Michael A. Malusis

Flow and transport through clay membrane barriers

Flux equations for liquid and solute migration through clay barriers that behave as semi-permeable membranes used in waste containment and remediation applications, known as clay membrane barriers (CMBs), are discussed. The results of a simplified analysis of flow through a geosynthetic clay liner (GCL) using measured values for the chemico-osmotic efficiency coefficient (ω) of the GCL indicate a total liquid flux that counters the outward Darcy (hydraulic) flux due to chemico-osmosis associated with clay membrane behavior of the GCL. Also, the solute (contaminant) flux through the GCL is reduced relative to the solute flux that would occur in the absence of membrane behavior due to chemico-osmotic counter advection and solute restriction. Since diffusion commonly controls solute transport through GCLs and other low-permeability clay barriers, the implicit (empirical) correlation between ω and the effective salt-diffusion coefficient of the migrating contaminant is an important consideration with respect to contaminant restriction in CMBs.

A laboratory apparatus to measure chemico-osmotic efficiency coefficients for clay soils

A laboratory apparatus for measuring the chemico-osmotic efficiency coefficient, ω, for clay soils in the presence of electrolyte solutions is described. A chemico-osmotic experiment is conducted by establishing and maintaining a constant difference in electrolyte concentration across a soil specimen while preventing the flow of solution through the specimen. The chemico-osmotic efficiency coefficient is derived from a measured pressure difference induced across the specimen in response to the applied concentration difference. The effective diffusion coefficient (D*) and retardation factor (Rd) of the electrolytes (solutes) also can be determined simultaneously by measuring the diffusive solute mass flux through the specimen until steady-state diffusion is achieved. Experimental results using specimens of a geosynthetic clay liner subjected to potassium chloride solutions indicate that the measurement of ω may be affected by soil-solution interactions, as well as by changes in the induced chemico-osmotic pressure difference due to solute diffusion. As a result, ω should be evaluated using the induced pressure difference at steady state. The time required to achieve a steady-state response in induced pressure difference is related to the time required to achieve steady-state diffusion of all solutes, and may be affected by the circulation rate at the specimen boundaries. The circulation rate should be sufficiently rapid to minimize changes in the boundary concentrations due to diffusion, but sufficiently slow to allow measurement of solute mass flux at the lower concentration boundary for evaluating D* and Rd.

Hydraulic Conductivity and Compressibility of Soil-Bentonite Backfill Amended with Activated Carbon

Flexible-wall permeability tests and rigid-wall consolidation/permeability tests were performed to evaluate the hydraulic conductivity and compressibility of a model soil-bentonite (SB) backfill amended with granular activated carbon (GAC) or powdered activated carbon (PAC). The tests were performed as part of an assessment of enhanced SB backfill with improved attenuation capacity for greater longevity of barrier containment performance. Backfill specimens containing fine sand, 5.8% sodium bentonite, and GAC or PAC (0, 2, 5, and 10% by dry weight) were prepared to target slumps of 125±12.5 mm . Hydraulic conductivity (k) and compressibility of backfill test specimens were measured in consolidometers as a function of effective stress, σ′ (24⩽ σ′ ⩽1,532 kPa) , whereas flexible-wall k was measured for backfill specimens consolidated to σ′ =34.5 kPa . The results indicate that addition of GAC has little impact on the hydraulic and consolidation properties of the backfill, whereas addition of PAC causes a dec...

Consolidation and Hydraulic Conductivity of Zeolite-Amended Soil-Bentonite Backfills

The effect of zeolite amendment for enhanced sorption capacity on the consolidation behavior and hydraulic conductivity, k ,o f a representative soil-bentonite (SB) backfill for vertical cutoff walls was evaluated via laboratory testing. The consolidation behavior and k of test specimens containing fine sand, 5.8% (dry weight) sodium bentonite, and 0, 2, 5, or 10% (dry weight) of one of three types of zeolite (clinoptilolite, chabazite-lower bed, or chabazite-upper bed) were measured using fixed-ring oedometers, and k also was measured on sep- arate specimens using a flexible-wall permeameter. The results indicated that addition of a zeolite had little impact on either the consolidation behavior or the k of the backfill, regardless of the amount or type of zeolite. For example, the compression index, Cc, for the unamended backfill specimen was 0.24, whereas values of Cc for the zeolite-amended specimens were in the range 0:19 ≤ Cc ≤ 0:23. Similarly, the k for the unamended specimen based on flexible-wall tests was 2: 4×1 0 � 10 m=s, whereas values of k for zeolite-amended specimens were in the range 1: 2×1 0 � 10 ≤ k ≤ 3: 9×1 0 � 10 m=s. The results of the study suggest that enhancing the sorption capacity of typical SB backfills via zeolite amendment is not likely to have a significant effect on the consolidation behavior or k of the backfill, provided that the amount of zeolite added is small (≤ 10%). DOI: 10.1061/(ASCE)GT.1943-5606.0000566.

Influence of adsorption on phenol transport through soil-bentonite vertical barriers amended with activated carbon.

The potential for enhanced containment of phenol by soil-bentonite (SB) vertical barriers amended with activated carbon (AC) was investigated. Results of batch equilibrium adsorption tests on model SB backfills amended with 0-10 wt.% granular AC (GAC) or powdered AC (PAC) illustrate that the backfills exhibited nonlinear adsorption behavior that was described well by both the Freundlich and Tóth adsorption models. The AC amended backfills exhibited enhanced phenol adsorption relative to unamended backfill due to hydrophobic partitioning to the AC. Adsorption capacity increased with increasing AC content but was insensitive to AC type (GAC versus PAC). Results of numerical transport simulations based on the measured adsorption behavior show that the Tóth model yielded similar or lower phenol breakthrough times than the Freundlich model for the range of source concentrations (C(o)) considered in the simulations (0.1-10 mg/L). Breakthrough time decreased with increasing C(o) but increased with increasing AC content. Predicted breakthrough times for an SB vertical barrier amended with 2-10 wt.% AC increased by several orders of magnitude relative to the theoretical case of a nonreactive (non-adsorbing) barrier. The findings suggest that AC may be a highly effective adsorption amendment for sustaining the containment performance of SB vertical barriers.

Critical review of coupled flux formulations for clay membranes based on nonequilibrium thermodynamics

Extensive research conducted over the past several decades has indicated that semipermeable membrane behavior (i.e., the ability of a porous medium to restrict the passage of solutes) may have a significant influence on solute migration through a wide variety of clay-rich soils, including both natural clay formations (aquitards, aquicludes) and engineered clay barriers (e.g., landfill liners and vertical cutoff walls). Restricted solute migration through clay membranes generally has been described using coupled flux formulations based on nonequilibrium (irreversible) thermodynamics. However, these formulations have differed depending on the assumptions inherent in the theoretical development, resulting in some confusion regarding the applicability of the formulations. Accordingly, a critical review of coupled flux formulations for liquid, current, and solutes through a semipermeable clay membrane under isothermal conditions is undertaken with the goals of explicitly resolving differences among the formulations and illustrating the significance of the differences from theoretical and practical perspectives. Formulations based on single-solute systems (i.e., uncharged solute), single-salt systems, and general systems containing multiple cations or anions are presented. Also, expressions relating the phenomenological coefficients in the coupled flux equations to relevant soil properties (e.g., hydraulic conductivity and effective diffusion coefficient) are summarized for each system. A major difference in the formulations is shown to exist depending on whether counter diffusion or salt diffusion is assumed. This difference between counter and salt diffusion is shown to affect the interpretation of values for the effective diffusion coefficient in a clay membrane based on previously published experimental data. Solute transport theories based on both counter and salt diffusion then are used to re-evaluate previously published column test data for the same clay membrane. The results indicate that, despite the theoretical inconsistency between the counter-diffusion assumption and the salt-diffusion conditions of the experiments, the predictive ability of solute transport theory based on the assumption of counter diffusion is not significantly different from that based on the assumption of salt diffusion, provided that the input parameters used in each theory are derived under the same assumption inherent in the theory. Nonetheless, salt-diffusion theory is fundamentally correct and, therefore, is more appropriate for problems involving salt diffusion in clay membranes. Finally, the fact that solute diffusion cannot occur in an ideal or perfect membrane is not explicitly captured in any of the theoretical expressions for total solute flux in clay membranes, but rather is generally accounted for via inclusion of an effective porosity, n(e), or a restrictive tortuosity factor, τ(r), in the formulation of Ficks first law for diffusion. Both n(e) and τ(r) have been correlated as a linear function of membrane efficiency. This linear correlation is supported theoretically by pore-scale modeling of solid-liquid interactions, but experimental support is limited. Additional data are needed to bolster the validity of the linear correlation for clay membranes.

Prediction of Earth Pressures in Soil-Bentonite Cutoff Walls

This paper presents a review of two models (i.e., arching and lateral squeezing) developed for predicting earth pressures in soil-bentonite (SB) cutoff walls. The assumptions of these existing models are discussed, a modified lateral squeezing (MLS) model is presented, and all three models are compared based on predicted horizontal stresses for representative field conditions. Each model predicts that the stress distribution within a SB cutoff wall may be considerably lower than a geostatic distribution, particularly at depth. The arching model yields the lowest stress distribution but may underestimate the true distribution due to the assumption of rigid trench sidewalls. The MLS model (1) allows sidewall deformation and (2) accounts for the stress-dependent nature of SB backfill compressibility. The study also finds that additional model development is needed to characterize the stress state of a SB cutoff wall in three dimensions.

A Miniature Cone for Measuring the Slump of Soil-Bentonite Cutoff Wall Backfill

Measurement of slump for soil-bentonite (SB) cutoff wall backfill using a standard ASTM C143-00 slump cone is widely employed to design SB backfill for workability and to evaluate field backfill quality during cutoff wall construction. The standard cone is particularly suitable for field testing, where large backfill quantities are available, but is less practical for laboratory testing of backfills during design. This paper describes the development and evaluation of a miniature slump cone that requires only ∼16 % of the backfill volume required for the standard cone. Results of slump tests performed on three model SB backfills with different solid compositions indicate that the miniature cone provides reproducibility similar to that given by the standard cone when the backfills are prepared to a standard slump of 100 mm to 200 mm. The empirical relationship between standard slump (SS) and miniature slump (SM) for all three model backfills is represented accurately by a single linear expression (i.e., SS=60+1.8SM, where SS and SM are in millimetres) that is nearly identical to the predicted correlation given by an analytical slump cone model (i.e., SS=64+1.8SM) for the range 100⩽SS⩽200 mm.

Hydraulic Conductivity of Sand-Bentonite Backfills Containing HYPER Clay

The hydraulic conductivity, k, of model sand-bentonite backfills containing HYPER clay (Na bentonite treated with carboxymethyl cellulose (CMC)) was investigated in this study. Flexible-wall tests were performed on backfill specimens composed of clean, fine sand and 2.7-5.6 % HYPER clay containing either 2 % CMC (HC2) or 8 % CMC (HC8). The geometric mean k to water (kw) for HC8 specimens decreased by nearly two orders of magnitude (from ~3x10 -9 m/s to ~3x10 - 11 m/s) with increasing HC8 content from 2.7 to 5.6 %. The geometric mean kw of 3x10 -11 m/s for the 5.6 % HC8 backfill was nearly an order of magnitude lower than kw reported previously for similar backfill specimens containing 5.7 % Na bentonite (Naturalgel ® (NG)) or 5.6 % multiswellable bentonite (MSB) due to the greater water absorption and swell capacity of HC8 relative to the NG and MSB. The 5.6 % HC2 specimens exhibited slightly lower kw relative to specimens containing 5.7 % NG or 5.6 % MSB. Also, whereas the NG and MSB specimens exhibited increases in k when the permeant liquid was changed to a 10-mM CaCl2 solution, no increases were observed for 5.6 % HC2. Although further testing is needed, the results illustrate the potential for HYPER clay to enhance the hydraulic performance of soil-bentonite vertical barriers.

Long Term In Situ Measurements of the Volumetric Water Content in a Soil Bentonite Slurry Trench Cutoff Wall

This paper presents the results of a field study to assess post-construction changes in the volumetric water content () of a soil-bentonite (SB) slurry trench cutoff wall. Time domain reflectometry (TDR) sensors were installed in a newly-constructed SB cutoff wall in the summer of 2008 and were used to monitor  as a function of depth within the SB backfill for approximately one year. The methods used to install the probes are described and the measured  distributions are presented and discussed. A general trend of decreasing  with time was observed at all sensor locations. Decreases in  within the portion of the wall below the water table are attributed to backfill consolidation, whereas the larger decreases in  near the top of the wall (above the water table) are likely due to a combination of backfill consolidation and backfill drying. A field sampling program is recommended to confirm the moisture profiles obtained from the TDR sensors.