i.e., axial dispersion coefficient, “DL” and the external mass transfer coefficient, “kf” are found in the order of 10?5–10?6?m2/s and 10?9–10?11?m/s, respectively.     

Field Performance of a Compacted Clay Landfill Final Cover at a Humid Site

A study was conducted in southern Georgia, USA, to evaluate how the hydraulic properties of the compacted clay barrier layer in a final landfill cover changed over a 4-year service life. The cover was part of a test section constructed in a large drainage lysimeter that allowed continuous monitoring of the water balance. Patterns in the drainage (i.e., flow from the bottom of the cover) record suggest that preferential flow paths developed in the clay barrier soon after construction, apparently in response to desiccation cracking. After four years, the clay barrier was excavated and examined for changes in soil structure and hydraulic conductivity. Tests were conducted in situ with a sealed double-ring infiltrometer and two-stage borehole permeameters and in the laboratory on hand-carved blocks taken during construction and after four years of service. The in situ and laboratory tests indicated that the hydraulic conductivity increased approximately three orders of magnitude (from ≈ 10?7?to? ≈ 10?4?cm?s?1) during the service life. A dye tracer test and soil structure analysis showed that extensive cracking and root development occurred throughout the entire depth of the barrier layer. Laboratory tests on undisturbed specimens of the clay barrier indicated that the hydraulic conductivity of damaged clay barriers can be underestimated significantly if small specimens (e.g., tube samples) are used for hydraulic conductivity assessment. The findings also indicate that clay barriers must be protected from desiccation and root intrusion if they are expected to function as intended, even at sites in warm, humid locations.     

Diffusive Transport of VOCs through LLDPE and Two Coextruded Geomembranes

The diffusive properties of two coextruded geomembranes, one with a polyamide inner core and the other with an ethylene vinyl-alcohol (EVOH) inner core, and a standard 0.53-mm (20-mil) linear low-density polyethylene (LLDPE) geomembrane were examined. Diffusion and sorption laboratory tests were performed to estimate the parameters controlling diffusive migration, including the partitioning, diffusion, and permeation coefficients of the geomembrane in both the aqueous and vapor phases. Results indicate a significant reduction in mass flux through the coextruded geomembranes compared to conventional LLDPE. The EVOH coextruded geomembrane had the lowest permeation coefficients (Pg) with a range of (2–6)×10?12?m2?s?1 for diffusion from the aqueous phase. These values for EVOH are upper bounds and the actual values may be lower than as stated. The polyamide (nylon) coextruded geomembrane had higher values than for EVOH, with a Pg range of (0.7–2.2)×10?11?m2?s?1 from the aqueous phase. The highest permeation coefficients were for the standard 20-mil LLDPE, which ranged from (0.6–1.1)×10?10?m2?s?1. Thus the permeation coefficient for LLDPE was about one order of magnitude greater than for the nylon coextruded and at least two orders of magnitudes higher than for the EVOH coextruded geomembrane. Both coextruded geomembranes showed decreased Pg values and therefore improved diffusive resistance to volatile organic compounds over traditional 0.56-mm PVC geomembranes. The EVOH geomembrane showed a 5–12-fold decrease in Pg in comparison to a 2.0-mm high density polyethylene geomembrane.     

Performance of Long-Driven H-Piles in Granitic Saprolite

There has been much advancement using conceptual models and analytical methods to explain various aspects of pile performance. They are mainly based on the findings of model tests and full-scale pile tests in fine-grained and coarse-grained soils, and driven piles on land are normally less than 40?m. Design methods developed from this data bank of pile geometries and soil conditions for long piles should be treated with caution. In this paper, 13 H-piles of 34–60?m and 7,096?kN capacity founded on granitic saprolite are studied. Among them, two piles were restriked at different time intervals. All piles were axially load tested statically using a maintained load method. In contrast to the short rigid piles founded on weaker soil, their load-transfer mechanism varied with the magnitude of applied load and pile length. They deformed almost linearly at small loads and might have buckled when the loads were large and the creep settlements were found to be length dependent. Existing criteria might not be able to interpret failure loads sometimes, but a pile dynamic analyzer was found to give the best estimate on pile capacity.     

Flow of Gasoline through Composite Liners

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 6.0×10?9?cm/s, 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.     

Membrane Behavior of Compacted Clay Liners

The containment function of clay barriers used for waste containment applications (e.g., landfills) can be enhanced if such clays exhibit membrane behavior or the ability to restrict the migration of solutes (e.g., contaminants). In this regard, compacted specimens of a locally available natural clay known as Nelson Farm Clay (NFC), as well as NFC amended with 5% (dry weight) sodium bentonite, were evaluated for hydraulic conductivity, k, and the potential for membrane behavior. The membrane efficiencies of specimens of both soils compacted such that k was less than 10?7?cm/s were measured by establishing steady salt (KCl) concentration differences, ?ΔCo, ranging from 3.9 to 47 mM across the specimens in a flexible-wall cell under closed-system boundary conditions. The measured membrane efficiency for the unamended NFC was negligible (i.e., ≤ 1.4%), even though the k was suitably low (i.e., k<10?7?cm/s). In contrast, compacted specimens of the bentonite amended NFC exhibited not only lower k but also significant membrane behavior, with membrane efficiencies as high as 97.3% for ?ΔCo of 3.9-mM KCl. The results suggest that natural clays typically suitable for use as compacted clay liners (CCLs) are not likely to behave as semipermeable membranes unless the clay is amended with bentonite or the clay is inherently rich in high swelling clay minerals (e.g., sodium smectite). The potential benefit resulting from membrane behavior in a CCL constructed with the bentonite amended NFC is illustrated analytically in terms of liquid flux.     

Interactions between Three Tropical Soils and Municipal Solid Waste Landfill Leachate

Three tropical soils from Ghana, West Africa, were investigated in the laboratory for their potential as liners for waste containment. The key characteristic evaluated was the impact of municipal solid waste landfill leachate on the geotechnical, mineralogical, sorptive and diffusive properties and hydraulic conductivity of the soils. The observed unique characteristics of the soils included their silica: sesquioxide ratios that allowed them to be classified as either lateritic (ratio of 1.33–2.0) or nonlateritic (ratio greater than 2.0). After 5–11 pore volumes of leachate permeation through the soils, the specific surface of each soil decreased due to mineralogical transformations, while the cation exchange capacity of the soils increased. Na+ and K+ present at the exchange sites of the soils increased at the expense of desorbed Ca2+. The effective diffusion coefficient, De, obtained for potassium was 1.3–2.0×10?10?m2/s while that of sodium was 7.3–14×10?10?m2/s. New minerals formed in the soils included hydroxyapatite, pyromorphite, ferrihydrite, hydroxypyromorphite, and strengite. The mineralogical transformations, however, did not adversely alter the hydraulic conductivity of the soils. This finding, along with the observed relatively low De values, suggests that the soils would be effective hydraulic barriers against the migration of potential contaminants in landfill leachate. The study also found that kaolinite and aluminum and iron oxyhydroxides with variable particle surface charge present in the soils allowed sorption of anions, such as, Cl?, that are generally considered conservative (nonreactive) in liner-leachate compatibility studies on soils from temperate regions.     

Behavior of Rigid Footings on Gravel under Axial Load and Moment

A laboratory testing program was conducted to study the settlement and rotation response of rigid square footings under combined axial load and moment. A total of 17 tests were performed in which the size of the footing, footing embedment, axial load, and load eccentricity were changed. The test soil consisted of a fine and well-graded gravel contained in a box with dimensions: 1.52×1.52?m (5×5?ft) cross section and 0.91?m (3?ft) deep. The soil was compacted in layers 150?mm (6?in.) thick to an approximate relative density of 84%. In each test, the axial load, moment, settlement at the center of the footing, and footing rotation were measured. Concentrically loaded footings with different sizes exhibited a similar behavior in terms of the applied stress-normalized settlement (settlement divided by size of footing) response. The analytical model proposed was based on such normalized response as an input, and it was calibrated to account for the change in soil stiffness with confinement. The formulation captures the inherent nonlinear deformations of the soil with load and the coupled nature of settlements and rotations of footings under axial load and moment. The model was tested by comparing calculated values with laboratory measurements from tests not included in its calibration. The comparisons showed a satisfactory agreement between calculations and measurements, bringing confidence in the analytical formulation proposed and the methodology used.     

Foundry Green Sands as Hydraulic Barriers: Laboratory Study

A laboratory testing program was conducted to assess the use of foundry sands from gray iron foundries, typically called green sands, as hydraulic barrier materials. Foundry green sands are mixtures of fine uniform sand, bentonite, and other additives. Specimens of foundry sand were compacted in the laboratory at a variety of water contents and compactive efforts and then permeated in rigid-wall and flexible-wall permeameters to define relationships between hydraulic conductivity, compaction water content, and dry unit weight. Additional tests were conducted to assess how hydraulic conductivity of compacted foundry sand is affected by environmental stresses such as desiccation, freeze-thaw, and chemical permeation. Results of the tests show that the hydraulic conductivity of foundry sand is sensitive to the same variables that affect hydraulic conductivity of compacted clays (i.e., compaction water content, and compactive effort). However, hydraulic conductivities <10?7 cm∕s can be obtained for many foundry sands using a broad range of water contents and compactive efforts, including water contents dry of optimum and at lower compactive effort. The hydraulic conductivity of foundry sand was generally unaffected by freeze-thaw, desiccation, or permeation with 0.1 N salt solution or municipal solid waste leachate, but was incompatible with acetic acid (pH = 3.5). Hydraulic conductivity of foundry sands correlates well with bentonite content and liquid limit, with hydraulic conductivity less than 10?7 cm∕s being achieved for bentonite content ≥6% and∕or liquid limit >20.