Vertical Cutoff Walls Research Articles

Hydraulic conductivity and microscopic properties of sodium hexametaphosphate-amended sand-sodium-activated calcium bentonite backfill in vertical cutoff walls exposed to lead-contaminated groundwater

Hydraulic conductivity and microscopic properties of sodium hexametaphosphate-amended sand-sodium-activated calcium bentonite backfill in vertical cutoff walls exposed to lead-contaminated groundwater

Study on the carbon sequestration performance and barrier mechanism of biochar cement-based vertical cutoff walls for phenol pollution in groundwater

Study on the carbon sequestration performance and barrier mechanism of biochar cement-based vertical cutoff walls for phenol pollution in groundwater

Adsorption characteristics and mechanism analysis of heavy metal Zn2+by cement-soil and alkali activated slag-bentonite-soil

With the increasing emphasis on low-carbon environmental protection, alkali activated cementitious materials are gradually being used as a substitute for cement in vertical cutoff walls for polluted sites. The composition of barrier materials significantly impacts the adsorption of heavy metals, thereby affecting the service life of cutoff walls. However, the adsorption characteristics and mechanisms of traditional cement-soil (CS) and alkali activated slag-soil (AASS) for heavy metals are still unclear. This study investigates the adsorption characteristics of Zn2+ using different mix ratios of barrier materials. Various mix proportions were designed, and qualitative and quantitative analyses were conducted using X-ray diffraction and thermogravimetric tests, respectively. The research results indicate that the adsorption of Zn2+ by barrier materials prepared from CS and slag-bentonite-soil (AASBS) is mainly chemical adsorption. The adsorption rate of heavy metals can be enhanced by increasing the cement content in the CS mix ratio. This is primarily because an increase in OH- in the hydration products encourages the formation of heavy metal precipitation. The generation of C-S-H and ettringite in the hydration products and ion exchange of heavy metals improve the adsorption of heavy metals. Increasing slag and alkali activator dosage, along with decreasing alkali activator modulus, promotes C-A-S-H generation and increases OH-, enhancing heavy metal precipitation. Increasing the content of bentonite in the AASBS mixture promotes the generation of C-A-S-H, and the interlayer structure of bentonite promotes the ion exchange of heavy metals. The findings of the study may offer a theoretical foundation for the installation of AASBS vertical cutoff walls in contaminated areas.

Centrifuge Modeling of Backfill Consolidation in Soil-bentonite Cutoff Walls

Soil-bentonite (SB) is commonly adopted for the backfill of vertical cutoff walls to contain contaminated groundwater. The permeability of SB backfill is dependent on its effective stress state, which is affected by the backfill consolidation process. This paper presents a geotechnical centrifuge model test to simulate the primary consolidation of typical SB cutoff walls with the advantages of reduced scale in time and size. The moisture content and earth pressure of the wall were monitored at varied depths. Test results showed that the rapid consolidation stage developed swiftly and various consolidation indicators exhibited significant development. Within the buried depth over the half (approximately two-thirds), the distribution of vertical effective stress was similar to the Bucknell field testing results, namely that it increased nonlinearly and then tended to stabilize. Also, the cumulative effect of lateral frictional resistance increased with depth and reached its peak value. In the deeper buried areas, lateral frictional resistance decreased sharply before increasing again with increasing depth, while vertical effective stress followed a pattern of significant increase followed by a decrease. This centrifuge test may be useful for achieving a comprehensive evaluation of the consolidation behaviour and mechanism of SB cutoff walls.

Sand-bentonite backfill amended with composite polymer exposed to organic acid: Hydraulic performance and microstructural properties

Soil-bentonite (SB) backfills in vertical cutoff walls are used extensively to contain contaminated groundwater. Previous studies show that the hydraulic conductivity of backfill can exceed the typically recommended maximum value (k = 1 × 10−9 m/s) if exposed to groundwater impacted by organic acids commonly released from uncontrolled landfills and municipal solid waste dumps. Polymer amended backfills exhibit excellent chemical compatibility to metal-laden groundwater. However, few studies to date have explored the effect of organic acid contaminated groundwater on hydraulic performance of polymer amended backfills. This study presents an experimental investigation on the hydraulic performance and microstructural properties of a composite polymer amended backfill used to contain flow of acetic acid-laden groundwater. A series of laboratory experiments were performed to evaluate free-swell indices of the composite polymer amended bentonites, liquid limits of the composite polymer amended and unamended bentonites, and slump heights and hydraulic conductivity (k) values of the amended backfills to acetic acid solutions with varying concentrations. The results were compared with those of the unamended bentonites and unamended backfills reported in a previous study. The results showed that the free-swell index and liquid limit of the amended bentonites were higher than those of the unamended bentonites. Permeation with acetic acid solutions with concentrations ranging from 40 mmol/L to 320 mmol/L conducted on the amended backfill only resulted in an increase in k of less than a factor of about 10 related to that based on permeation with tap water (1.68 × 10−10 -4.41 × 10−11 m/s to acetic acid solution versus 1.65 × 10−11 m/s to tap water). Mechanisms contributing to enhanced chemical compatibility of amended backfill were ascertained based on scanning electron microscopy, mercury intrusion porosimetry, and zeta potential analyses.

Numerical Simulation of the Solute Migration Regulated by Vertical Cutoff Wall based on FEFLOW

Cutoff wall has been widely used in desalination of coastal aquifers in many coastal regions and remediation of industrial waste water pollution. In recent years, the excessive exploitation of resources in the coastal brine distribution area leads to the salinization of deep soil. At the same time, the mine water pollution caused by improper discharge of industrial waste water into the abandoned mining area is a new form of pollution in Shandong, China. However, due to the complex geological conditions of abandoned mining areas, loose collapse of overlying strata in roadway and gob layer, it is difficult to prevent and control groundwater pollution in deep gob layer. It is a major difficulty in practical engineering to evaluate the cutoff interception performance of vertical cutoff wall on the pollution of gob layer in such mining areas and optimize cutoff interception measures. In this paper, a typical polluted abandoned mining area in Shandong, China is taken as an example, and COD is used as the main simulation factor for numerical simulation. The numerical simulation results show that in the process of groundwater pollution control, the cutoff wall with appropriate thickness and hydraulic conductivity can effectively block the diffusion of solute. The horizontal transport of solute can be significantly weakened when the thickness of the cutoff wall is 90 cm and the hydraulic conductivity is 1.0×10-4 m/d. Compared with increasing the thickness, reducing the hydraulic conductivity of the cutoff wall is more effective in controlling the migration of solute in the site. When the hydraulic conductivity is reduced by an order of magnitude, the diffusion range of solute is reduced by 19.74% compared to before. Therefore, priority should be given to reducing the hydraulic conductivity of the cutoff wall in order to optimize its influence on pollutant migration.

Study on hydraulic conductivity of dredged sediment-attapulgite vertical cutoff walls under confining pressure

Landfills require effective containment systems to prevent the leakage of leachate into the underground environment. Cutoff walls are commonly employed for this purpose, with options including rigid and flexible designs. In areas where structural strength is not a primary concern, flexible cutoff walls offer lower permeability and environmental benefits due to their lack of cement content, thereby reducing CO2 emissions. This study investigates the use of dredged sediment and attapulgite as materials for flexible cutoff walls. Through analyses of bound water content, free water content, hydraulic conductivity, and scanning electron microscopy, we explore the impact of confining pressures on cutoff wall permeability.Our findings reveal that the consolidation induced by confining pressure does not significantly alter the bound water content within the cutoff wall. Instead, changes in water content are predominantly attributed to variations in free water content. Under identical confining pressures, we observe a positive correlation between permeability and hydraulic gradient, with permeability increasing as the hydraulic gradient rises, and anti-permeability decreasing accordingly. Additionally, when holding the hydraulic gradient constant, increasing confining pressure leads to a continuous decrease in permeability. Microscopic analyses highlight that high confining pressure not only compresses pore diameter but also alters pore morphology, thereby influencing permeability.This study contributes to the understanding of cutoff wall behavior under different conditions. Our results demonstrate that increasing confining pressure during soil consolidation effectively reduces cutoff wall permeability to meet design standards. However, the influence of high leachate head on permeability should also be considered. These findings provide a more environmentally friendly and lower permeability option for landfill sites, which is significant for the design and enhancement of containment systems in landfill sites.

Engineering properties of microcapsules self-healing geopolymer cutoff wall backfill under dry-wet cycles

Vertical cutoff wall has been employed for decades to control groundwater flow and subsurface contaminant transport. Environmental stress fluctuations induced by alternating dry and wet conditions impair the permeability and durability of the cutoff wall, leading to the dispersion of contaminants. The objective of this study is to incorporate self-healing microcapsules into existing geopolymer cutoff wall backfill (GCWB) to form self-healing geopolymer cutoff wall backfill (SHGCWB) that hold promise for better durability and performance. The in-situ polymerization method was used to develop single-walled and double-walled microcapsules. The microcapsules use sodium silicate as the healing agent encapsulated in single-walled polyurethane (PU) and double-walled polyurethane/melamine-formaldehyde (PU/MF) microcapsules. The effect of microcapsules on the unconfined compressive strength (UCS) and hydraulic conductivity of SHGCWB were elaborated. The durability and hydraulic conductivity variation of SHGCWB in dry-wet cycle was thoroughly investigated by Scanning Electron Microscopy (SEM) test for understanding the self-healing mechanism. The overall performance demonstrated the significant potential of the use microcapsules as a self-healing approach for cutoff walls.

Durability of geopolymer cutoff wall backfill incorporating reactive MgO particles under dry-wet cycles

Vertical cutoff wall has been employed for decades to control groundwater flow and subsurface contaminant transport. The barrier performance of cutoff wall material may significantly deteriorate under dry-wet cycles due to precipitation, drought, and groundwater fluctuation. This study adopted active magnesium oxide (MgO) particles to enhance the durability of geopolymer cutoff wall backfill (GCWB) under dry-wet cycles. The influence of particle size and proportion of MgO on the unconfined compressive strength (UCS), hydraulic conductivity, self-healing capacity and durability of GCWB were comprehensively examined. The results indicate that the incorporation of active MgO particles hindered the geopolymerization process of GCWB to a certain extent, leading to an increase in hydraulic conductivity and a decrease in UCS. But the overall performance of GCWB was still favorable. The hydraulic conductivity of samples with MgO gradually increased to more than 1 × 10−8 m/s after at least two dry-wet cycles. In the subsequent cycles, the samples with MgO exhibited a pattern of first gradually declining then ascending in hydraulic conductivity. This is due to hydration and carbonization of the remained MgO in matrix during the dry-wet cycles, which converts into oxygen-rich carbonates, forming well-ramified networks and reducing the proportion of macropores. Thus, the durability of GCWB with MgO is significantly more favorable than that devoid of MgO. In particular, large particle size MgO can exert more persistent self-healing effect, endowing the samples with superior durability under the prolonged dry-wet cycles.

Study on the hydraulic conductivity of plastic concrete vertical cutoff walls

With the rapid development of global economy and the accelerating process of urbanization, the prevention and control of groundwater pollution has received great challenges. In the remediation of contaminated sites, the traditional method is to use isolation barriers for in-situ remediation. However, due to the influence of various factors, the permeability of barriers will be changed to different degrees. This paper takes plastic concrete as the research object, through the permeability, water content and SEM test, discusses the water-binder ratio, no pollution and the presence of Ba2+ conditions on the permeability. The results show that in the case of no pollution, the decrease in the water-binder ratio and the increase of bentonite content will decrease the permeability, which decreases with the increase of age and satisfies the exponential relationship. Both Ba2+ and pH had adverse effects on the permeability. Ba2+ inhibits the formation of hydration products and the bonding between aggregate and bonding of cementitious materials, resulting in the decrease of compactness and the increase of porosity, which adversely affects the permeability. The addition of Ba2+ and the increase of pH will have adverse effects on the permeability, and these influencing factors need to be considered in the actual project.

Containment of phenol-impacted groundwater by vertical cutoff wall with backfill consisting of sand and bentonite modified with hydrophobic and hydrophilic polymers

A novel soil-bentonite backfill is proposed for use in vertical cutoff walls to contain phenol in groundwater at contaminated sites. The backfill consists of sand and bentonite modified with tetramethylammonium and carboxymethylcellulose, labeled as STCMB backfill. Flexible-wall permeability and double-reservoir diffusion tests were conducted to investigate the impact of phenol solution on hydraulic conductivity (k), effective diffusion coefficient (D*) and partition coefficient (Kp) of the backfill, respectively. The permeability results showed k of the STCMB backfill decreased by 0.91 times when the permeating liquid was changed from tap water to phenol solution. The diffusion testing results showed that D* values for the STCMB and conventional backfill (labeled as SCB backfill) were 4.0 × 10−10 m2/s and 3.0 × 10−10 m2/s, respectively, whereas Kp values for the STCMB and SCB backfills were 2.0 mL/g and 0.75 mL/g, respectively. The octanol-water partition coefficient model is suitable for estimating Kp for nonpolar organics. Furthermore, a series of solute transport simulations using Pollute V7 program was performed to evaluate the performance of vertical cutoff walls comprising STCMB and SCB backfills in containing phenol in lateral flowing groundwater. Overall, the STCMB backfill has demonstrated superior effectiveness in containing phenol in groundwater.

Material characteristics and pollutant diffusion simulation of cutoff walls made of alkali activated slag/bentonite

To prevent heavy metal pollution, vertical cut-off walls made of cement soil are often used as barriers. This article developed a cementitious material with a “low-carbon footprint” (alkali activated slag) to replace cement. The performance of alkali activated slag soil and cement soil was comprehensively explored through unconfined compressive strength, hydraulic conductivity, Zn2+ adsorption, and chloride ion diffusion tests. And the micro pore structure and micro morphology characteristics of the cutoff wall were explored through nuclear magnetic resonance testing and scanning electron microscopy testing. Finally, considering the normal distribution of pollutants, the diffusion process of pollutants in the transition layer, cut-off wall and aquifer is simulated by using the COMSOL Multiphysics 5.6. Research has found that alkali activated slag soil meets the requirement of low permeability in the early stage of maintenance, making up for the shortcomings of cement soil maintenance in the early stage. After adding bentonite to alkali activated slag soil, it was found that the hydraulic conductivity and effective diffusion coefficient showed a decreasing trend, while the adsorption rate of Zn2+ was significantly increased. Due to the double layered structure of bentonite and the hinge of C-A-S-H generated by alkali activated slag, it promotes an increase in the proportion of small pores and the generation of free water with poor fluidity, thereby improving the blocking effect of the cut-off wall on pollutants. Through simulation results, it was found that the breakthrough time of alkali activated slag soil as a cutoff wall is 3.07 times that of cement soil. After adding 6% and 12% bentonite to the alkali activated slag soil, the breakthrough time of the cut-off wall increased by 1.74 times and 4.25 times, respectively.

Engineering Properties of Novel Vertical Cutoff Wall Backfills Composed of Alkali-Activated Slag, Polymer-Amended Bentonite and Sand.

The workability, hydraulic conductivity, and mechanical properties are essential to contaminant containment performance of cementitious backfills in vertical cutoff walls at contaminated sites. This study aims to investigate the engineering properties of a novel vertical cutoff wall backfill composed of reactive magnesia (MgO)-activated ground granulated blast furnace slag (GGBS), sodium-activated calcium bentonite amended with polyacrylamide cellulose (PAC), and clean sand (referred to as MSBS-PAC). Backfills composed of MgO-activated GGBS, sodium-activated calcium bentonite, and clean sand (referred to as MSBS) were also tested for comparison purposes. A series of tests were conducted which included slump test, flexible-wall hydraulic conductivity test, and unconfined compression test. The pore size distributions of two types of backfills were investigated via the nuclear magnetic resonance (NMR) technique. The results showed the moisture content corresponding to the target slump height was higher for MSBS-PAC backfill than that for MSBS backfill. The MSBS-PAC backfill possessed lower pH, dry density, and higher void ratio at different standard curing times as compared to MSBS backfill. The unconfined compressive strength and strain at failure of the MSBS-PAC backfill were noticeable lower than those of the MSBS backfill. In contrast, the hydraulic conductivity of MSBS-PAC backfill was approximately one order of magnitude lower than that of the MSBS backfill, which was less than 10-9 m/s after 28-day and 90-day curing. Lower hydraulic conductivity of MSBS-PAC backfill was attributed to the improvement of pore structure and pore fluid environment by PAC amendment.

Transport of Organic Contaminants in Composite Vertical Cut-Off Wall with Defective HDPE Geomembrane.

Soil-bentonite vertical cut-off wall is an emergency technique used for contaminant control in geo-environmental engineering, high-density polyethylene (HDPE) geomembrane (GM) with an extremely low-permeability coefficient is expected to enhance the contaminant barrier effect of the vertical cut-off wall. To evaluate the barrier performance of the composite barrier composed of GM and soil-bentonite mixture towards organic contaminant, while also quantitively revealing the impact of GM defects and placement, a one-dimensional transport model for organic contaminants in composite barrier is solved under semi-infinite boundary conditions. The proposed transport model is validated by numerical simulations using COMSOL Multiphysics 5.4, and the effects of GM defect rate, placement within the composite isolation wall, and contact level with soil-bentonite on contaminant transport behavior are further studied. The results show that as the average frequency of GM defects increases from 2.5 to 50 holes per hectare, the breakthrough time of organic contaminants through composite barrier decreases by almost 70%. Poor contact level between GM and soil-bentonite mixture may reduce the breakthrough time of the composite cut-off wall by 65%. Although the selection of GM placement has limited impact on the transient flux of contaminants, it does affect the total flux of contaminants over a certain period of time. The effects of permeability coefficient, effective diffusion coefficient, distribution coefficient, and hydraulic head of the composite cut-off wall can be considered by the proposed analytical solution, which would provide guidance and reference for the design and service performance evaluation of the composite cut-off wall.

Using alkali-activated dredge sludge ash-bentonite for low permeability cutoff walls: The engineering properties and micromechanism

Although vertical cutoff walls can effectively contain groundwater pollution, vertical barriers constructed using conventional cement-bentonite-soil (CBS) mixtures exhibit poor permeability and produce considerable CO2 during cement production. The dredged sludge ash (DSA) has potential to be used as an eco-friendly source for vertical barriers building because of the remarkable pozzolonicity. In this study, a mixture consisting of MgO-activated dredged sludge ash (DSA) and bentonite was used to develop a low permeable MgO-activated DSA-bentonite-sand (MDBS) barrier material for application to vertical cutoff walls. The mechanical properties of MDBS and CBS mixtures were investigated and the hydration products and microstructure were examined using scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), and mercury intrusion porosimetry (MIP). The results showed that the MDBS mixture exhibited excellent impermeability: the hydraulic conductivity of MDBS cured for 60 d reached the minimum (1.83 × 10−8 cm/s), and considerably superior to the common requirement (lower than 10−7 cm/s) for cutoff walls. The unconfined compressive strength of the MDBS cured for 60 d increased by approximately 112% compared to the samples cured for only 7 d. The hydration products in MDBS were mainly magnesium silicate hydrate and hydrotalcite, which filled the material pores more effectively than the hydration products of CBS (calcium silicate hydrate calcium hydroxide and ettringite). The hydration products, bentonite swelling, and pore size distribution determined the MDBS mechanical properties and permeability. It is verified that activated dredge sludge ash can be used as an ideal cutoff walls material instead of cement.

Experimental study on barrier performance and durability under dry-wet cycles of fly ash based geopolymer cutoff wall backfill

Vertical cutoff wall is one of the most effective technologies to restrict the migration of contaminants. In this study, a geopolymer cutoff wall backfill (GCWB) consisting of reactive sodium silicate, fly ash, sand, and water was developed, which does not need Ordinary Portland Cement (OPC) and bentonite. The basic macroscopic properties of GCWB were tested, in terms of workability, unconfined compressive strength, hydraulic conductivity, and chemical compatibility, then the optimal material ratio was determined. In order to evaluate the barrier performance of GCWB under dry-wet cycles, the hydration products and microstructural characteristics were measured by X-ray Diffraction (XRD), Mercury Intrusion Porosimetry (MIP), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). According to the orthogonal analysis, the optimal material ratio with the highest unconfined compressive strength was 30FA1.2M12S and that with the lowest hydraulic conductivity was 40FA1.2M12S. The unconfined compressive strength and hydraulic conductivity of GCWB can achieve 6.62 MPa and 4.83 × 10−11 m/s after cured for 28 days, respectively. The chemical compatibility of GCWB was overall favorable in Na2SO4 and CaCl2 solutions. Under the effect of dry-wet cycles, the variation of hydraulic conductivity, mass and volume in contaminant solutions were basically the same as that in water. The hydraulic conductivity increased sharply by two orders of magnitude after the 1st dry-wet cycle and tended to be stable (slightly larger than 1 × 10−8 m/s) in the subsequent dry-wet cycles. Therefore, the barrier performance and durability of GCWB was desirable in water and contaminant solutions, indicating that it is a promising antifouling cutoff wall material.

Swelling Behavior and Flow Rates of a Novel Hydrophilic Gasket Used in Composite Geomembrane Vertical Cutoff Walls and Infrastructures Exposed to Contaminated Groundwater

The swelling capacity of novel hydrophilic gaskets used in geomembrane cutoff walls and infrastructures is critical for decreasing the flow rates of contaminated groundwater. This study investigated the swelling behavior, relaxation characteristics, flow rates, and micro-morphology of a hydrophilic gasket with different testing liquids. The radial swelling tests were performed using a device modified from single-lever consolidation instrument. A flow rates model apparatus was manufactured and employed to measure the flow rates of the poor-sealing hydrophilic gasket. According to the test results, the swelling ratio of the hydrophilic gaskets soaked in the DIW were the highest, followed by the NaCl solution, the MSW landfill leachate, and the CaCl2 solution. Relaxation phenomena appeared in all the specimens regardless of the testing liquids. The flow rates of the specimens penetrated with DIW, NaCl, and CaCl2 decreased to a stable state, and then increased extremely slowly to stable values. Moreover, self-healing of the hydrophilic gasket was observed. The micro-morphology indicated that sodium polyacrylate (PAAS) with insufficient expansion could separate from the matrix under high multivalent ionic strength or loading pressure conditions. Therefore, it is critical to develop the modified hydrophilic gasket with resistance to contaminated groundwater for a better barrier performance for use in contaminated sites and infrastructures.

Analytical solution for transport of degradable contaminant in cut-off wall and aquifer

An analytical solution for contaminant transport through a vertical cut-off wall and aquifer system is developed, which can consider the advection, dispersion and degradation processes. The uniqueness of the solution is that it includes hyperbolic eigenfunctions, considers the degradation process and is suitable for an aquifer with finite thickness due to the existence of a river or lake. The solution is first verified against an existing analytical solution and a numerical model using the Comsol software program. The three important aspects are then investigated and some important conclusions are obtained. In some scenarios, the hyperbolic eigenfunctions must be considered; otherwise, the obtained results are unreasonable. When the aquifer thickness is more than a critical value (approximately ten times the cut-off wall thickness in this study), it is reasonable to regard the aquifer as semi-infinite. Contaminant degradation has significant influence on contaminant transport. A higher degradation intensity leads to a lower contaminant concentration but a larger mass flux in the cut-off wall. Therefore, the present solution is suitable for an aquifer with arbitrary thickness and has better applicability than the existing analytical solutions. It is promising to be used as an applicable tool for the design of vertical slurry cut-off walls.

Pollution characteristics and vertical cutoff wall optimization at an industrial contaminated site in China

Vertical cutoff walls have been widely used in the remediation of contaminated sites. However, determining the best method for evaluating the long-term barrier performance of vertical cutoff walls presents a major difficulty in actual projects. Here, a case study is presented for a typical electroplating, medical, and chemical industrial park in China. Based on the analysis of groundwater pollution characteristics at the site, pollutants included metals (Ni, Al), ammonia nitrogen, and 1,2-dichloroethane. Finite element model simulations of Ni transport at the site showed that a vertical cutoff wall with a thickness of 60 cm and a hydraulic conductivity of 1.0 × 10−8 cm/s could significantly attenuate pollutant transport in the horizontal direction. Compared with other methods such as reducing the hydraulic conductivity or increasing the adsorption retardation factor of the vertical cutoff wall, increasing the thickness was more effective in controlling pollutant transport at the study site. Doubling the thickness would cause the Ni leakage concentration to decrease by more than 98% and the breakthrough time to increase by more than 47 years. It is recommended that the thickness of cutoff walls be maximized to optimize their effects on pollutant transport.

Hydraulic Conductivity of Sand/Biopolymer-Amended Bentonite Backfills in Vertical Cutoff Walls Permeated with Lead Solutions

Hydraulic Conductivity of Sand/Biopolymer-Amended Bentonite Backfills in Vertical Cutoff Walls Permeated with Lead Solutions