Browsing by Author "Shackelford, Charles D., advisor"
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Item Open Access Limiting membrane and diffusion behavior of a compacted sand-bentonite mixture for hydraulic and chemical containment(Colorado State University. Libraries, 2017) Fritz, Cameron John, author; Scalia, Joseph, IV, advisor; Shackelford, Charles D., advisor; Ronayne, Michael J., committee memberSodium-bentonite (Na-bentonite) commonly is used either as an additive component or as the sole component of engineered barriers used for waste containment applications, because the tendency of Na-bentonite to exhibit high swell can result in the restriction of advective and diffusive contaminant transport. Additionally, compacted mixtures of Na-bentonite and sand can be an effective and economical alternative to barrier materials consisting only of natural clay (e.g., compacted clay liners) if the use of natural clay is not logistically or economically feasible. The existence of membrane behavior, i.e., the ability of a porous material to exhibit selective restriction of migrating chemical species from the clay pores, previously has been shown for typical engineered bentonite-based barriers commonly used in hydraulic and chemical containment applications, including compacted sand-bentonite (SB) mixtures. However, the extent to which clay membrane behavior may persist in the presence of highly concentrated chemical solutions, which have been shown to have an adverse effect on the magnitude of membrane behavior in clays, remains largely unknown, with few studies having quantified the limiting membrane and diffusion behavior of bentonite-based barrier materials. Moreover, the limiting membrane and diffusion behavior of compacted SB mixtures has not yet been evaluated. Based on these considerations, the purpose of this study was to quantify the limiting membrane and diffusion behavior of two specimens of a compacted SB mixture comprising 15 % Na-bentonite (by dry weight) by determining the threshold salt concentration at which measurable membrane behavior was eliminated. The specimens were exposed to a series of boundary monovalent salt solutions with increasingly higher source concentrations, Cot, until measured values of the membrane efficiency coefficient, ω (0 ≤ ω ≤ 1), were effectively nil (i.e., 0.000), representing the limiting condition at which measurable membrane behavior was eliminated. Overall, ω decreased from an average of 0.032 to 0.000 as Cot increased from 160 mM KCl to 3.27 M NaCl, resulting in a threshold concentration between 1.63 M and 3.27 M NaCl for both specimens that was much higher than the range of salt concentrations for which measurable membrane behavior previously was thought to exist. Effective diffusion coefficients, D*, for nonreactive chloride (Cl-) also were measured during membrane testing to evaluate possible changes in diffusion behavior corresponding to the progressive destruction of membrane behavior. However, D* was relatively constant throughout all testing stages (2.1 x 10-10 m2/s ≤ D* ≤ 3.0 x 10-10 m2/s), indicating that the corresponding decrease in ω from 0.032 to 0.000 had little to no effect on the diffusion of Cl-.Item Open Access Membrane and diffusion behavior of a compacted sand-bentonite mixture for hydraulic and chemical containment applications(Colorado State University. Libraries, 2016) Meier, Amara Joy, author; Shackelford, Charles D., advisor; Bareither, Christopher A., committee member; Butters, Gregory L., committee memberDue to the ability of sodium bentonite (Na-bentonite) to exhibit high swell, low hydraulic conductivity, k (≤ 10-10 m/s), and semipermeable membrane behavior when exposed to water and dilute chemical solutions, Na-bentonite is commonly used as a component for engineered barriers (e.g., geosynthetic clay liners (GCLs)), or as an engineered barrier (e.g., compacted Na-bentonite buffers) used to contain solid wastes and liquid contaminants. Compacted sand-bentonite (SB) mixtures typically comprising 5 to 20 % Na-bentonite (by dry weight) are commonly used as alternatives to compacted clay liners (CCLs) for containment of solid wastes and liquid contaminants when a suitable natural clay source is not readily or economically available. In addition, membrane behavior, or the ability of a porous material to selectively restrict the passage of dissolved chemical species (solutes), has been shown to exist in many of these bentonite-based barriers, including GCLs, bentonite amended natural clays used for CCLs, and soil-bentonite backfills for in situ vertical cutoff walls. However, compacted SB mixtures suitable for use as engineered hydraulic and chemical containment barriers previously have not been evaluated for membrane behavior. As a result of these considerations, the purpose of this study was to evaluate simultaneously the membrane and diffusion behavior of a SB mixture that would be suitable for use as an engineered barrier for hydraulic and chemical containment applications. Accordingly, membrane tests were conducted on duplicate specimens of a compacted SB mixture comprising 15 % bentonite that was shown to exhibit sufficiently low k (≤ 2.7 x 10-11 m/s) to be suitable for use as a hydraulic and chemical containment barrier. In addition, the simultaneous diffusion of the principal salt species evaluated in the study, viz., Cl- and K+, was evaluated for one of the specimens. The results indicated that both specimens exhibited virtually the same magnitude of membrane behavior, with measured values of the membrane efficiency coefficients, ω, ranging from 0.395±0.053 to 0.063±0.012 when exposed to KCl solutions with source concentrations, Cot, ranging from 5 mM KCl to 80 mM KCl, respectively. In addition, the diffusion of both Cl- and K+ was found to be restricted relative to the case in which the specimen would not exhibit membrane behavior (i.e., ω = 0). Despite the imposition of chemical conditions in the tests that were more complex than those imposed previously, the measured values of ω and the effective diffusion coefficients, D*, for Cl- were in good agreement with those reported in the literature for other bentonite-based engineered barriers when exposed to similar or the same types of salts and salt concentrations. Thus, this study provides the first results to illustrate that a compacted SB mixture that is suitable for use as a hydraulic and chemical containment barrier behaves as a semipermeable membrane that can restrict aqueous-phase diffusion of chemical species to an extent that the chemical containment function of the barrier is improved.Item Open Access Membrane behavior and diffusion in unsaturated sodium bentonite(Colorado State University. Libraries, 2015) Sample-Lord, Kristin M., author; Shackelford, Charles D., advisor; Bareither, Christopher A., committee member; Butters, Gregory L., committee member; Lu, Ning, committee member; Sale, Thomas C., committee memberSodium-bentonite (Na-bentonite) is a highly active clay commonly used as a barrier or a component of a barrier for chemical containment applications (e.g., landfills, waste impoundments, vertical cutoff walls) due to the ability of Na-bentonite to limit solute (contaminant) transport resulting from high swell and low hydraulic conductivity. However, Na-bentonite also may exhibit semipermeable membrane behavior or solute restriction, which can result in enhanced performance of the barrier by reducing liquid and contaminant flux. Experimental studies to date have focused on the correlation between membrane behavior and diffusion of solutes almost exclusively under fully saturated conditions (i.e., degree of water saturation, S, of 1.0). However, clay barriers can exist at various degrees of water saturation (S < 1.0), and, based on our current, conceptual understanding of the mechanisms causing membrane behavior in saturated clays, the influence of membrane behavior on solute transport is likely to be even more significant in clays under unsaturated conditions. Based on these considerations, an innovative testing apparatus was developed to allow for the simultaneous measurement of membrane behavior and diffusion in unsaturated Na-bentonite. The test specimens were prepared using a dialysis method that allowed for control of the cation species on the exchange complex of the bentonite, removal of excess soluble salts, and estimation of diffusion properties. Membrane efficiencies (ω) and effective diffusion coefficients (D*) of bentonite specimens with S ranging from 0.79 to 1.0 were measured by performing multistage tests using solutions of potassium chloride (KCl). The source concentrations (Cot) of the KCl solutions were 20 mM, 30 mM, and 50 mM, which resulted in average concentrations in the specimen at steady-state diffusion (Cave) of approximately 10 mM, 15 mM, and 25 mM. For all values of S, a decrease in S correlated with an increase in ω and a decrease in D*. For example, for Cot of 50 mM, ω increased from 0.31 to 0.41 and D* for chloride decreased from 4.1 x 10-10 m2/s to 3.1 x 10-10 m2/s as S decreased from 1.0 to 0.84. The results of this study advance our fundamental understanding of solute transport mechanisms in Na-bentonite and contribute to the base of knowledge that must be established prior to incorporating membrane behavior effects in the design of barriers for chemical containment facilities.Item Open Access Membrane behavior of clay liner materials(Colorado State University. Libraries, 2008) Kang, Jong Beom, author; Shackelford, Charles D., advisorMembrane behavior represents the ability of porous media to restrict the migration of solutes, leading to the existence of chemico-osmosis, or the flow of liquid in response to a chemical concentration gradient. Membrane behavior is an important consideration with respect to clay soils with small pores and interactive electric diffuse double layers associated with individual particles, such as bentonite. The results of recent studies indicate the existence of membrane behavior in bentonite-based hydraulic barriers used in waste containment applications. Thus, measurement of the existence and magnitude of membrane behavior in such clay soils is becoming increasingly important. Accordingly, this research focused on evaluating the existence and magnitude of membrane behavior for three clay-based materials that typically are considered for use as liners for waste containment applications, such as landfills. The three clay-based liner materials included a commercially available geosynthetic clay liner (GCL) consisting of sodium bentonite sandwiched between two geotextiles, a compacted natural clay known locally as Nelson Farm Clay, and compacted NFC amended with 5% (dry wt.) of a sodium bentonite. The study also included the development and evaluation of a new flexible-wall cell for clay membrane testing that was used subsequently to measure the membrane behaviors of the three clay liner materials. The consolidation behavior of the GCL under isotropic states of stress also was evaluated as a preliminary step in the determination of the membrane behavior of the GCL under different effective consolidation stresses.Item Open Access Membrane behavior, diffusion, and compatibility of a polymerized bentonite for containment barrier applications(Colorado State University. Libraries, 2012) Bohnhoff, Gretchen L., author; Shackelford, Charles D., advisor; Benson, Craig H., committee member; Borch, Thomas, committee member; Sale, Thomas C., committee memberConventional (untreated or unmodified) bentonites are commonly used in hydraulic containment barriers to contain liquid flow and contaminant transport, because of the ability of bentonite to swell and achieve low hydraulic conductivity to water, substantial membrane behavior, and low solute diffusion coefficients. However, conventional bentonites also have been shown to be affected adversely by environmental conditions that promote multivalent-for-monovalent cation exchange. In this study, the membrane behavior and diffusive properties of a polyacrylic acid modified bentonite referred to as a bentonite polymer nanocomposite, or BPN, were determined through the simultaneous measurement of membrane efficiency coefficients, ω, and solute diffusion coefficients, D*, during combined multi-stage membrane and diffusion tests using either potassium chloride (KCl) with concentrations ranging from 4.7 mM to 54 mM or calcium chloride (CaCl2) with concentrations ranging from 5 mM to 20 mM. The BPN exhibited substantial membrane behavior when exposed to KCl with values of ω that were higher than those previously reported for conventional (unmodified) bentonite under similar testing conditions. For example, the ω value measured in this study for a BPN specimen contained within a rigid-wall cell and based on circulation of 20 mM KCl was 0.43, whereas that previously reported for a GCL specimen containing a conventional bentonite under similar testing conditions except at a lower porosity (0.74 vs. 0.92) was only 0.30. Also, in contrast to previously reported results for conventional bentonite, the membrane behavior of the BPN was sustained when exposed to 5 mM CaCl2, and values of ω for the BPN were higher than those previously reported for conventional and other modified bentonites. For example, the value of ω for the BPN tested in a rigid-wall cell with 5 mM CaCl2 was 0.95, whereas the ω values for an anionic polymer modified bentonite, known as Hyper clay, and a GCL were 0.13 and 0, respectively. However, exposure of specimens of the BPN to 10 mM CaCl2 for a test conducted in a rigid-wall cell and 20 mM CaCl2 for a test conducted in a flexible-wall cell did ultimately result in complete destruction of the membrane behavior. The destruction of the membrane behavior of the specimen in the rigid-wall test was attributed to short-circuiting along the side-walls of the rigid cell after shrinkage of the BPN specimen, whereas the destruction of the membrane behavior of the specimen in the flexible-wall test correlated with the time to reach steady-state diffusion of calcium (Ca2+). Similar to a previous study involving a conventional bentonite, the diffusive properties of the BPN also were shown to correlate well with the membrane behavior of the BPN, such that that the diffusive solute mass flux decreased as the membrane efficiency of the BPN increased. However, in contrast to previous test results, the steady-state values of D* for K+ and Ca2+ were not only not equal to but also lower than the D* value for Cl- at steady state, although the differences between the D* for K+ or Ca2+ versus that for Cl- diminished with increasing source concentration of KCl or CaCl2, respectively. This inequality between salt cation and salt anion D* values at steady state was attributed to the complicating existence of significant excess Na+ that was initially present within the specimen of BPN prior to testing and contributed to satisfying the requirement for electroneutrality, a contribution that diminished with time as the Na+ diffused out of the specimen. Finally, the use of BPN in soil-bentonite (SB) backfills of vertical cutoff walls was investigated. The hydraulic conductivity, k, to tap water, the consolidation behavior, and the chemical compatibility (Δk) based on permeation with CaCl2 solutions of SB backfills amended with BPN were evaluated and compared with those for a backfill comprised of a conventional bentonite. Although the backfills containing BPN were more sensitive to stress conditions than the backfill containing conventional bentonite, the overall hydraulic performance of a backfill containing 5 % dry BPN was better than that of the backfill containing 5 % dry conventional bentonite by approximately two orders of magnitude in terms of k. Overall, the BPN exhibited improved membrane and diffusion properties relative to conventional and other modified bentonites previously tested under similar conditions. However, the improved membrane behavior of the BPN was ultimately destroyed upon exposure to 10 mM CaCl2 in a rigid-wall cell and 20 mM CaCl2 in a flexible-wall cell. Also, despite an overall lower k of the sand-BPN backfills relative to a backfill comprised of the same sand but a conventional bentonite upon permeation with a 50 mM CaCl2 solution, the chemical resistance of the sand-BPN backfills in terms of changes in k was not any better than that for the sand-conventional bentonite backfill. Thus, the beneficial behavior of the BPN was not unlimited nor without issues, such that any perceived benefit of polymerized bentonites must first be properly characterized on a case-by-case basis prior to use.Item Open Access Numerical evaluation of one-dimensional large-strain consolidation of mine tailings(Colorado State University. Libraries, 2015) Agapito Tito, Luis Angel, author; Bareither, Christopher A., advisor; Shackelford, Charles D., advisor; Sutton, Sally J., committee memberThe objective of this study was to evaluate the applicability of commercially-available, one-dimensional (1-D) large-strain consolidation programs (FSConsol and CONDES0) for predicting mine tailings consolidation to estimate storage capacity of tailings storage facilities (TSFs). This study consisted of the following tasks: (i) consolidation modeling of well-known benchmark examples from literature, (ii) parametric study to assess the influence of input parameters (i.e., constitutive relationships, initial void ratio, impoundment geometry, and tailings production rate) on consolidation behavior and storage capacity, and (iii) consolidation and storage capacity prediction for a full-scale copper TSF. A benchmark example that represented instantaneous deposition of tailings (Townsend and McVay 1990) was evaluated with CONDES0 and FSConsol and indicated that both models are appropriate for predicting the consolidation behavior of tailings that are deposited instantaneously. Both models yielded similar temporal settlement curves and void ratio profiles. A gradual tailings deposition benchmark example (Gjerapic et al. 2008) was evaluated with both programs and suggested that FSConsol was more applicable for problems dealing with continuous discharge of tailings. In particular, FSConsol was more applicable when the tailings discharge rate varied temporally, which is a key constraint to modeling a full-scale TSF. The parametric study results suggested that the initial tailings void ratio and constitutive relationships (i.e., void ratio versus effective stress, e-σ', and hydraulic conductivity versus void ratio, k-e) had more pronounced effects on consolidation behavior relative to impoundment geometry and tailings production rate. In particular, a comparison between rapidly consolidating mine tailings (copper tailings) and slowly consolidating mine tailings (mature fine tailings from oil sands) indicated that a decrease in hydraulic conductivity by four orders of magnitude can extend the time required for consolidation by more than 200 yr. Changes in impoundment geometry and tailings production rate had limited effects on impoundment capacity for the range of side slopes (1.0H:1V to 4.5H:1V) and production rates (50 mtpd to 300 mtpd) evaluated in this study. FSConsol modeling results from the full-scale copper mine TSF were compared to field data and suggest that a 1-D consolidation model can yield a satisfactory prediction of in-situ consolidation behavior of copper tailings. Comparison between the actual average tailings dry density (ρd) during the first 4 yr of operation and predicted average ρd yielded coefficients of determination (R2) as high as 81 % and 93 % for Operation and Design assessments, respectively. In addition, predicted tailings height within the TSF showed good agreement with actual impoundment heights for the first 6 yr of operation; R2 = 99.1 % for the Operation assessment and cyclone operation time (COT) of 70 %, which was the average actual COT. A procedure was developed to predict average ρd of a full-scale TSF using a 1-D consolidation model that includes the following considerations: (i) estimate total tailings volume in the TSF based on predicted impoundment height and (ii) use this total volume with dry tailings mass discharged into the TSF to compute ρd. The main finding from this study was that the modeling of gradual tailings deposition with FSConsol provides a reliable prediction of impoundment height and impoundment capacity.Item Open Access Selected factors affecting measurement of the hydraulic conductivity of geosynthethic clay liners(Colorado State University. Libraries, 2018) Popang, Monika Aprianti, author; Scalia, Joseph, IV, advisor; Shackelford, Charles D., advisor; Ronayne, Michael J., committee memberGeosynthetic clay liners (GCLs) are thin (~7 to 10 mm), factory manufactured hydraulic barriers typically comprising a layer of sodium bentonite sandwiched between two geotextiles. Upon hydration and permeation with water at low effective stress (σ´) (e.g., typically ≤ ~30 kPa [4 psi]), the bentonite in GCLs, which typically is initially in an air-dried condition, swells to form a low hydraulic conductivity (k) layer (i.e., k of ~2-3×10-11 m/s) that is suitable for use as a barrier in hydraulic and chemical containment applications. However, adverse physico-chemical interactions between the bentonite in GCLs and both the hydrating and permeating liquids may yield substantially higher k than what is typically acceptable for design (i.e., k ≤ 1×10-9 m/s). Accordingly, this study pertained to evaluating the effects of the type of permeant liquid and the magnitude of σ´ on the measurement of k of two GCLs, a higher grade needle-punched (HGN) GCL and a lower grade needle-punched (LGN) GCL. The permeant liquids included tap water (TW), conservative water (CW), and several calcium chloride (CaCl2) solutions, and the σ´ included 27.3 kPa (4 psi) and 61.7 kPa (9 psi). The resulting measured ratios of final k for the HGN GCL relative to the LGN GCL (kf HGN/kf LGN) at ~24-27 kPa (4 psi) were ~28 (1.5 orders of magnitude), ~194 (2.3 orders of magnitude), and ~1975 (3.3 orders of magnitude) based on permeation with 5, 10, and 20 mM CaCl2, respectively. Thus, an increase in the fiber bundles density of the needle-punched fibers of the GCL adversely impacted the k of this GCL. Tests using dyed permeant liquids revealed that the high k was attributable to preferential flow along the fiber bundles of the GCL. Also, an increase in σ´ from 27.3 kPa (4 psi) to 61.7 kPa (9 psi) did not appreciably impact the measured k. Finally, permeation of the HGN GCL with the more dilute liquids (TW, CW, 1 and 2.5 mM CaCl2) resulted in consistently low k of ~2×10-11 m/s. The results of this study also illustrate the importance of achieving not only hydraulic equilibrium but also chemical equilibrium before terminating the k tests, as the measured k at hydraulic equilibrium based on the higher ionic strength solutions (i.e., 5, 10, and 20 mM CaCl2) typically were lower and, therefore, more unconservative than the measured k at chemical equilibrium. This study also evaluated the use of different methods to measure the k of the HGN GCL, including the falling headwater, constant tailwater method and the constant rate-of-flow method using a flow pump with pressure transducers. The results indicated that neither method proved substantially effective at achieving chemical equilibrium faster, although employing higher hydraulic gradient (i) was shown to expedite the attainment of chemical equilibrium. This result is associated with flushing action in the intergranular pore spaces that effectively maintaining higher concentration gradient between within and outside the bentonite granules. Also, prehydrating the specimens with the permeant liquid tended to enhance the osmotic swell of the bentonite resulting in a lower k at lower pore volumes of flow. Finally, diffusion of solutes from interlayer to intergranular pore spaces within the GCL specimens permeated with CW was shown to be the rate-limiting mechanism for attaining chemical equilibrium.Item Open Access Standardized hydraulic conductivity testing of compacted sand-bentonite mixtures(Colorado State University. Libraries, 2015) Tong, Shan, author; Shackelford, Charles D., advisor; Bareither, Christopher A., committee member; Borch, Thomas, committee memberTo view the abstract, please see the full text of the document.Item Open Access Time and scale effects in laboratory permeability testing of compacted clay soil(Colorado State University. Libraries, 1989) Javed, Farhat, author; Shackelford, Charles D., advisor; Jameson, Donald A., advisor; Doehring, Donald O., committee member; Abt, Steven R., committee memberPermeability (hydraulic conductivity) testing of clays in the laboratory typically requires a significant amount of time. It is hypothesized that the time required for clay permeability test can be reduced substantially through a statistical modelling technique known as "time series analysis". In order to test this hypothesis, permeability tests were performed on compacted samples of a silty clay soil in a standard Proctor mold (9.4 x 10-4 m3). The soil was separated into five different fractions representing five ranges in precompaction clod sizes. Constant-head permeability tests were performed on each of these five fractions. Tests were replicated five times for the time series analysis. The results of analysis indicate that time series modelling can significantly reduce statistical error associated with permeability data. It is demonstrated that the time required for clay permeability test can be reduced appreciably through time series modelling. Permeability tests also were performed on four soil fractions in a large-scale (0.914 m x 0.914 m x 0.457 m) double-ring, rigid-wall permeameter. The results of small-scale (Proctor mold) permeability tests indicate that the soil permeability does not vary much with a change in the precompaction clod size. Presence of large clods (> 25 mm), however, may result in side-wall leakage. The large-scale tests indicated that permeability is strongly related to the precompaction clod sizes. Permeability of the soil increased more than two orders-of-magnitude as the maximum precompaction clod size increased from 4.75 mm to 75 mm. Comparison of the results from the small-scale and the large-scale tests indicated that, for all soil fractions, the large-scale permeability was higher by more than an order-of-magnitude. As a result, there appears to be a scale-effect associated with laboratory permeability testing. This scale effect is more significant when soil contains considerable quantity of clods that are large relative to the size of permeameter. These results imply that the large-scale test is more capable of accounting for the hydraulic defects resulting from large clods. A more realistic evaluation of the field permeability of a compacted clay, therefore, may be possible in the laboratory if the permeameter is fairly large relative to the maximum precompaction size of clods present under field conditions.Item Open Access Zeolite-amended backfills for enhanced metals containment via soil-bentonite vertical cutoff walls(Colorado State University. Libraries, 2016) Hong, Catherine SooJung, author; Shackelford, Charles D., advisor; Bareither, Christopher A., committee member; Malusis, Mike A., committee member; Sale, Thomas C., committee member; Stromberger, Mary E., committee memberLow hydraulic conductivity (k), soil-bentonite (SB) vertical cutoff walls are commonly used to contain contaminated groundwater in geoenvironmental applications. The low k of the SB cutoff walls is attributed, in part, to the high swelling property of the bentonite component of the backfill. In addition, the high cation exchange capacity (CEC) of the bentonite, typically on the order of 80 to 150 cmolc/kg, imparts some intrinsic attenuation capacity to the backfill for cations (e.g., metals) via cation exchange. However, due to the low amounts of bentonite in typical SB cutoff walls (i.e., < 10 % by dry weight), this attenuation capacity is limited in traditional SB cutoff walls. Therefore, consideration has been given to amending SB backfills with zeolites to enhance the attenuation or adsorption capacity. Zeolites are naturally occurring aluminosilicates with high CEC (180 to 400 cmolc/kg) and a cage-like structure that allow the zeolites to perform as a molecular sieve and as adsorbents for ammonium, heavy metals, cations, and radioactive wastewater. In this study, three types of zeolites (two types of chabazite and a clinoptilolite) were used as amendments for SB backfills to enhance the adsorption capacity with respect to two metals, viz., potassium (K) and zinc (Zn). The results of measurements of the slump, consolidation behavior, and k of the unamended and zeolite-amended SB backfills with ≤ 10 % zeolite (by dry weight) confirmed that the zeolite-amended SB backfills exhibited similar physical properties compared to those for the unamended SB backfill, including the low k (≤ 1.0×10-9 m/s) typically required for SB vertical cutoff walls. The results of batch equilibrium adsorption tests (BEATs) indicated that the added zeolite increased the adsorption capacity of the SB backfill, but the effectiveness differed for different types of zeolite and the different metals (i.e., K and Zn). The results of numerical simulations for transport of K and Zn through a hypothetical 1-m thick model cutoff wall based on the results of the BEATs indicated that the barrier containment durations increased relative to that for the unamended SB backfill by as much as 108 yr and 228 yr for backfills with 5 and 10 % zeolite amendment, respectively. Finally, the results of long-term column tests (1.05 to 3.75 yr) indicated that the retardation factor (Rd) for K with the 5 % zeolite-amended SB backfills was 2.4 to 3.2 times greater than that for the unamended SB backfill, whereas Rd for Zn was 1.4 to 2.2 times greater than that for the unamended SB backfill. Based on the results of this study, the addition of small amounts of zeolite (≤ 10 % by dry weight) to traditional SB backfills can significantly enhance the adsorption capacity of the SB backfills for metals, thereby enhancing the containment performance of vertical cutoff walls comprising zeolite-amended SB backfills. However, the magnitude of any enhanced containment is dependent on both the adsorption capacity and the adsorption behavior of the specific metal with the specific backfill, and will be dependent on both the type and amount of the added zeolite.