Hydraulic Performance of Geosynthetic Clay Liners in a Landfill Final Cover

Percolation from a landfill final cover containing a geosynthetic clay liner (GCL) as the hydraulic barrier is described. The GCL was covered with 760?mm of vegetated silty sand and underlain with two gravel-filled lysimeters to monitor percolation from the base of the cover. Higher than anticipated percolation rates were recorded in both lysimeters within 4–15?months after installation of the GCL. The GCL was subsequently replaced with a GCL laminated with a polyethylene geofilm on one surface (a “composite” GCL). The composite GCL was installed in two ways, with the geofilm oriented upwards or downwards. Low percolation rates (2.6–4.1?mm/year) have been transmitted from the composite GCL for more than 5?years regardless of the orientation of the geofilm. Samples of the conventional GCL that were exhumed from the cover ultimately had hydraulic conductivities on the order of 5×10?5?cm/s. These high hydraulic conductivities apparently were caused by exchange of Ca and Mg for Na on the bentonite combined with dehydration. The overlying and underlying soils likely were the source of the Ca and Mg involved in the exchange. Column experiments and numerical modeling indicated that plant roots and hydraulic anomalies caused by the lysimeters were not responsible for the high hydraulic conductivity of the GCL. Despite reports by others, the findings of this study indicate that a surface layer 760?mm thick is unlikely to protect conventional GCLs from damage caused by cation exchange and dehydration. Accordingly, GCLs should be used in final covers with caution unless if cation exchange and dehydration can be prevented or another barrier layer is present (geomembrane or geofilm).     

Gas Transport Parameters for Compacted Reddish-Brown Soil in Sri Lankan Landfill Final Cover

Gas exchange through the compacted final cover soil at landfill sites plays a vital role for emission, fate, and transport of toxic landfill gases. This study involved measuring the soil-gas diffusivity (Dp/Do, the ratio of gas diffusion coefficients in soil and free air) and air permeability (ka) for differently compacted soil samples (reddish-brown soil) from the final cover at the Maharagama landfill in Sri Lanka. The samples were prepared by either standard Proctor compaction or hand compaction to dry bulk densities of 1.60–1.94??g?cm-3. Existing and modified models for predicting Dp/Do and ka were tested against the measured data. The simple, single-parameter Buckingham model predicted measured Dp/Do values across compaction levels equally well or better than a dry bulk density (DBD) dependent model and a soil-water retention (SWR) dependent model. The measured ka values for differently compacted samples were highly affected by the compaction level and the sample moisture preparation method. Also, for air permeability, a single-parameter Buckingham-type ka model was most accurate in predicting ka in the differently compacted soil samples. Equivalent air-filled pore diameters (the effective diameter of the drained pores active in leading air through the sample) for gas flow, deq, were calculated from the measured Dp/D0 and ka values. The deq increased with compaction level, suggesting that a very high compaction level creates well-connected macropores in the reduced total pore space of the cover soil. This is an important consideration when designing cover soils for optimally low water and high oxygen exchange while minimizing climate and toxic gas emissions from the waste layer to the atmosphere.     

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.     

Results from 18 Years of In Situ Performance Testing of Landfill Cover Systems in Germany

The water balances and the long-term performance of different landfill cover systems have been measured in situ in large-scale lysimeters on the landfill Hamburg-Georgswerder, Germany since 1988. The cover systems including different barrier components for water transport were constructed with state-of-the-art technology and have been excavated at several occasions especially to inspect the structure of the barriers. For the first time, the irreversible impact of crack formation in cohesive soil barriers and geosynthetic clay barriers due to desiccation, shrinkage, ion exchange, and plant root penetration has been observed and quantified in this study. After four years of good performance, these covers began to leak between 90 and 200 mm/year (average precipitation of 860 mm/year). The hydraulic conductivity of the cohesive soil barriers increased from 2×10?10 to 9×10?8?m/s, the daily peaks of the leakage through the geosynthetic clay barriers from initial values around 2×10?11?to?2×10?7?m3/(m2?s). The composite barriers with geomembranes above cohesive soil barriers performed very well, showing no leakage and only very little thermally induced water transport. A capillary barrier also performed well (average annual leakage of 18 mm/year). The data of the past 10 years prove that evapotranspiration can be increased significantly by planting bushes, which also limits the potential leakage through barrier layers.     

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.     

Hydraulic Conductivity of Geosynthetic Clay Liners Exhumed from Landfill Final Covers

Samples of geosynthetic clay liners (GCLs) from four landfill covers were tested for water content, swell index, hydraulic conductivity, and exchangeable cations. Exchange of Ca and Mg for Na occurred in all of the exhumed GCLs, and the bentonite had a swell index similar to that for Ca or Mg bentonite. Hydraulic conductivities of the GCLs varied over 5 orders of magnitude regardless of cover soil thickness or presence of a geomembrane. Hydraulic conductivity was strongly related to the water content at the time of sampling. Controlled desiccation and rehydration of exhumed GCLs that had low hydraulic conductivity (10?9?to?10?7?cm/s) resulted in increases in hydraulic conductivity of 1.5–4 orders of magnitude, even with overburden pressure simulating a 1-m-thick cover. Comparison of these data with other data from the United States and Europe indicates that exchange of Ca and/or Mg for Na is likely to occur in the field unless the overlying cover soil is sodic (sodium rich). The comparison also shows that hydraulic conductivities on the order of 10?6?to?10?4?cm/s should be expected if exchange occurs coincidently with dehydration, and the effects of dehydration are permanent once the water content of the GCL drops below approximately 100%. Evaluation of the field data also shows that covering a GCL with a soil layer 750–1,000?mm thick or with a geomembrane overlain by soil does not ensure protection against ion exchange or large increases in hydraulic conductivity.     

Extreme Compaction Effects on Gas Transport Parameters and Estimated Climate Gas Exchange for a Landfill Final Cover Soil

Landfill sites have been implicated in greenhouse warming scenarios as a significant source of atmospheric methane. In this study, the effects of extreme compaction on the two main soil-gas transport parameters, the gas diffusion coefficient (Dp) and the intrinsic air permeability (ka), and the cumulative methane oxidation rate in a landfill cover soil were investigated. Extremely compacted landfill cover soil exhibited negligible inactive soil-air contents for both Dp and ka. In addition, greater Dp and ka were observed as compared with normal compacted soils at the same soil-air content (ε), likely because of reduced water-blockage effects under extreme compaction. These phenomena are not included in existing predictive models for Dp(ε) and ka(ε). On the basis of the measured data, new predictive models for Dp(ε) and ka(ε) were developed with model parameters (representing air-filled pore connectivity and water-blockage effects) expressed as functions of dry density (ρb). The developed Dp(ε) and ka(ε) models together with soil-water retention data for soils at normal and extreme compaction (ρb = 1.44 and 1.85??g?cm-3) implied that extremely compacted soils will exhibit lower Dp and ka at natural field-water content (-100??cm H2O of soil-water matric potential) because of much lower soil-air content. Numerical simulations of methane gas transport, including a first-order methane oxidation rate, were performed for differently compacted soils by using the new predictive Dp(ε) model. Model results showed that compaction-induced difference in soil-air content at a given soil-water matric potential condition is likely the most important parameter governing methane oxidation rates in extremely compacted landfill cover soil.     

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.     

Fugitive Methane Emissions from Landfills: Field Comparison of Five Methods on a French Landfill

The study presented in this paper has been initiated by the Veolia Environment research center for waste management and supported by the French Environmental Agency. A comparison of five field-scale measurement methods for measuring fugitive methane emissions has been conducted on a French landfill site. The five methods evaluated consisted of a tracer gas technique, laser radial plume mapping, inverse modeling technique, differential absorption light detection and ranging (LiDAR), and helicopter-borne spectroscopy. These methods are evaluated on their abilities to measure emissions from a practical user-oriented aspect (metrological, technical, and economical criteria). High disparities in terms of quantitative results and applicability were observed from all methods. Techniques that used Gaussian dispersion modeling appeared less applicable to landfill sites due to topographic complexity and did not provide high confidence in the results. However, the method using optical remote sensing (radial plume mapping) methods showed that a spatially detailed analysis is achievable (cell level), and the LiDAR method showed very promising approach and technical performances.     

Thickness and Hydraulic Performance of Geosynthetic Clay Liners Overlying a Geonet

Experimental results from physical testing are reported to examine the thickness and hydraulic performance of three geosynthetic clay liners (GCLs) overlying a geonet when subjected to vertical stresses (e.g., as may be found in a secondary leachate collection layer or hydraulic control layer in solid waste landfills). The GCL was found to intrude into the underlying geonet and the effects of GCL type and water content, temperature, applied pressure, and test duration on the final GCL thickness are examined. The results are consistent with GCL deformation from the beneficial consolidation of bentonite as opposed to lateral extrusion of bentonite. Results from fixed ring flow tests suggest that the indentations in the GCL caused by intrusion into the underlying geonet do not appear to negatively impact the hydraulic performance (permittivity or resistance to internal erosion) of the particular GCLs tested for the conditions examined. The flow capacity of the geonet in these tests was found to depend not only on the amount of GCL intrusion but also on the orientation of the geonet relative to the flow direction.     

Pilot-Scale Simulation of Landfill Bioreactor and Controlled Dumping of Fresh and Partially Stabilized Municipal Solid Waste in a Tropical Developing Country

Four pilot-scale lysimeters were used to study the benefits of landfill operation with and without leachate recirculation in tropical weather conditions. Young and old landfills were simulated by filling lysimeters with a segregated fraction of fresh municipal solid waste (MSW) and MSW mined from an open dump site, respectively, and periodically monitoring leachate quantity and quality and biogas quality. For each substrate, one lysimeter was operated as a bioreactor with leachate recirculation and another lysimeter was operated as a controlled dump, for a period of 10 months. Densities between 652 and 825??kg/m3 could be achieved with fresh and mined MSW. Despite such compaction during waste placement, bioreactor technology helps in leachate management, especially in the case of the young landfill lysimeter operated in tropical weather. The benefits of leachate recirculation in the young landfill lysimeter were evident from the significant decrease in chemical oxygen demand (COD) (86%), biochemical oxygen demand (BOD) (82%), dissolved organic carbon (DOC) (85%), and volatile solids (75%) in leachates. However, ammonia nitrogen (amm-N) and chlorides in the leachates accumulated in bioreactor landfills. Operating an old landfill lysimeter as a bioreactor seemed to have no exceptional advantage in the context of leachate management, although leachate recirculation enhanced the methane potential of both fresh and mined MSW.     

Characterization of Methane Flux and Oxidation at a Solid Waste Landfill

Methane emissions were measured at several locations at a typical solid waste facility using a static chamber technique. At the entire facility, methane flux varied from ?13.6?to?1,755?g?m?2?day?1. The flux data had an arithmetic mean value of 71.3?g?m?2?day?1 and a geometric mean value of 18.6?g?m?2?day?1. At this site, methane emission was generally lower on the side slopes relative to the flat areas of the landfill. The spatial variability of methane flux was characterized by point kriging and inverse distance weighing (IDW) in an intensive study of a 61×61-m area. The geospatial means in this area obtained by both methods were almost identical (20.9 versus 20.8?g?m?2?day?1). These geospatial means for the area were also similar to the arithmetic mean (24.5?g?m?2?day?1), but 3.4 times the geometric mean (6.5?g?m?2?day?1). Methane oxidation was evaluated at the surface of the landfill and at several depths within the cover soil using stable isotope techniques. The δ?13C of CH4 averaged ?55.4% in the anoxic zone. Methane collected in chambers and in surficial soil probes exhibited more 13C enriched values, ranging from ?55.4 to ?34.5%, due to the preferential uptake of 12CH4 by methanotrophic bacteria. Methane oxidation at the landfill averaged 22% and occurred in the upper 70?cm of the landfill cover soil. Oxidation occurred in all tested locations of the landfill and for all ages of buried waste.     

Field Data and Water-Balance Predictions for a Monolithic Cover in a Semiarid Climate

Water-balance predictions made using four codes (UNSAT-H, VADOSE/W, HYDRUS, and LEACHM) are compared with water-balance data from a test section located in a semiarid climate simulating a monolithic water-balance cover. The accuracy of the runoff prediction (underprediction or overprediction) was found to affect the accuracy of all other water-balance quantities. Runoff was predicted more accurately when precipitation was applied uniformly throughout the day, the surface layer was assigned higher saturated hydraulic conductivity, or when Brooks-Corey functions were used to describe the hydraulic properties of the cover soils. However, no definitive or universal recommendation could be identified that would provide reasonable assurance that runoff mechanisms are properly simulated and runoff predictions are accurate. Evapotranspiration and soil-water storage were predicted reasonably well (within ≈ 25?mm/yr) when runoff was predicted accurately, general mean hydraulic properties were used as input, and the vegetation followed a consistent seasonal transpiration cycle. However, percolation was consistently underpredicted (>3?mm total) even when evapotranspiration and soil-water storage were predicted reliably. Better agreement between measured and predicted percolation (or a more conservative prediction) was obtained using mean properties for the soil-water characteristic curve and increasing the saturated hydraulic conductivity of the cover soils by a factor between 5 and 10. Evapotranspiration and soil-water storage were predicted poorly at the end of the monitoring period by all of the codes due to a change in the evapotranspiration pattern that was not captured by the models. The inability to capture such changes is a weakness in current modeling approaches that needs further study.     

Operation and Performance of Horizontal Wells for Leachate Control in a Waste Landfill

This paper presents data on gas and leachate flow rates and leachate levels obtained during a 600?day pumping trial on three retrofitted horizontal wells in a domestic waste landfill at Rainham, United Kingdom. The changes in gas and leachate flow rate with time and atmospheric pressure, and the interaction between the two flows, are discussed. The spatial variability of the response of the leachate levels within the landfill is explored with reference to the anisotropy and heterogeneity of the permeability of the waste. It is shown that horizontal wells can be an effective means of controlling leachate heads near the base of a landfill, and that leachate levels must be measured using piezometers with a discrete response zone rather than fully screened observation wells if meaningful results are to be obtained. It is argued that the large amounts of gas pumped from the nominally saturated zone of the landfill must have come from the ongoing degradation of the waste within the zone of influence of the well.     

Application of Fuzzy Logic to Estimate Flow of Methane for Energy Generation at a Sanitary Landfill

This paper proposes the use of a multicriteria assessment technique to evaluate the methane flow during gas extraction from a sanitary landfill. A number of parameters determine the gas generation and the feasibility for its extraction from a landfill. These parameters form a complex set of information with unknown mathematical interrelationships making potential gas flow evaluations difficult and elusive. In addition, the data available for a particular landfill are very often imprecise, uncertain, or subjective, making it even more difficult to evaluate the potential for gas extraction without conducting pilot tests. The method proposed in this paper uses fuzzy composite programming that allows for the use of imprecise information. A landfill gas potential index has been defined, which can be determined by easily obtainable climatological, geological, and landfill parameters. The landfill gas (LFG) potential index is related to the landfill gas flow using an empirical equation. The LFG potential model was calibrated and verified using data obtained from 61 landfills where gas extraction is being conducted. A sensitivity analysis was done to study the impact of variations in the input data on model output.     

Performance of a Stiff Support System in Soft Clay

The performance of an excavation support system for a subway station renovation project in Chicago and its effects on an adjacent, shallow-foundation supported building are presented. The 13-m-deep excavation was made through soft to medium stiff clays and was supported by a 900-mm-thick secant pile wall, one level of cross-lot bracing, and two levels of tiebacks. Design considerations are discussed and construction procedures are summarized. Field performance data were collected, including lateral soil movements at five locations, building settlements along the exterior wall and interior columns, support system loads, and observations of building damage. As planned in the design, minor damage occurred to nonload bearing portions of the building. Of the 38 mm of maximum lateral movement adjacent to the building, 9 mm occurred during wall installation, 16 mm developed as the soil was excavated, and 13 mm occurred during tunnel demolition and station renovation as a result of soil creep and reduction of wall stiffness. Settlements extended beyond the secant pile wall a distance approximately equal to the depth of the secant pile wall. The effect of excavation was to cause larger settlements within the affected zone, but not to expand the width of the settlement trough. When distortions exceeded approximately 1/960, damage began to manifest itself in the nonload bearing portions of the building.     

Effect of Uncertain Hydraulic Conductivity on the Simulated Fate and Transport of BTEX Compounds at a Field Site

Monte Carlo simulations were conducted to investigate the effect of uncertain hydraulic conductivity (K) on the natural attenuation of BTEX compounds (benzene, toluene, ethyl benzene, and xylenes) through aerobic respiration, denitrification, Fe(III) and sulfate reductions, and methanogenesis observed at a field site on Hill Air Force Base, Utah. First, the uncertainty in the ln?K field was represented by multiple realizations of spatially correlated random fields. The simulated BTEX plumes resulting from a light nonaqueous phase liquid source were analyzed for mass distributions and the relationships among various factors such as dissolved BTEX mass, plume spreading, and depletions of electron acceptors or productions of Fe2+ and methane. Second, additional K realizations with the same mean but different variances and correlation lengths were used to determine how the model responds to varying degrees of uncertainty in the K field. The methodology and insights are of general interest and applicable to fuel-hydrocarbon natural-attenuation sites.     

Assessing of Natural Attenuation and Intrinsic Bioremediation Rates at a Petroleum-Hydrocarbon Spill Site: Laboratory and Field Studies

Natural attenuation is a passive remedial approach that depends upon natural processes to degrade and dissipate contaminants in soil and groundwater. Intrinsic bioremediation is believed to be the major process among the natural attenuation mechanisms that account for the reduction of contaminant concentrations. In this study, a mass flux approach was used to calculate the contaminant mass reduction at a petroleum-hydrocarbon spill site. The mass flux technique is a simplified mass balance procedure, which is accomplished using the differences in total contaminant mass flux across two cross sections of the contaminant plume. The mass flux calculation results show that up to 86% of the dissolved total benzene, toluene, ethylbenzene, and xylene (BTEX) isomers removal was observed via natural attenuation at this site. Evidence for the occurrence of natural attenuation was the decreased contaminant mass flux through the plume cross sections along the transport path and limited spreading of the BTEX plume. Evidences for the BTEX biodegradation include: (1) decreased BTEX concentrations along the transport path; (2) depletion of dissolved oxygen, nitrate, and sulfate; (3) production of dissolved ferrous iron, sulfide, methane, and CO2; (4) deceased pH in the spill source area and increased pH in iron-reducing area; (5) increased alkalinity and microbial populations; and (6) preferential removal of certain BTEX components along the transport path. The effectiveness of intrinsic bioremediation on BTEX removal was evaluated by the in situ tracer method. Results reveal that approximately 74% of the BTEX removal was due to the intrinsic biodegradation process. The first-order decay model was applied for the natural attenuation and intrinsic bioremediation rates calculation. Results show that the biodegradation capacity (34.5 mg/L) for BTEX was much higher than the detected contaminants within the plume. The calculated total BTEX first-order natural attenuation and intrinsic bioremediation rates were 0.025 and 0.017% 1/day, respectively. Results of polymerase chain reaction, denaturing gradient gel electrophoresis, and nucleotide sequence analysis reveal that some petroleum-hydrocarbon degraders (Flavobacterium capsulatum, Xanthobacter sp., Xanthobacter flavus, Xanthomonas codiaei, Pseudomonas boreopolis, Methylobacterium sp., Reichenowia pictae) might exist at this site, which might contribute to the BTEX biodegradation. Results suggest that the natural attenuation mechanisms can effectively contain the plume, and the mass flux method is useful in assessing the occurrence and efficiency of the natural attenuation and intrinsic bioremediation processes.