Containment System Performance
Observational data and predictive models for the performance of engineered barrier systems—including liner systems, cover systems, leachate collection systems, and vertical barriers—and the overall performance of containment systems are evaluated in this chapter. Unfortunately, few direct observational data on performance are available for most of these systems and none of the data extend beyond three decades. Consequently, predictions of long-term performance generally rely on extrapolations from relatively short-term data (see Box 1.2 for definitions of performance periods) and on assumptions based on the long-term performance of barrier system elements (described in the previous chapter).
An example of the types of performance information that are available on waste landfills and impoundments is given in Box 5.1. The information focuses on liners and covers; comparable information on vertical barriers for waste containment has not been summarized.
OBSERVATIONS OF PERFORMANCE
Overall Liner System Performance
The best-available information on the overall performance of liner systems comes from monitoring data for the environment surrounding the liner system. The New York Department of Environmental Conservation (NYDEC) reviewed groundwater monitoring data at all modern municipal solid waste (MSW) and hazardous waste landfills in the state (letter to the committee from Stephen Hammond, Director, NYDEC, August 30, 2006). For New York, “modern” means since 1988, when the state issued new regulations for MSW landfills that require double-liner systems. The number of facilities reviewed includes 27 MSW landfills and 4 hazardous waste landfills. Of these 31 landfills, 28 have double-composite liners, while 3 have double liners with a single geomembrane in the primary liner. In total these landfills comprise 1,100 acres of barrier systems and 450 years of operation. Considering that landfill cells are developed gradually over a period of years, the landfills assessed correspond to 7,000 to 10,000 acre-years of operation.
Based on groundwater monitoring data from onsite monitoring wells, NYDEC did not find a single instance of an adverse impact to groundwater that could be attributed to leakage through a containment system at any one of these facilities. NYDEC did find several instances where groundwater was impacted by older unlined portions that were also present at some of the landfill sites and by onsite activities not related to the barrier system, such as a leaking seal in a leachate conveyance line outside the landfill cell.
In addition, NYDEC reviewed water quality monitoring data from pressure relief systems, which existed at 20 of the 27 MSW landfills included in this study. These systems directly underlie the base liner, so they potentially provide direct information on leakage through the containment system. At all but 4 of the 20 landfills with pressure relief systems, the pressure relief systems covered the entire footprint of the barrier system. NYDEC also did not find a single instance where these data indicated the presence of contaminants that had been released from the overlying barrier system into the pressure relief system.
Fluid Leakage Through Liner Systems
Leakage of fluid through the primary liner system in a double-liner system can be estimated using measured pumping or flow rates from the secondary leachate collection (leak detection) system. The flow rate reflects how well both the leachate collection system and the primary barrier layers are working. The measurement is actually the pumping rate required to maintain a constant head in the sump of the secondary collection system; therefore, clogging in the secondary collection system could also affect the measured flow rate.
Types of Data Available on Waste Containment Landfills and Impoundment System Performance
Few data exist on containment system performance. One of the most comprehensive studies on short- and medium-term performance of containment systems was conducted by Bonaparte et al. (2002), based on information collected from publications and from discussions with facility owners, operators, designers, and regulators. The study reported performance problems from multiple sources, including operators, at approximately 2,000 U.S. landfills and impoundments designed and constructed under Resource Conservation and Recovery Act (RCRA) Subtitles C and D. A total of 85 “problems” were identified at 79 landfill and 5 surface impoundment facilities (listed in Table 5.1 and shown graphically in Figure 5.1). Care must be taken in drawing conclusions from these data, since the search for problem facilities was not exhaustive.
TABLE 5.1 Types of Problems Encountered at Waste Containment Facilities
In the landfill performance assessment discussed in Box 5.1, flow rates through primary liners into the leak detection system were reported for active cells with double liner systems with top geomembrane liners (single or composite; Bonaparte et al., 2002). Facilities that used conventional construction quality assurance (CQA) programs had substantially lower leakage rates through geomembrane liners than facilities that did not.
Table 5.2 summarizes the mean, maximum, and minimum average monthly flows for single geomembrane, geomembrane-compacted clay liner (GM-CCL), and geomembrane-geosynthetic clay liner (GM-GCL) composite systems overlain by sand liquid collection layers for the initial, active, and postclosure periods. Of the three systems, leakage was by far the greatest through the single geomembrane. Leakage through the GM-GCL systems was generally equal to or less than that through the GM-CCL systems, and the rates were dramatically lower (typically by an order of magnitude) during the active life and postclosure period for the GM-GCL systems compared to the GM-CCL systems. Similar data were reported for GM-GCL systems compared to GM-CCL systems when a geonet liquid collection layer was used.
The findings discussed above and the relatively low leakage rates (even with a single geomembrane, provided a good CQA program is in place) indicate that modern geomembrane and composite liners are working reasonably well. A composite liner has a significantly lower leakage rate than a single geomembrane. The data suggest that a composite liner with a GCL leaks less than one with compacted clay, but more care must be taken during construction of the liner system and placement of the waste when a GCL is used.
Figure 5.2 shows the flow rates over time of the primary leachate collection and removal system and the secondary leak detection system in a Pennsylvania MSW landfill. Leachate generation, as represented by leachate collection and removal system flow rate measurements, is highest during the initial period of landfilling. Flow rates decrease as the waste thickness increases and daily and intermediate covers are applied (i.e., active period of operation) and become almost negligible once the final cover is placed and the landfill enters the postclosure period. The monitoring data also show that over time the flows in the leak detection system are small to negligible, confirming that the composite liner system performed efficiently over the 7 years monitored.
Diffusion Through Bottom Liner Systems
Diffusive flux may constitute a significant portion of total flux through a liner system, particularly for well-constructed composite liners and low hydraulic conductivity (less than about 1 × 10−10 m/s) compacted clay liners. Diffusion is driven by chemical gradients, that is, by the difference in the concentrations of a chemical or compound above and below the barrier layer. Geomembranes are an excellent barrier to diffusion of ionic contaminants (e.g., salts, metals, volatile fatty acids), but they will readily allow diffusion of volatile organic compounds (Rowe, 2005). Because the concentration of volatile organic compounds in the leachate in modern MSW landfills is typically low (generally less than 1,000 parts per million and often less than tens of parts per million), diffusive flux through a composite liner and underlying attenuation layer will not generally pose a significant environmental hazard while the geomembrane remains intact. However, in hazardous waste facilities and at MSW landfills where a sensitive receptor is in close proximity to the liner, diffusive flux can be a concern.
TABLE 5.2 Liquid Collection Rates for Double-Liner Leak Detection Systems
Medium-term data on the chemical composition of leachate have been reported from lysimeters installed beneath single- and composite liner systems (Bonaparte et al., 2002; King et al., 1993). Exhumation and undisturbed sampling have also been used to obtain chemical concentrations in compacted clay liners (Reades et al., 1989; King et al., 1993; Rowe, 2005). These studies showed that chloride diffused over a distance of approximately 0.75 m over 4.3 years through a 0.3-m-thick clogged sand layer underlain by a 1.2-m-thick compacted clay liner. A diffusion coefficient of 6 × 10−10 m2/s provided a good fit for the field concentration profile of chloride and is consistent with the diffusion coefficient measured in the laboratory. Sodium was also retarded at a low level (Na+ migrated about 35 cm into the compacted clay liner), and potassium was retarded at a high level (K+ migrated about 5 cm into the liner) through the same liner system. Desorption of calcium was observed in the concentration profile.
The results of an ongoing study on leachate chemistry for single- and composite liner systems in Wisconsin for periods exceeding 20 years indicate that a wide variety of volatile organic compounds in various concentrations have appeared at different frequencies in the liquid effluent collected from lysimeters beneath the liners (see Section 4.1.4 and Klett et al., 2006). The exact mechanism for the transport of these volatile organic compounds is not yet known with any certainty and could be leakage or diffusion through the liner, gas migration around the liner, or both. In general, the arrival of the volatile organic compounds occurred approximately 10 years after waste placement. This timing is generally consistent with the observation that volatile organic compounds (especially toluene) diffused to a depth of about 0.6 m in
4.3 years at the Keele Valley landfill (Barone et al., 1993; Rowe, 2005) and the rate of diffusion of ions to depths of over 1 m in low-permeability clay (Quigley and Rowe, 1986; Lake and Rowe, 2005b). Overall, these studies indicate that the potential for diffusive flux, including the long-term flux of volatile organic compounds, should be considered when designing a facility.
Gas Migration Control for Bottom and Side Slope Liner Systems
Modern MSW landfill liner systems are intended to control both gas and liquid (leachate) migration. However, gas migration pathways around even a gas-tight liner system can adversely impact the gas containment effectiveness of the liner system. The California Integrated Waste Management Board’s (CIWMB’s) 2003 landfill facility compliance study examined 16 landfills with base liners employing geomembrane barriers over the entire waste footprint (Geosyntec Consultants, 2003). Six of the landfills were reported to be in “corrective action,” indicating that monitoring systems detected a release of contaminants from the waste mass. In five of these six cases, analysis of the groundwater chemistry indicated that the release was related to landfill gas, while in the sixth case the cause of the release was undetermined but was suspected to be related to construction.
The results of the CIWMB study suggest that there may be a systematic flaw with respect to gas containment in modern geosynthetic liner systems. Kavazanjian and Corcoran (2002) discussed the occurrence of landfill gas impacts on groundwater at MSW landfills with geomembrane liner systems. They attribute these gas impacts to geomembrane liner termination details that allow landfill gas that has accumulated in the leachate collection and removal system to travel around the buried edge of the liner termination and penetrate into the ground. This pathway is illustrated in Figure 5.3. This problem can be mitigated in a number of ways, including placing the leachate collection system under vacuum, placing gas collection trenches in the waste adjacent to the side slope (taking care not to draw too much oxygen into the system), and/or modifying the termination detail for the side slope leachate collection layer to include a geomembrane flap or other type of cap around the end of the leachate collection and removal system.
Mechanical Damage to or Deterioration of Bottom Liner Systems
The performance of bottom liner systems can be affected significantly by damage or deterioration during operation or after closure. Box 5.2 illustrates how damage to a composite liner during filling increased the leakage rate dramatically. This example also demonstrates the value of redundancy in a liner system; while the primary liner was compromised significantly, the secondary liner remained intact and prevented an adverse impact to the environment.
The 2002 Environmental Protection Agency (EPA) database of observed problems in liner systems at modern U.S. municipal solid waste and hazardous waste landfills lists eight instances of liner system leakage caused by damage. In five cases the liner was a single geomembrane that was damaged during construction or operation, and in three cases it was a geomembrane or composite liner that leaked at a leachate collection pipe penetration. In all but one case the leakage was detected using measured flow rates from the leak detection system. An important point is that only a small
Case History on Mechanical Damage to a Geosynthetic Composite Liner
This case history illustrates how the leakage rate through a composite geomembrane-geosynthetic clay liner can increase significantly due to relatively large through-going holes. The barrier was a double-liner system for a hazardous waste landfill located in the Midwest (Daniel and Gilbert, 1996). The primary liner was a composite liner with a geomembrane overlying a geosynthetic clay liner. The rate of pumping from the leak detection system averaged about 10 lphd for the first 3 months of operation, which is typical for a composite liner. However, the pumping rate increased to more than 30,000 lphd after the third month of operation. The suspected cause of the increased leakage rate was damage to the primary liner during waste placement. An extensive field investigation, which ultimately involved removing the waste and uncovering the liner system, identified 28 holes that were apparently caused by a backhoe moving waste in the landfill. The holes extended through the geosynthetic clay liner (Figure 5.4), meaning that the composite system performed similarly to a single geomembrane liner with holes. The holes penetrated through the secondary geomembrane and a small distance (less than 3 cm) into the secondary compacted clay liner. However, the secondary clay liner was still intact and able to contain the waste at the location of the holes. The holes were repaired and pumping rates into the leak detection system dropped back to about 10 lphd. This case history demonstrates the value of redundancy in a barrier system; the damage to the primary liner was immediately detected and leakage through it was contained by the secondary detection system and barrier.
fraction (less than 10 percent) of the approximately 2,000 landfills in the database contained double-liner systems with data from a leak detection system. In one case, damage to a geomembrane was detected during construction using an electrical leak location survey. Therefore, mechanical damage to the liner system occurred in approximately 5 to 10 percent of the facilities where it could be detected.
In the Bonaparte et al. (2002) study, other types of liner damage were observed that did not necessarily result in leakage because they were detected immediately. The vast majority of these problems involved slope failures; in fact, nearly one-half of the problems identified in the facilities surveyed were the result of slope failures or excessive displacement in the liner system (Figure 5.1). In addition, two instances of liner damage were caused by landfill fires, two were caused by installing gas wells, and one was caused by desiccation cracking when the compacted clay liner was left exposed for 3 years prior to waste placement.
Degradation of leachate collection and leak detection systems was also observed in the Bonaparte et al. survey. The observed problems included clogging of geotextile filters (two cases) by fines, clogging of the sand drainage layer and pipes (one case), uncontrolled leachate seeps due to perched leachate in waste (one case), and clogging or problems with flow rate measuring systems such as the one described in Box 4.4 (four cases).
Temperature variations above and within a liner system can be substantial, as illustrated by the case history in Box 5.3. Elevated temperatures can accelerate degradation of geosynthetic liner materials and increase the hydraulic conductivity of clay liners (Rowe, 2005). In addition, thermal gradients can affect the transport of liquids and gases through liner systems.
Temperatures of 30 to 40°C have been observed on base liners after 5 to 10 years, and higher temperatures can develop even earlier when there is moisture augmentation (Koerner and Koerner, 2005). Thus, while primary composite liners have performed well to date, the long-term performance is in some doubt. This, as well as the problems noted above regarding damage to primary liners, suggests the desirability
Case History on the Temperature Environment for a Geosynthetic Composite Liner
This case history illustrates how temperatures within a liner system vary with time and are influenced by waste placement conditions. Data were collected from a Michigan landfill with a bottom liner that included (from top to bottom) a 45-cm-thick protective sand layer, a geotextile-geonet composite leachate collection layer, a geomembrane, and a geosynthetic clay liner (GCL). Temperatures were measured in three cells using thermocouple sensor arrays placed above the leachate collection system within the protective sand layer and immediately below the GCL on the subgrade for variable periods from approximately 1 year prior to waste placement and up to 5 years thereafter (Yesiller and Hanson, 2003; Hanson et al., 2005a). Under exposed conditions, temperatures in the liner system followed seasonal temperature variations. The thickness of the protective sand layer was insufficient to prevent excessive temperature variations and freezing of the liner system. Seasonal temperature fluctuations were dampened after the first lift of waste was placed over the liner systems, then temperatures increased with time. Temperatures in the liner system in the middle of the landfill had reached 20°C to over 30°C within 2 to 4 years after waste placement. The effects of seasonal variations were still observed at locations within approximately 20 m of cell perimeters (Figure 5.5). The average absolute thermal gradient was 26°C/m for the exposed liner and 16°C/m for the waste-covered liner.
of double-lined systems for ensuring satisfactory long-term performance of the overall barrier system. However, even with double liners high temperatures on the secondary liner are possible, and consideration should be given to designing the secondary system to accommodate high temperatures (Rowe and Hoor, 2007).
Overall Cover System Performance
The overall performance of cover systems may be assessed in terms of their effectiveness in providing the basic environmental protective functions of a cover. These include infiltration control, isolation of waste from the environment, protection against inadvertent intrusion, protection against wind and water erosion, and control of gases. The use of cover systems, or capping, as the selected alternative (“final remedy”) for remediation of RCRA and Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) sites has been increasing, and the committee did not find any reports of capping remedies that have to be reconsidered because of cover system failure. Although these observations suggest that the great majority of cover systems have performed well in at least the short and medium terms, it is possible that the absence of reported cover system
failures may simply reflect the lack of systematic studies. A case history illustrating how a cover system can be used effectively to remediate a pre-RCRA hazardous waste landfill is presented in Box 5.4.
The evidence suggests that properly designed and constructed cover systems at MSW and hazardous waste landfills and RCRA and CERCLA sites can effectively isolate waste and contaminated soil and groundwater from the environment. In most cases, failure of a cover system to perform this function would be readily obvious to even casual observers. Thousands of cover systems have been constructed around the world in the past 30 years, and many of these systems are inspected regularly. Although cover systems are occasionally eroded by water or wind action, they generally appear to be performing satisfactorily.
The CIWMB study (Geosyntec Consultants, 2003) included a limited evaluation of overall cover system effectiveness in landfills. The environmental compliance records of 224 landfills were surveyed for the time period from January 1998 through December 2001, including 211 landfills that had received waste since October 9, 1993, when the EPA Subtitle D landfill regulations took effect, 10 landfills that had closed, and 3 landfills that had ceased to accept waste prior to October 9, 1993. In Phase I of the study, 48 of the landfills were described as fully covered, including the 10 that had closed prior to October 9, 1993. With the exception of landfill gas migration problems, the environmental performance of fully covered sites was superior to that of sites that were not fully covered. This conclusion is consistent with numerous observations that construction of an effective gas-tight cover without adequate enhancement of the gas collection system exacerbates lateral gas migration by forcing gas that had been diffusing vertically through the top of the landfill toward the sides of the landfill and downward to groundwater.
In Phase II of the CIWMB study, 53 landfills, including the 13 that had closed prior to 1993, were examined in more detail (Geosyntec Consultants, 2004). The study noted that closed landfills have a significantly lower occurrence of surface water actions and concluded that “construction of an approved final cover system can reduce the potential for surface water impacts” (p. 18). The degree of effectiveness of cover systems could not be judged from the report and the committee was unaware of corroborating studies regard-
Case History on Final Cover Performance for Site Remediation
This case history illustrates how a final cover system that includes geosynthetic components, when coupled with appropriate leachate management measures, can be used to effectively manage postclosure maintenance costs and to mitigate impacts to shallow groundwater. The approximately 51-ha North Parcel of the Acme Landfill operated from the 1950s through the late 1980s and accepted municipal solid waste, designated waste, and limited amounts of RCRA hazardous waste (RMC Geoscience, 1998). The unlined landfill is located near the margin of San Francisco Bay on relatively soft and compressible bay mud deposits, and saline groundwater occurs at or near the ground surface.
The approximate quantity of leachate generated in the landfill was evaluated using a water balance of the general form ∆S = ∆VL(in)− ∆VL(out), where ∆S is the change in leachate volume (or storage) within the landfill for a given period of time, ∆VL(in) is the sum of water inflows to the refuse (volume of leachate generation) for a given period of time, and ∆VL(out) is the sum of water outflows from the refuse (volume of leachate removed) for a given period of time.
Leachate generation sources were assumed to include subsurface flow into the refuse fill from consolidation of the compressible bay mud deposits and infiltration of precipitation through the intermediate cover of the landfill. Outflow from the landfill was assumed to include active leachate extraction, leachate seepage or evapotranspiration at the perimeter of the landfill, and leachate outflow in the subsurface. Subsurface inflow was evaluated using both a numerical model and consolidation calculations. Infiltration of precipitation was evaluated using a modified version of the Hydrologic Evaluation of Landfill Performance model. Surface water inflow to the parcel was assumed to be negligible. Evapotranspiration and subsurface outflow were evaluated using a numerical model. The change in storage over time was calculated using data from a number of leachate piezometers located throughout the fill.
Closure design evaluations and monitoring indicated that leachate within the parcel contributed to shallow groundwater contamination. The data and water balance evaluation further showed that leachate within the parcel was generated by an equal combination of the water squeezed out of the consolidating bay mud and infiltration of precipitation through the interim cover, and hydrographs showed that leachate elevations were increasing throughout the fill at a relatively constant rate. Numerical modeling suggested that future impacts to groundwater could be mitigated by reducing the amount of leachate within the parcel. As a result, interim closure activities in 1992 included installation of several interior leachate extraction wells and construction of an onsite leachate treatment plant.
Final closure design for the North Parcel also focused on leachate management and control and included (1) installation of a perimeter leachate extraction system, (2) installation of a combination geosynthetic clay liner and geomembrane final cover system, and (3) evaluation of upgrades to the treatment plant. A target 95 l/min extraction rate for the treatment plant was ultimately selected based largely on the estimates of leachate generation and the amount of time required to reduce an approximately 3.7 m mean sea level average leachate level within the fill to an average elevation of 0 m mean sea level. The assumption was that the final cover would, for all practical purposes, eliminate leachate generation from infiltration.
Final cover closure construction began in 1997 and was substantially completed in 1998. Leachate monitoring data collected during construction showed relatively direct and measurable response to heavy precipitation, particularly when portions of the waste were exposed during construction. Leachate elevation monitoring data collected since the cover was completed show measurable decreases in leachate volume, as shown in Figure 5.6. The reduction in total leachate volume is consistent with the elimination of infiltration as a source of leachate generation. Leachate extraction and piezometer monitoring data indicate that the current leachate generation rate in the parcel is on the order of 45 l/min and results principally from groundwater inflow from consolidation of the underlying bay mud. The results have, within a relatively short period of time, allowed the landfill operator to adjust leachate extraction from the parcel and to better manage the overall costs associated with postclosure monitoring and maintenance. Equally significant, groundwater monitoring since the final cover was completed shows that groundwater quality has improved at the perimeter of the parcel as leachate levels in the parcel have decreased.
ing the effectiveness of capping as a remedy at RCRA or CERCLA sites.
The observed and inferred good performance of cover systems does not suggest that periodic maintenance to repair erosion or other forms of cover distress (e.g., cover veneer stability failures) is not required. In fact, most engineered cover systems rely on regular maintenance to maintain their integrity. Some systems (e.g., for low-level radioactive and mixed waste) are designed to survive for 1,000 years or more without regular maintenance. In these cases, cover systems are often designed to emulate natural landscapes that have resisted erosion, and perhaps limited infiltration and intrusion, for thousands of years. “Natural analog” cover systems typically have low profiles to provide stability and minimize erosion, and they are populated by a natural succession of native plant species until the native landscape is emulated (Caldwell and Reith, 1993). The Fernald mixed waste landfill in Ohio has such a cover system. This system has an essentially flat barrier layer, with a top deck grade not greater than 6 percent and side slopes with a maximum inclination of 6H:1V.
In summary, while the overall performance of covers that have been constructed to date is generally satisfactory, erosion and instability continue to be persistent problems for relatively steep (i.e., steeper than 4H:1V) covers. Most cover systems rely on continuing maintenance for good operation. Maintenance-free covers have not yet been demonstrated to work.
Percolation Through Cover Systems
A primary function of most (but not all) cover systems is minimization (or sometimes simply control) of percolation through the cover and into the underlying waste or impacted
soil. Measured percolation rates through cover systems can provide a variety of insights on the performance of the cover system, including the effectiveness of the surface at promoting runoff, the effectiveness of soil layers above or within the barrier at storing and removing moisture, the effectiveness of drainage layers at minimizing the hydraulic head on the underlying barrier layers, and the effectiveness of evapotranspirative barrier layers at minimizing leakage.
Percolation rates for cover systems containing single compacted soil layers have been measured using pan lysimeters in test plots in four climatic regions for durations up to 7 years (Benson, 1999, 2001). The compacted clay layers were between 600 and 900 mm thick and they were overlain by varying thicknesses of vegetative layers. Percolation through the cover systems generally increased at all of the test sites during the respective study periods. Percolation rates for the compacted clay layers were initially between 10 and 50 mm/year in humid climates (approximately 1 to 4 percent of the total precipitation) and approximately 1 to 4 mm/year in semiarid climates (approximately 1 to 2 percent of precipitation). Percolation increased if cracks developed in the clay layers: rates of 100 to 150 mm/year were measured in humid climates (10 to 20 percent of precipitation) and approximately 30 mm/year in semiarid climates (approximately 10 percent of precipitation). Percolation as high as 500 mm/year was observed through a cover that included a cracked single compacted clay liner at a landfill located in a warm humid area. These data are consistent with other work showing that desiccation, freeze/thaw, root penetration, animal intrusion, and settlement-induced cracking were major factors affecting the performance of covers with compacted clay layers (Bonaparte et al., 2002). The Bonaparte et al. study recommended against using compacted clay layers alone in the final cover systems of landfills, especially landfills with wastes that settle significantly.
Compacted clay layers are also used in conjunction with geomembranes to form composite barriers in cover systems. Percolation rates obtained from lysimeters beneath cover systems containing composite barriers in test plots in a humid region were summarized by Benson (1999). The data show that percolation through the covers generally increased with time over the 7-year study period. The measured percolation rate at the end of the study period was 2 to 3 mm/year, which was significantly lower than the percolation rate of approximately 150 mm/year measured in test plots with a single compacted clay liner cover at the same site. No cracking of the clay liner beneath the geomembrane was observed at the site, consistent with findings from the Bonaparte et al. (2002) study. Additional studies indicated that percolation through composite barriers was generally less than 10 mm/year, with values generally in the range of 1 to 3 mm/year in humid climates and 0.1 to 1 mm/year in semiarid climates (Benson, 2001).
Another indirect measure of cover system performance is the reduction in leachate pumping rates following capping of a landfill containing a leachate collection and removal system. Data in Bonaparte (1995) and Bonaparte et al. (2002) indicate that a well-designed and well-constructed final cover system can significantly reduce leachate generation rates, and by inference percolation rates, by over an order of magnitude in MSW landfills. A drop in flow rate from 60 percent of the annual rate of precipitation when the landfill cells were receiving waste to 13 percent and 1 percent of annual precipitation after 1 and 10 years of closure, respectively, was reported in a hazardous waste containment facility (Haikola et al., 1995). The landfills in the facility were covered with a barrier system that included a composite liner. Statistical analysis of the data indicated that flow into the leachate collection system was generally independent of precipitation subsequent to placement of a final cap over the wastes.
Gas emissions from landfills are highly variable in both space and time. The variations reflect differences in waste conditions (type and decomposition rates), type and thickness of cover materials, atmospheric conditions, and measurement techniques. Point measurements of methane emissions in MSW landfills yield values that vary over seven orders of magnitude (from 0.0004 to 4,000 g/m2d; Bogner et al., 1997).
Bonaparte et al. (2002) reported systematic problems with gas emissions for landfills with compacted clay liner covers attributed to cracking in the clay due to desiccation and settlement. Comparisons of measured cover emissions and of methane recovery rates (from landfill gas collection systems) for a single compacted clay liner (1 m thick), single GCL, and a single geomembrane liner were reported by Spokas et al. (2006). All of the barrier materials were covered with 300 mm of topsoil. The GCL was underlain by a sand layer, and the geomembrane was underlain by a gravel layer. The study was conducted at three landfills in France in test cells that were filled with similar wastes but capped with the different cover configurations. The emissions were significantly higher and the recovery rate was significantly lower for the GCL cover than for the other covers.
In laboratory studies, Vangpaisal and Bouazza (2004) showed that GCL gas permeability was sensitive to variations in water content in the GCLs. Decreases of five to seven orders of magnitude in the gas permeability of the GCLs at water contents above approximately 70 percent were measured. Decreases were generally lower for GCLs tested subsequent to hydration under unconfined conditions than if hydrated under confined conditions (σ = 20 kPa), but gas permeability still decreased by several orders of magnitude. Gas permeability was also affected by GCL structure (needle-punched, stitch-bonded, etc.) and the form of bentonite (powder or granular), with higher permeabilities obtained for stitch bonded and granular GCLs. These findings suggest that, when GCLs are used in covers, they should
have a sufficient thickness (900 mm or greater) of soil above them to provide sufficient overburden pressure to minimize the effects of freeze/thaw or desiccation.
Data on the effectiveness of evapotranspirative cover systems in controlling gas migration are sparse, but these types of covers are expected to be more transmissive to gas transport than systems containing compacted clay layers, hydrated GCL, and geomembrane barrier materials. However, both gas transport potential and gas generation rates must be considered in assessing the effectiveness of alternative covers for gas migration control. Furthermore, methanotropic bacteria can oxidize methane in vegetated soil covers (Bogner at al., 1997; Liptay et al., 1998; Borjesson et al., 2001; Park et al., 2005). Comparison of emissions from a Florida landfill with a lightly vegetated cover over relatively young wastes, a heavily vegetated cover over relatively old wastes, and a daily cover with no vegetation on fresh wastes indicated that emissions were highest from the lightly vegetated cover and lowest from the highly vegetated cover (Abichou et al., 2006). Emissions were generally higher in flat areas than in sloped areas. In addition, emissions were relatively uniformly distributed across the lightly vegetated cover but were generally localized to defects (cracks) in the thickly vegetated cover. Methane oxidation was also higher in the thickly vegetated cover than in the lightly vegetated cover (Abichou et al., 2006). The low emission rates in the nonvegetated areas may be related to periodic scarifying and recompacting of the soil cover in these areas, a common operational practice at landfills.
In the long term, gas-generating waste in a landfill will degrade to the point where it no longer generates significant quantities of gas (from an environmental protection standpoint). The length of time required for this stabilization to occur depends on waste composition, climate, and landfill operational practices. For typical MSW landfills operated in compliance with EPA Subtitle D standards, the amount of time required to deplete 80 percent of the landfill’s gas generation capacity is expected to be approximately 30 years at a temperate site and 80 years at an arid site (Bonaparte et al., 2002). However, because the decomposition process decays exponentially with time for a homogeneous mass and because different sections of large landfills have varying decomposition rates, the remaining 20 percent of the degradation capacity may take significantly longer. Furthermore, construction of a cover system that essentially prevents percolation of moisture into the waste can slow and virtually halt degradation. However, any subsequent breach of the cover system that allows renewed infiltration can restart or accelerate the degradation process. The long duration over which gas generation remains an environmental protection consideration is part of the motivation behind research to intentionally add oxygen and liquids to MSW landfills to accelerate decomposition processes.1
Deterioration of Cover Systems
A smaller but unknown subset of the landfills surveyed by Bonaparte et al. (2002) had a final cover system that had been constructed and in operation for more than several years. Of the 79 landfills for which problems were reported, 24 of the problems were related to performance of the cover system. Of these problems the vast majority (18) were caused by slope failures, mainly related to rainfall events. In addition, two instances of erosion were identified: one of a topsoil layer and one of an erosion control mat, both of which were on long, 3H:1V slopes. Two problems were caused by differential settlement; in one case, settlement caused high-density polyethylene geomembrane boots (sleeves that maintain a seal between a geomembrane and a through-passing pipe) to tear where gas wells penetrated through the cover. In the other case, settlement caused GCL panels to separate at the overlapped seams. Finally, two problems were caused by construction. Deterioration also led to increases of one to two orders of magnitude in the saturated hydraulic conductivity of the clay barrier layer within 5 years of placement (Benson and Khire, 1997; Albrecht and Benson, 2002; Benson et al., 2007; EPA, 2007).
Temperature variations have significant effects on cover systems because the soil components are susceptible to cracking from desiccation and freeze/thaw cycles. High temperatures can affect the durability of geosynthetic components in covers, and thermal gradients can affect the transport of liquids and gases through cover systems. The direction of heat flow through cover systems varies with the seasons; upward heat flow occurs in winter and downward heat flow occurs in summer. Exceptions may occur for covers in locations with extreme hot or cold climates. Measured thermal trends in a cover system at an MSW landfill in Michigan are illustrated in Box 5.5.
Leakage Through Vertical Barriers
The primary function of most vertical waste containment barriers is to control the lateral subsurface migration of liquids, either hazardous liquids themselves or contaminated groundwater. The performance of vertical walls used to contain contaminated soils and groundwater was assessed at 24 sites across the United States, most of which had been operational for less than 10 years (EPA, 1998). The contaminants at these sites were hazardous in nature and included heavy metals, volatile organic compounds, and polycyclic aromatic hydrocarbons. Seventeen of the sites included active containment systems, which maintained an inward gradient across the barrier walls. The other seven sites (usually the oldest systems) had passive containment systems with essentially no gradient across the walls. The wall types
For example, see <http://www.epa.gov/garbage/landfill/bioreactors.htm>.
Case History on the Temperature Environment for a Composite Cover System
This case history illustrates how temperatures in a cover system vary with time. Data were collected from a Michigan landfill with a cover system that included (from top to bottom) an approximately 900-mm-thick vegetative and protective soil layer, a geotextile-geonet composite, a geomembrane, and an approximately 900-mm-thick compacted clay layer. Temperatures have been measured through the cover profile and into the underlying soil foundation layer and top of the waste layer using thermocouple sensors (the depth of each sensor is presented in the legend in Figure 5.7) since 2002 (Yesiller and Hanson, 2003; Hanson et al., 2005b). Temperatures in the cover system followed seasonal variations and demonstrated phase lag and amplitude decrement with depth. The temperature of barrier components (geomembrane and compacted clay) varied seasonally by over 15°C. The average absolute thermal gradient was over 20°C/m at the location of the compacted clay component of the cover system.
included soil-bentonite (21 sites), cement-bentonite, clay, and vibrating beam walls. Performance was evaluated mainly using groundwater quality and hydraulic head criteria. The study found that 83 percent of the 20 sites met design objectives (e.g., wall hydraulic conductivity and compatibility, wall thickness, key-in depth) and performed satisfactorily. The most significant factor in poor performance was leakage near areas where the walls are keyed into underlying low-permeability barrier layers. The leakage was attributed to poor construction of the keys. Data from four sites with seepage cutoff walls indicated that all of the walls constructed using soil-bentonite, concrete, and plastic concrete performed their intended functions well (EPA, 1998). A case history illustrating field performance of a vertical barrier is presented in Box 5.6.
Few problems with vertical waste containment barrier walls have been reported. However, this may simply reflect the paucity of data, either good or bad, on the actual performance of these walls. Construction defects in soil-bentonite and soil-cement-bentonite slurry walls, nonuniformities that develop during their placement, and subsequent settlement of wall materials could all lead to zones of higher hydraulic conductivity and increased contaminant transport. In the case of concrete panel walls, geosynthetic panel walls, and sheet pile walls, improperly filled joints between adjacent panels can be sources of leakage. Inadequate seals or keys into natural barriers beneath the wall can also be a problem for all types of vertical barrier walls.
Extraction trenches and vertical walls also serve as vertical barriers in some containment systems. However, the
Case History on Vertical Wall Performance
This case illustrates how, under the right conditions, a gravel-filled trench can serve as an effective vertical barrier against offsite migration of hazardous and toxic wastes. About 75 million liters of liquid hazardous and toxic industrial wastes were disposed of at the Hardage Site near Oklahoma City, from 1972 to 1980. Following Superfund designation of this site and a court-ordered excavate, incinerate, and reentomb remedy for its remediation, the Hardage Steering Committee, a consortium of the potentially responsible parties for the site, developed an alternative remedy that it believed would better protect the environment and be more cost effective than the EPA remedy. A trial before a federal district judge in late 1989 resulted in a decision in favor of the steering committee’s remedy.
The Hardage disposal area is underlain by fine-grained sandstones and siltstones, which are sufficiently strong to allow deep vertical trenches without lateral support. Remediation actions included construction of an 820-m-long, 20-m-deep (on average), 0.9-m-wide gravel-filled trench keyed into an underlying low-permeability layer to intercept contaminated fluids and a low-permeability composite cap over the disposal area. The gravel-filled interceptor trench, a unique feature of this waste containment system, is shown in plan view in Figure 5.8 and in cross section in Figure 5.9. Originally a plastic concrete (cement bentonite) cutoff wall was planned. However, EPA concerns about the possibility of fractures in the Stratum III rock, the buildup of fluid pressure against the cutoff wall, and leakage through the wall led to adoption of the V-shaped gravel-filled interceptor trench.
Numerical modeling of groundwater flow was used to establish the trench location. The trench bottom is sloped to a series of liquid recovery sumps, with pumps for removal of the captured flow. The performance of the trench segments is monitored by water level measurements in the recovery sumps and piezometers located along the trench between the sumps. The site remedy has been operational since September 1995. The First Five Year Review (in 2002) indicated that all immediate threats at the site had been addressed and that the remedy components are expected to continue to protect human health and the environment (EPA, 2002b). The use of a cutoff trench appears to be effective as a cutoff wall for containment of liquid wastes at this site. It offers the additional advantages that liquids reaching the trench can be removed and treated, quantities are known, and an inward gradient is maintained.
committee was unable to find published data on the performance of these systems. At a visit to the Love Canal site in New York, the committee was told that the vertical extraction trench used to control contaminant migration at the site was performing well but that no published data were available. Since extraction trenches and wells are active systems, their long-term performance relies on continued operation of the associated pumping system as well as the ability of the trench or well to resist clogging.
PREDICTING THE PERFORMANCE OF BARRIER SYSTEMS
Prediction of the properties of the components of engineered barrier systems was discussed in Chapter 4. This section describes predictive models of how the individual components of the system interact to contain contaminant transport. Both elements are necessary in order to adequately predict containment system behavior.
Predicting the Performance of Liner Systems
Assessing Leakage Through Smooth Composite Liners
Leakage through composite liners can be calculated using empirical equations, analytical equations, or numerical analysis (Rowe, 2005). The analyses assume that holes are present in geomembrane components of the barrier systems. Empirical equations are established by curve-fitting families of solutions from analytical equations (Giroud and Bonaparte, 1989; Giroud, 1997); example results are provided in Table 5.3. Analytical equations involve either the assumption of perfect contact (Rowe and Booker, 2000) or lateral migration in a transmissive zone below the geomembrane combined with one-dimensional flow into the underlying soil (Jayawickrama et al., 1988; Rowe, 1998; Touze-Foltz et al., 1999, 2001a). Numerical methods allow modeling of actual three-dimensional conditions near a hole (Foose et al., 2001) or the complete variability of the interface topography if it is known (Cartaud et al., 2005a).
The assumption of perfect contact between the geomembrane and the subsoil can provide a lower bound to the leakage through holes in a geomembrane over a compacted clay liner or GCL. Assuming there is a uniform transmissive zone between the geomembrane and clay liner, Rowe’s (1998) direct contact analytical equation or Giroud’s (1997) empirical charts can be used to calculate leakage. The assumption of one-dimensional flow in the liner is not correct, however. Nonetheless, for a geomembrane over a GCL, Rowe’s direct contact analytical solution provided good agreement with the results of numerical analysis for typical reported transmissivity values (Foose et al., 2001). Similarly, Rowe (2005) found
TABLE 5.3 Representative Leakage Rates for Single Geomembrane and Composite Liners
Steady-State Leakage Ratea (lphd)
Hole Frequency (per hectare)
Single Geomembrane Linersb
100 to 100,000
1 to 100
1,000 to 1,000,000
1 to 1,000
10,000 to 10,000,000
10 to 10,000
aFor range of hole diameters from 1 to 10 mm and hydraulic heads from 0.1 to 10 m.
bAssumes steady state flow through an orifice with a coefficient of discharge of approximately 1.
cFor an underlying clay layer that has a thickness of 0.915 m, a hydraulic conductivity of 1 × 10−9 m/s, and a freely draining boundary at its base. Assumes that holes in the geomembrane do not extend through the underlying clay liner, a gap with a uniform thickness between the geomembrane and the clay of 0.01 to 0.05 mm (i.e., no wrinkles), and a no-flow boundary equidistant between holes.
excellent agreement (error of less than 4 percent) between the direct contact analytical solution and an axisymmetric numerical analysis for interface transmissivities within the practical range of interest for both a GM-GCL composite liner and a GM-CCL composite liner.
Both the numerical and analytical solutions assume a uniform transmissivity of the interface. Analytical solutions have also been developed for regular variations in the transmissivity of the interface (Touze-Foltz et al., 2001a), and numerical methods are available to model more complex situations (Cartaud et al., 2005b). Although these may be useful in interpreting laboratory tests where the actual interface topography is well defined, in practical situations the interface topography will vary significantly (e.g., as is evident from the work of Cartaud et al., 2005a, 2005b) and is unknown at the location of the (assumed) holes used in design calculations. Thus, the more simplified approaches used in conjunction with a range of likely transmissivities will provide the information needed for design purposes provided there are no significant wrinkles in the geomembrane. If there are wrinkles, as is often the case, predictions made using simplified approaches will not be consistent with the observed leakage and the likely number of holes/ha for either GM-CCL or GM-GCL systems (Rowe, 2005).
Assessing Leakage Through Composite Liners with Wrinkles
Wrinkles in a geomembrane exacerbate the effect of holes on leakage rates if the wrinkles coincide with the holes (Rowe, 2005). The potential for contaminant migration increases through a hole in the geomembrane at or near the wrinkle. Future holes are also more likely to develop due to stress cracking at points of high tensile stress in the wrinkle. Wrinkles in a geomembrane arise both during construction and, in particular, from thermal expansion when the geomembrane is heated by the sun after placement. These wrinkles do not disappear when the geomembrane is loaded (Stone, 1984; Soong and Koerner, 1998; Brachman and Gudina, 2002). Pelte et al. (1994) reported wrinkles that were 0.2 to 0.3 m wide and 0.05 to 0.1 m high at a spacing of 4 to 5 m. Touze-Foltz et al. (2001b) reported wrinkles that were 0.1 to 0.8 m wide and 0.05 to 0.13 m high at a spacing of 0.3 to 1.6 m. Rowe et al. (2004) report a case of 1,700 wrinkles/ha.
Validating Models for Leakage Through Liner Systems
Techniques for calculating leakage through composite liners can be validated using data from landfills with leak detection systems. Predictions from equations that assume holes are in direct contact between the geomembranes and the underlying clay liners (Rowe, 2005) were compared with leakage rates reported by Bonaparte et al. (2002). The comparison showed that 20 to 30 holes/ha (with a hole radius of 10 mm and depth of fluid above the geomembrane of 0.3 m) would be required to match the observed leakage rates if contact with the compacted clay liner (GM-CCL) is poor (as defined by Giroud and Bonaparte, 1989) and that 90 to 100 holes/ha would be required if contact with the compacted clay liner is good. Many more than 100 holes/ha would be required to explain the maximum flow observed. For composite liners with a GCL (GM-GCL), about 40 to 100 holes/ha would be required to explain the typical range of flow values (Rowe, 2005). The maximum observed leakage was more than an order of magnitude greater than the predictions of leakage from 100 holes/ha. Although this leakage could be explained by a large number of holes, the number of holes predicted by these calculations is much higher than can be reasonably expected. Thus, the use of calculations that assume direct contact (or even poor contact) between geomembranes and clay liners and that do not explicitly consider wrinkles is not appropriate for estimating leakage through composite liners unless the landfill is constructed with negligible wrinkles. Wrinkles are common in North American landfills. The extent of wrinkles has recently been illustrated by Chappel et al. (2007).
When holes in wrinkles are factored into the calculation, the typically observed leakage could be readily explained by 12 to 22 holes in 10-m-long wrinkles/ha or as few as 1 to 3 holes in 100-m-long wrinkles/ha (Rowe, 2005). (The length of a wrinkle is the total linear distance that fluid can migrate along a wrinkle and its interconnections.) A similar) A similar comparison for a geomembrane over a GCL indicated that the observed leakage could be explained by 2 to 3.5 holes in 10-m-long wrinkles/ha, while the maximum leakage can be explained by about 5 holes in 100-m-long wrinkles/ha. Thus, the typical observed leakage for composite liners with both compacted clay liners and GCLs can be readily explained by holes in wrinkles for the typical number of holes/ha and reasonable combinations of other parameters (Rowe, 2005).
Leakage is a function of the size and number of holes (especially in wrinkles, as discussed above) and the head on the liner. The head on the liner depends on the rate at which leachate is pumped from the primary leachate collection system and the hydraulic conductivity of the drainage layer. A numerical model was developed by Gilbert (1993) to predict the rate of leakage through a liner system as a function of the pumping rate from the primary collection system versus time (as a proxy for the infiltration rate into the drainage system from the overlying waste) and the geometry and configuration of the leachate collection system and liner. The side slopes, base slopes, pipes and sumps, and the properties of the materials in the liner system were taken into account. This model was calibrated with measured pumping rates for double-liner systems at 16 hazardous waste landfill cells. The leakage rate through the liner system was approximately proportional to the square root of the flow rate into the leachate collection system. The average proportionality constant was about (e.g., a flow rate of 5,000 l/day into the collection system produced an average leakage rate of about 50 l/day into the leak detection system).
The above model results show that to obtain accurate predictions of leakage through composite liners it is necessary to take into account holes in wrinkles and elevated head.
Diffusive Contaminant Transport Through Liner Systems
Contaminant transport via diffusion has been well documented. Examples include the migration of chloride to a depth of about 0.75 m in 4.25 years (Reades et al., 1989), the migration of volatile organic compounds up to 0.6 meters in about 4.25 years at the Keele Valley Landfill (Barone et al., 1993), and the migration of heavy metals less than 0.1 m in 15 years at the Confederation Road Landfill (Yanful et al., 1988). The extent of diffusion through thick clay deposits over a period of 10,000 to 12,000 years has been successfully predicted by a number of investigators (e.g., Quigley et al., 1983; Desaulniers et al., 1981; Rowe and Sawicki, 1992) using the diffusion-advection equation (where diffusion dominates). Diffusion of contaminants through clay liners beneath waste has also been successfully predicted (Rowe, 2005).
A composite liner containing a 1.5-mm high-density polyethylene geomembrane over a 3-m-thick compacted clay liner was investigated after 14 years of use as a leachate lagoon liner (Rowe, 2005; Lake and Rowe, 2005b). The geomembrane had no overlying protection layer. Inspection at decommissioning revealed numerous holes in the geomembrane, and cores through the compacted clay showed that contaminants had diffused about 1.7 m in 14 years. Observed and predicted diffusion rates in the clay would match if it is assumed that the geomembrane failed in the first 4 years of operation. A more positive example of composite liner performance was reported by Rowe (2005) for two test sections at the Keele Valley Landfill. For the section with only a compacted clay liner, a clear diffusion profile is evident with ion migration to a depth of more than 1.1 m (data limited by the depth of the lowest monitor at 1.1 m) over 12 years. The composite-lined section showed no evidence of a concentration profile for ionic species over the same time period, and the measured conductivity was representative of background values. Both findings are consistent with predictions based on laboratory-determined parameters. This suggests that (1) there is negligible advective flow (leakage) through the geomembrane near the conductivity sensors, and (2) there has been negligible diffusion of ionic species through the geomembrane in 12 years.
Overall, the available data suggest that current techniques for predicting diffusive contaminant transport give reasonable results when compared with observed field behavior. Consideration needs to be given to diffusion in well-designed and well-constructed barrier systems where advective transport (leakage) is small.
Vertical Barrier Performance Modeling
The performance of vertical barriers can be modeled using two-dimensional solutions for advective and diffusive flow. Advection and diffusion can be modeled separately or together using differential equations for combined advective diffusive flow (Krol and Rowe, 2004). There are a limited number of closed-form solutions to these equations, and most solutions used in practice today are numerical. They are generally based on the assumption of saturated flow. The required input parameters include the geometry of the problem, the material properties (e.g., saturated hydraulic conductivity, diffusivity of the materials with respect to the migrating compounds of interest, sorption/desorption coefficients, reactive flow parameters), and the boundary conditions (e.g., hydraulic heads or fluid flux, chemical concentration or chemical compound flux). Furthermore, changes in material properties and boundary conditions with time are required to predict long-term performance. These changes with time are perhaps the most difficult parameters to evaluate given the sensitivity of the material properties to environmental impacts, including chemical concentrations in the pore fluid, deformations of the barrier system, and temperature. Moreover, diffusive and advective flow modeling of cementitious vertical barriers is further complicated by the difficulty of separating the material/physical coefficients from the chemical coefficients. Studies show that, while it is feasible to predict contaminant transport through vertical barrier walls around contaminated sites (e.g., Krol and Rowe, 2004), few field data are available to evaluate the actual performance of the walls that have been constructed to date.
Predicting the Performance of Covers
Performance evaluations of waste containment covers are generally based on predictions of the amount of water that
percolates through the cover. Percolation through the cover is the primary performance index because this water migrates into and possibly through the underlying waste, generating gas, solid waste leachate, and/or acid drainage that can lead to ground and surface water contamination or other adverse environmental impacts. Percolation is commonly predicted from a water balance analysis that includes processes such as surface runoff, evaporation, transpiration, internal lateral drainage within the cover or intralayer flow, and soil water storage.
Cover performance assessments also can be based on the gas flux. A gas flux criterion may be important, for example, when minimizing the ingress of oxygen into sulfidic mine tailings would prevent or minimize the generation of acid drainage, when preventing the egress of methane gas generated from biodegradation of solid wastes, or when minimizing the egress of radon gas from radioactive waste materials, such as uranium mine tailings (Shackelford, 1997).
The performance of cover systems also depends on the system integrity. Cover system integrity is maintained by providing sufficient resistance to wind and surface water erosion, providing adequate stability for the cover on side slopes, minimizing the amount of differential settlement of the cover to prevent excessive cracking and leakage, and minimizing the effects of environmental distress of the cover, such as desiccation cracking due to wet/dry cycles and/or freeze/thaw cycles.
Models for Predicting Cover Performance
Some of the models typically used to predict the percolation performance (i.e., water balance) of cover systems are listed in Table 5.4. The most widely used model is probably the Hydrologic Evaluation of Landfill Performance (HELP) model (Schroeder et al., 1994). Unlike most other water balance models, HELP assumes unit gradient flow and an unsaturated hydraulic conductivity that varies with water content in accordance with Campbell’s equation for all layers except “barrier layers.” For clay barrier layers, a saturated hydraulic conductivity is used and the hydraulic gradient is computed based on the depth of liquid pooled on the surface and an assumed pore water pressure of zero at the base of the clay layer. The gradient is set equal to zero when no water is pooled on the surface of the clay barrier. Composite barriers are simulated using a Giroud-type equation. This relatively simple assumption for barrier layers in HELP, together with relatively simple algorithms for routing the water balance, minimizes data input requirements and shortens computational times but sacrifices accuracy and versatility (flexibility) with respect to evaluating the
TABLE 5.4 Models for Predicting Percolation Performance of Waste Containment Covers
different factors influencing cover performance (Khire et al., 1997).
Comparisons of models with field-measured water balance data have shown that the water-routing algorithms incorporated in HELP do not accurately simulate the complex hydrodynamics of landfill covers (e.g., see Table 5.5). Accurate predictions of water balance in covers typically require algorithms that use much more sophisticated models for unsaturated flow that are based on Richards’s equation and that use nonlinear functions to describe the distribution in unsaturated hydraulic conductivity (e.g., the van Genuchten-Mualem function) and soil water characteristic curves (e.g., Brooks-Corey or van Genuchten functions). All of the water balance models listed in Table 5.4 except HELP employ these more sophisticated unsaturated flow equations and functions and evapotranspiration models. Of course, the greater data input requirements and the longer computational times (days or weeks in some cases) make the use of these more sophisticated models less desirable for many practical applications.
Overall, the available evidence suggests that the HELP model provides relatively poor predictions of the performance of water balance systems, such as earthen final covers. Accurate predictions of the performance of water balance systems require more sophisticated models with greater input data and possibly longer run times.
Issues and Limitations
The ability to predict the performance of waste containment covers is limited by two problems in particular: (1) the existence of time-varying properties and processes (e.g., climate, vegetation, soil) and (2) the role of heterogeneities on flow through cover systems (Sleep et al., 2006). Because covers are exposed to the environment and are under relatively low confining stresses, they are susceptible to the impacts of surface and climatic processes. All models of cap performance require climatic data, including precipitation, temperature, and solar radiation to determine infiltration and evapotranspiration. Although historical data are available for many locations, methods for estimating extreme values of these variables are not well developed.
Physical deterioration must be considered when modeling cover percolation (e.g., see Benson et al., 2007). Changes in surface vegetation affect surface runoff, erosion, and evapo-
TABLE 5.5 Selected Studies Comparing Predicted and Field-Measured Performance of Water Balance Systems
transpiration. Covers are (1) penetrated by roots or burrowing animals, resulting in the generation of highly conductive pathways for water infiltration; (2) cracked because of settlement; and (3) desiccated through environmental stresses, such as freeze/thaw cycles and wet/dry cycles (e.g., Albrecht and Benson, 2001). At present there is no reliable way to predict the occurrence or effects of such time-dependent changes in material properties and processes.
Finally, a capability to predict the occurrence and impact of local heterogeneities in soil on the flow through cover systems does not yet exist. Most predictions are based on models that assume the properties of each soil layer in a cover system are homogeneous. However, the existence of local heterogeneities resulting from compaction, settlement-induced cracking, and desiccation can result in significant differences between predicted and actual performance.
Predicting Gas Transport Through Containment Systems
Transport of gases through individual barrier materials as well as through barrier systems in bottom and cover liners and vertical barriers has received far less attention than the transport of liquids through these materials and systems. In general, it is assumed that the trends in hydraulic conductivity apply broadly to gas conductivity (i.e., low hydraulic conductivity indicates low gas conductivity) and that conditions and mechanisms that change hydraulic conductivity cause a comparable change in gas conductivity. Advective transport of gas is expected to control gas movement through porous materials (e.g., leachate collection layers), and diffusive transport is expected to be the dominant mechanism for geomembranes. For low-permeability soil barrier materials (e.g., compacted clay liners, GCLs), the dominant gas transport mechanism is not clear and may depend on the degree of saturation of the material. Although laboratory tests have been reported on gas transport in porous barrier materials (e.g., Izadi and Stephenson, 1992; Vangpaisal and Bouazza, 2004), these data are not normally collected. Water and solvent vapor transmission rates for geomembranes are reported by Matrecon, Inc. (1988), and diffusive flux of methane for high-density polyethylene is reported in Spokas et al. (2006). Regulations require emissions data (e.g., 500 ppmv for methane), not gas conductivity or diffusion rates for individual barrier materials or barrier systems (EPA, 2005).
As discussed above, an effective cover (with respect to liquid percolation) does not necessarily ensure that gas will not escape; the details of the leachate collection/removal system are important to make sure the gas does not bypass the containment system.
Predicting the “Active” Lifetime of Waste
The active lifetime of waste in a landfill (i.e., the time span over which the waste can actively generate gas or leachate with potentially harmful constituents) is an important factor in evaluating the long-term performance of engineered barriers, not only because it defines the desired service life of the containment system but also because gas and leachate can interact with containment system components and thereby affect their longevity. Analysis of leachate and gas data from 50 municipal solid waste landfills in Germany with ages up to 30 years indicated that concentrations of individual leachate constituents rather than gas generation controlled the duration of the postclosure care period (Kruempelbeck and Ehrig, 1999). Extrapolations of temporal variations of measured leachate quality in the 50 German landfills as well as other studies suggested that postclosure care periods were highly variable (<10 to 1,700 years).
The active life of other waste types may be significantly different than the active life of municipal solid waste. The active life of hazardous and low-level radioactive waste is typically on the order of centuries to a thousand years (GAO, 2005; NRC, 2006). For hazardous waste remediation sites, however, the active life may vary from tens of years (e.g., when capping is an interim solution accompanied by source control measures) to hundreds or thousands of years (e.g., for dense nonaqueous-phase liquids in fractured rock strata) and must be determined on a case-specific basis.
In summary, landfills, especially some of the larger ones, are likely to require attention for centuries, not decades.
Local and Global Slope Stability
The integrity and performance of landfill liner, cover, and vertical barrier systems can be affected by both global and local slope stability. Of the 85 problems with landfill containment systems identified in Bonaparte et al. (2002), 14 percent involved liner instability and 21 percent involved cover system instability. Stability issues associated with vertical barrier construction are discussed in Filz et al. (2004). Stability failure of a barrier system can be defined in terms of two different performance states:
complete loss of stability or function (e.g., waste slope or slurry trench side wall failure), also known as a global stability failure; and
impairment of a structure’s function (e.g., deformation of a landfill liner leading to loss of function, local soil slumping during slurry trench excavation for a vertical barrier), also known as a local stability failure.
Global stability considerations include static and seismic stability of the foundation, waste mass, and cover systems for landfills (Mitchell and Mitchell, 1992) and side wall stability for vertical barriers (Fox, 2004). Figure 5.10 illustrates different global stability modes for landfills. A global stability failure in a landfill is likely to breach any barrier layer that it crosses. A global stability failure during construction of a vertical barrier system will cause the excavated trench to
close, and a failure after construction would likely breach the barrier layer. Even if the barrier is not breached, a global failure may significantly diminish its capacity to withstand external loads (e.g., soil slipping off of a geosynthetic barrier layer, exposing it to ultraviolet radiation and other external loads). Generally, limit equilibrium stability analyses are conducted to determine a factor of safety against various modes of global slope stability failure. Barrier layers may be particularly susceptible to global stability failure as they create a planar surface along which the shear strength may be less than that of the material on either side.
Perhaps the best-known global landfill failure is that of the Kettleman Hills hazardous waste landfill in 1988, described in Box 5.7. Kettleman Hills was the first double-lined landfill constructed in the western United States, and procedures for landfill stability analyses were not yet well developed. Back analyses of the Kettleman Hills case history (Seed et al., 1990) indicated that a critical factor contributing to the failure was the low shear strength of the interface between the geomembrane base liner and the underlying compacted clay soil. Ironically, the composite geomembrane low-permeability soil barrier system developed to minimize advective transport of contaminants from the landfill provided a plane of weakness along which the failure surface developed. Subsequent to the Kettleman Hills failure, evaluation of global stability along barrier system interfaces using the results of laboratory interface shear testing became standard practice for the design of geosynthetic barrier systems. However, global stability failures along geosynthetic interfaces continued to occur in both cover and liner systems because
of inadequate attention to slope stability in design, construction, and operation.
An important set of field performance data for interface stability was obtained from 14 full-scale test plots with GCLs in their covers in Cincinnati, Ohio, in 1994 (Daniel et al., 1998). Test plots were constructed using a variety of GCL cover configurations at inclinations of 3H:1V and 2H:1V. Based on limit equilibrium analyses using the laboratory interface shear test data, all of the 3H:1V test plots were expected to be stable, but some of the 2H:1V test plots were expected to be unstable (i.e., a factor of safety of less than 1.0). Field performance of the test plots was in substantial agreement with the stability analyses based on the laboratory interface shear data. Cover configurations with a factor of safety of less than 1.0 failed, while all but one that was predicted to be stable remained stable. The one exception
Case History of Stability Failure at Kettleman Hills Landfill
This case history illustrates the importance of considering stability in designing engineered barriers that perform effectively. Landfill B-19, Phase 1A, was a hazardous waste landfill located near Kettleman City, California, operated by Chemical Waste Management, Inc. Construction of the landfill was completed in February 1987, and approximately 450,000 m3 of waste had been placed into the cell at the time of failure in March 1988. The aerial photograph in Figure 5.11 shows the landfill just prior to the failure. The waste was placed to a height of about 28 m on a relatively flat base surrounded by excavated side slopes. The barrier consisted of a double-liner system on the side slopes and base; the primary liner on the base was a composite liner, and the secondary liner on the base was underlain by a tertiary liner system (i.e., a vadose zone monitoring system). On March 15, 1988, the entire mass of waste slid horizontally about 10 m away from the side slopes.
The slope failure caused extensive damage to the liner system. Figure 5.12 shows one of the side slopes after the failed waste mass was removed from the landfill. The waste mass slid along interfaces in the barrier system and ripped through the primary collection system, the primary liner, and the secondary collection (or leak detection) system. It cost tens of millions of dollars to repair the damage. However, there was no release of waste from the barrier system due to substantial redundancy; both the secondary and tertiary liners on the base remained intact and contained the waste even after the failure.
Failure occurred primarily because the possibility of a stability failure during the waste placement period was not taken into account: The shear strengths for interfaces between materials in the barrier system were not measured, and the stability calculations were simplistic. Analyses of the failure showed that several of the interfaces had very low shear strengths and exhibited strain-softening behavior, meaning that once equilibrium was lost there was the potential for large movement before the waste mass regained stability (Mitchell et al., 1990; Seed et al., 1990; Byrne et al., 1992; Stark and Poeppel, 1994; Gilbert et al., 1998).
was a test plot in which stability was predicted from the assumption that the bentonite in the GCL would not hydrate because it was encapsulated by geomembranes. Postfailure investigation indicated that the bentonite in the GCL hydrated sufficiently to lower the factor of safety to less than 1, possibly because of construction defects in the test section. A subsequent test section constructed at the same location to remove this problem remained stable throughout the test period.
Based on studies following the Kettleman Hills failure, the Cincinnati GCL test plots, and similar investigations, procedures for evaluating global interface stability are now well established in engineering practice. With proper study and analysis, it should be possible to avoid short-term global interface stability failure for most waste containment systems. All of the stability problems identified in Bonaparte et al. (2002) were attributable to design or construction errors. Significant uncertainties, however, remain about the long-term performance of containment system elements (e.g., the long-term durability of reinforced GCLs, the long-term performance of leachate collection and removal layers and cover system drainage layers), as well as about certain aspects of waste shear strength (e.g., the shear strength of saturated waste in a bioreactor landfill, the shear strength of waste held in containers in a hazardous waste landfill).
Local instability considerations may be important when large deformations adjacent to or across landfill barrier systems are anticipated (e.g., in side slope liner systems of MSW landfills). Local instability during vertical barrier construction is generally associated with local collapse of small underground sections of the excavated trench prior to back-filling, leading to “windows” of native material within the vertical barrier. Figure 5.13 illustrates how waste settlement can lead to loss of the integrity of geosynthetic liner system elements on shallower slopes and cracking and bulging of the low-permeability soil liner on steeper slopes, even when the global stability is adequate. Relatively complex numerical stress deformation analyses are generally required to assess local stability. Jones and Dixon (2005) used numerical modeling to investigate the local stability of a side slope liner system in response to waste settlement. However, while sophisticated analyses may be required to predict these effects, simple engineering measures can often be used to mitigate local instability risk (e.g., use of a “slip sheet” above a side slope liner to mitigate downdrag on the liner due to waste settlement). Although local stability assessments are not generally employed in engineering practice and monitoring systems are generally not designed to identify local stability failures, analyses suggest that local instability may affect the integrity of subsurface barrier systems in steep-sided land-
fills and other facilities where large deformations adjacent to or across the barrier system are anticipated.
Impact of GCL Strength on Stability
The internal shear strength of GCLs is of particular concern for local and global stability of landfills because of the potential for low strengths upon hydration of the bentonite. Reinforcement was introduced into GCLs as a means of mitigating this problem. Most modern reinforced GCLs used in waste containment employ needle-punched reinforcement. Reinforced GCLs have high peak strengths but relatively low postpeak shear strengths at large displacements (75 mm or larger). The postpeak in-plane strength of hydrated GCLs is likely to be the lowest in-plane strength in a barrier system that employs a GCL (Bouazza et al., 2002). Even when the GCL is reinforced, the postpeak shear strength of a hydrated GCL in a landfill liner system may be represented by a friction angle on the order of 8 to 10 degrees. However, strengths this low are likely to be associated with higher overburden pressures (i.e., pressures representative of base liner conditions), where the interface plane of weakness often makes a transition from the geomembrane-GCL interface to an internal GCL failure plane (Sharma et al., 1997).
As discussed by Gilbert (2001), interface failures generally occur along the interface with the lowest peak strength in the barrier system. Therefore, if the peak strength of the GCL is higher that the peak strength of another interface in the liner system, the postpeak strength of that other interface will govern stability. This suggests that the internal shear strength of a reinforced GCL will only affect stability in barrier systems where the peak strength of the GCL is the lowest peak strength in the barrier system (e.g., in liner systems at relatively high overburden pressures). This is consistent with the results of the Cincinnati GCL test sections (Daniel et al., 1998), wherein hydrated reinforced GCLs with 0.9 m of soil overburden slopes remained stable on 3H:1V and failed on 2H:1V slopes along geotextile-geomembrane interfaces rather than internally.
A new generation of reinforced GCLs with thermal-locked fibers has shown significantly higher postpeak shear strengths in laboratory tests than previous GCLs, with failure occurring at the geotextile-geomembrane interface over a wide range of overburden pressure, even when the GCL is hydrated under low overburden pressures prior to testing (Kavazanjian et al., 2006b). The peak strengths developed in these GCLs are high enough to suggest that in many cases a reinforced GCL may not impact stability even after hydration.
It is common practice to assume that a GCL deployed in the field will eventually hydrate and therefore to base the interface stability assessment of a barrier system that employs a GCL on the hydrated strength of the GCL. It has been suggested that GCLs can be encapsulated with two geomembranes to inhibit hydration and thus enhance the inplane shear strength of a barrier system in which stability is governed by the hydrated GCL shear strength (Giroud et al., 2004). Results of the Cincinnati GCL test section indicate that, with proper attention to design details, encapsulation can inhibit hydration and enhance the shear strength of the GCL, at least in the short term. Although calculations suggest that encapsulation can inhibit hydration for hundreds to thousands of years (Giroud et al., 2004), encapsulated GCLs in liner and cover systems in a number of landfills (Kavazanjian et al., 2006b) have not been used long enough to confirm the calculations.
The long-term stability of GCLs may be affected by degradation of the reinforcement. The physical and chemical degradation processes for polypropylene and polyethylene fibers that are used in needle-punched and stitch-bonded reinforced GCLs were studied by Hsuan and Koerner (2002). They suggested possible performance and index test methods for monitoring the polymeric degradation and concluded that, when GCLs are subjected to long-term shear stresses, fiber durability is important, particularly for sloping surfaces and
canyon-type landfill liners. Factors involved in fiber durability are stress level, environmental conditions (e.g., oxygen level), required lifetime, and polymer formulation. The key to the polymer formulation is the manufacturing process for the fibers and the type and amount of antioxidants.
Since the strength of GCLs is a function of the strength of the reinforcing fibers, an understanding of the long-term behavior of the GCL reinforcement is important. Thomas (2002) tested the long-term oxidative stability of a polypropylene textile made from fibers used to reinforce a commercial needle-punched GCL and estimated that the reinforcing materials would retain 50 percent of their strength for 30 years when exposed to air at 20°C. However, when accounting for the effects of oxygen limitation, a service lifetime approaching 100 years for the reinforcing fibers has been estimated for buried applications (Salman et al., 1998). Lower service lifetimes can be expected under elevated temperature conditions, such as those that occur at MSW landfills.
Degradation of polymer and slow disentanglement of fibers can differentially compromise the strength of a reinforced GCL (Thies et al., 2002). Some needle-punched GCLs, in which the needle-punched fibers were thermally bonded to the carrier geotextile in an attempt to enhance the reinforcing fiber anchorage, actually failed sooner in elevated temperature aging tests. The enhanced anchorage resulted in failure accompanied by breaking of the reinforcing fibers from their anchoring, rather than simple disentanglement. In addition, GCLs made with polypropylene took 20 to 60 times as long to fail as those made from polyethylene. This result corroborated data from short-term peel tests that measure the strength required to peel the geotextile off of the bentonite core (Müller et al., 2004), demonstrating the need for long-term shear tests to properly assess the lifetime of GCLs.
In summary, field observations indicate that global stability can be a significant threat to the short-term integrity of liner and cover systems and that cover stability can be a concern in the medium term. Clogging or undercapacity of drainage systems on the side slopes of covers appears to be the most significant factor affecting the medium-term global stability of cover systems. No data exist on the long-term stability of modern liner and cover systems because few of these systems have been in place for more than 30 years. However, the long-term stability of liner and cover systems that rely on the strength of a geosynthetic element may become an issue as the polymers degrade with age. The long-term stability of side slope cover systems in seismically active regions may also be a significant concern, as unconditional stability of side slopes steeper than 5H:1V may not be attainable in areas of even moderate seismicity because of the high initial static shear stress acting on the slope and the potential for amplification of seismic motions at the landfill cover. The initial static shear stress on a 5H:1V slope can lead to yield accelerations as low as 0.1 g for typical geosynthetic interface friction angles in the absence of cohesion. Furthermore, amplification can result in peak accelerations in landfill cover as high as 0.3 g for earthquakes with maximum horizontal accelerations (free-field peak ground accelerations) as low as 0.1 g. Whenever the peak acceleration at the landfill cover exceeds the yield acceleration, the cover is not unconditionally stable and a seismic deformation analysis is required. Local stability of geosynthetic liner systems, particularly in steep-sided landfills, is an issue that is not often considered in landfill design. However, the high compressibility of municipal solid waste may impose significant loads on side slope liner systems, which can create local stability problems that impact the integrity of the containment system in the short and medium terms.
Modeling Concrete Barrier Performance
A variety of models have been developed to predict different aspects of concrete barrier performance. Models of concrete barrier performance must consider complex interactions among the physical properties of the barrier (e.g., permeability, porosity, crack structure) and advection and diffusion of contaminants (e.g., radioactive species, volatile organic compounds), chemical species that affect the barrier properties (e.g., sulfates, chloride), and transport media (solid, liquid, and gas phases). A one-dimensional model based on micromechanics theory and the diffusion-reaction equation can predict the expansion of mortar bars (Krajcinovic et al., 1992). Models have also been developed to address nonlinear diffusion-reaction conditions that lead to cracking, changes in diffusivity, and degradation of concrete subjected to external sulfate sources (Tixier and Mobasher, 2003a, 2003b). In the STADIUM model, chemical and physical phenomena are described by a general equation expressing the variation in concentration of ionic species through a permeable material (Marchand et al., 1999a, 1999b). Dissolution of portlandite and decalcification of calcium silicate hydrate (the “glue” that binds aggregate together in Portland cement concrete) are among the effects predicted by the model. Atkinson and Hearne (1989) developed a model in which the rate of spalling of a concrete barrier is expressed as a function of the elastic and fracture properties of concrete, its intrinsic sulfate diffusion coefficient, the external sulfate concentration, and the concentration of ettringite (an expansive mineral).
4SIGHT is used to model the degradation of buried concrete vaults due to advective and diffusive transport of sulfate and chloride ions (Snyder and Clifton, 1995; Snyder et al., 1995). The model predicts both the bulk hydraulic conductivity of the concrete and the structural integrity of the concrete and reinforcement. The model includes precipitation and dissolution of salts caused by changes in pore fluid pH as well as their impact on the porosity of the concrete. Input to the model includes the initial crack density and crack geometry in the concrete; the spacing of joints in the concrete and the permeability of any joint-filling compound; the properties of the concrete; and external ion concentrations, including
sodium, chloride, potassium, calcium, chloride, and sulfate. The key properties controlling the diffusion of chloride and sulfate ions through the concrete include the porosity of the concrete and the formation factor (the ratio of the electrical conductivity of the pore fluid to the bulk electrical conductivity of the concrete). The pore fluid conductivity is a function of the various ionic species present and their relative concentrations, which can be established from chemical equilibrium considerations.
The 4SIGHT model can be modified to make probabilistic predictions of concrete service life using Monte Carlo simulations (Snyder, 2001). The laboratory data used by Snyder to validate the probabilistic model had a maximum duration of less than 100 days, and field data that were sufficiently quantitative spanned only one or two decades. One advantage of the probabilistic approach is that it allows uncertainty about the properties of the concrete to be incorporated into the model. Therefore, while models exist for the complex interactions that govern concrete barrier performance, considering the relatively short duration for which field data are available, the ability to predict the degradation of PCC barriers for periods longer than several decades remains largely unproven.
PREDICTING THE OVERALL PERFORMANCE OF CONTAINMENT SYSTEMS
Given the absence of observations and performance data over the long term, models are used to predict the long-term performance of waste containment systems. However, few field data exist to calibrate or validate these models. Moreover, it is difficult to model these systems when so many of the parameters have a wide range of possible values. Because waste containment systems often need to be effective for timescales that stretch across decades or even centuries, long-term performance predictions must be an essential part of containment design. Unfortunately, long-term performance models often rely on extrapolations of data using time-temperature superposition and other assumptions that introduce significant uncertainty into the models. Because of these uncertainties, it will be necessary to continue to monitor the performance of critical systems and to compare observations to long-term predictions for the foreseeable future.
Prediction of the overall performance of containment systems requires a combination of predictive elements. Cover performance analyses are required to predict the generation of leachate and landfill gas (although both of these can also be predicted empirically) and the migration of gas through the cover and laterally (if contained by the cover). Liner performance analyses are required to predict the advective transport of leachate and gas and diffusive transport of chemical compounds through the liner. Advective-diffusive-dispersive transport models are required to predict contaminant transport across vertical barriers. Models for the active life of the waste and for changes in the properties of the containment system elements with time also are required. Once all of these models are integrated, it should be possible to predict the rate of transport of constituents of concern across the boundaries of the containment system. However, even after the rate of transport is established, additional analyses will be required to predict the advective and diffusive transport of these constituents to the “point of compliance” (i.e., the point at which containment system performance is evaluated) and beyond to a point where the constituents of concern may have an impact on a sensitive receptor.
One-Dimensional Contaminant Transport Models
A limited number of closed-form solutions are available for one-dimensional advective-diffusive-dispersive flow. The closed-form one-dimensional solutions are generally available for situations with well-posed boundary conditions, that is, for constituent concentrations and fluxes or advective potentials at the boundaries of the domain that are known and either constant or that conform to a well-behaved mathematical function. Common boundary conditions for one-dimensional contaminant transport modeling are discussed in Rabideau (1995) and Khandelwal et al. (1997). Rabideau also presents graphical solutions for some well-posed cases where diffusion and advection dominate. However, even if a solution to the advective-dispersive-reactive equation (ADRE) is available, estimation of the governing parameters for that equation is no simple task. In particular, evaluation of the effective diffusion coefficient, D*, which includes the effect of both diffusion and dispersion, and the retardation factor, Rd, which governs sorption, may require batch equilibrium or column testing in the laboratory. Some guidance on diffusion coefficients and sorption is available in the literature (e.g., Rowe et al., 2004). The accuracy of ADRE modeling is further complicated by unsaturated flow, coupled processes, nonlinear and/or rate-dependent sorption, cation and anion exchange, matrix diffusion, temperature effects, and the other chemical and biological processes mentioned previously. In many but not all cases these factors lead to attenuation of the contaminants, and ignoring them is conservative with respect to the rate of contaminant migration. However, some of the processes can lead to the generation of harmful daughter products, and desorption and dissolution can also have adverse effects.
Migration from a landfill liner or cover system or through a barrier wall can often be modeled as one-dimensional transport. In such cases the closed-form solutions provide a basis for assessing barrier performance. Furthermore, one-dimensional solutions can often be used effectively in quasi-two- and three-dimensional models, as described below.
Multidimensional Contaminant Transport Models
Numerical solutions to the advective-dispersive-reactive equation are generally required for multidimensional flow.
Multidimensional ADRE modeling adds additional complexity to an already complex problem. Lateral dispersion creates additional uncertainty of the effective diffusion and cannot be modeled well in laboratory column experiments. In general, numerically complex finite element or finite difference models are required to solve the ADRE in more than one dimension. However, several simplified models for multidimensional contaminant transport are available. POLLUTE (Rowe and Booker, 1986, 2005; Rowe et al., 2004) is a widely used computer program that models finite layer contaminant migration for landfill design employing a “one and one-half dimensional solution to the advection-dispersion equation.” POLLUTE also considers radioactive and biological decay, phase changes (allowing modeling of diffusion through geomembranes and in unsaturated leak detection systems), and transport through fractures. Applications of POLLUTE to assess contaminant transport from a landfill are described by Lo (1992), Rowe (1998), Simms et al. (2001), and Lake and Rowe (2005b). It has been approved in many jurisdictions for comparing alternative barrier systems.
The Multimedia Exposure Assessment Model (MULTIMED) is an EPA-developed hybrid model for simulating the movement of contaminants leaching from a waste disposal facility (Salhorta et al., 1995; Sharp-Hansen et al., 1995). The program consists of modules for contaminant transport through the subsurface, on the surface, and in water and air. A semianalytical one-dimensional transport model is used to transmit contaminants from the landfill vertically through the vadose zone (Figure 5.14) using either (1) an analytical model that considers the effects of longitudinal dispersion, linear adsorption, and first-order decay or (2) a numerical model that includes longitudinal dispersion, nonlinear adsorption, first-order decay, time-variable infiltration, and arbitrary initial chemical concentrations in the vadose zone. The vadose zone transport model is coupled with a semianalytical saturated zone transport model that considers one-dimensional uniform flow, three-dimensional dispersion, linear adsorption, first-order decay, and dilution. MULTIMED can consider parameter uncertainty, both steady state and transient flow, and up to 11 different chemical species simultaneously.
MULTIMED can be coupled with the HELP model to evaluate contaminant transport from geomembrane-lined landfills, although it does not correctly model the diffusion of organic contaminants through geomembranes. The EPA has approved the use of MULTIMED for demonstrating that alternatives to the prescriptive liner system meet RCRA Subtitle D MSW landfill performance requirements. However, EPA recommendations for MSW landfill applications (Sharp-Hansen et al., 1995) impose several conservative restrictions on the MULTIMED analyses, including (1) no decay of the contaminant source, (2) the contaminant concentration is calculated at the top of the aquifer, (3) only steady state transport, (4) the concentration of the contaminants entering the aquifer system is constant with time, (5) the contaminant pulse is continuous and constant for the duration of the simulation, (6) the point of compliance is located directly down gradient of the facility and intercepts the center of the contaminant plume, and (7) a Gaussian source geometry is assumed for the contaminant plume. Two case histories of the application of MULTIMED to demonstrate compliance of an alternative liner system with Subtitle D MSW landfill performance standards are described in Dobrowolski and Kavazanjian (2003).
Observations of the performance of liner and cover systems suggest that when properly designed and constructed these systems do a good job of limiting the migration of harmful contaminants over the 10 to 20 years for which data are available. However, the data are limited and interpretations rely more on the absence of observed adverse impacts on the environment than on direct observations of barrier system performance. In the case of vertical waste containment barriers, little information is available to evaluate their performance or to predict their integrity and effectiveness over the long term. The adequacy of systems for monitoring environmental impacts of each of these barrier systems may be questionable, particularly in the long run. Synthesized, publicly available data on the performance of landfills are sparse. Notable exceptions include the reports of Bonaparte et al. (2002) and Rowe (2005) and the letter to the committee from the New York Department of Environmental Conservation, which contain key information from up to 20 years of landfill monitoring. Models are available to predict the long-term performance of containment systems, but they rely heavily on predictions of the long-term integrity of containment system elements. Thus, if it is accepted that containment systems are performing satisfactorily in the short and medium terms, maintaining the integrity of containment system elements over the long term (i.e., for the active life of the wastes they contain) appears to be the most significant requirement to assure satisfactory long-term performance of engineered barrier systems.
The key findings regarding the performance of engineered barrier systems can be summarized as follows:
Liner systems: Liners appear to be working reasonably well over periods of up to 20 years. A composite liner limits leakage significantly better than a single geomembrane. A composite liner with a GCL has a lower leakage rate than one with compacted clay, but more care must be taken during construction. An additional attenuation layer may be required to control diffusive transport in liner systems involving a GCL or thin compacted clay liner.
The lifetime of a primary liner is related to the temperature on the liner, with higher temperatures causing greater likelihood of desiccation cracking and degradation of geosynthetics. The addition of a secondary liner provides a substantial increase in the ability of the barrier system to contain contaminants. The potential for diffusive flux, including the long-term flux of volatile organic compounds, should be considered when designing a facility.
Cover systems: Although cover systems that employ a single clay or GCL barrier layer have been known to crack, desiccate, or otherwise degrade, liquid percolation rates suggest that cover systems that employ geomembrane barriers are generally performing well. The only somewhat persistent problems with covers employing geomembrane elements are side slope instability, erosion, and gullying, often caused by clogging or insufficient capacity of the cover drainage layer. Although evapotranspirative covers perform well in the short term in arid and semiarid climates, their long-term performance and their performance in temperate climates have not been demonstrated. Finally, most cover systems rely on continuing maintenance for good operation. Maintenance-free covers have not been demonstrated to be effective.
Vertical barriers: Little information is available to evaluate the performance of vertical barriers or to predict their integrity and effectiveness over the long term. More monitoring is required to determine whether these systems are performing adequately.
Barrier integrity: Local slope stability and global slope stability are significant short-term concerns but can be mitigated with proper attention during design and operations. More work has to be done on local stability, especially on steep slopes.
Principal findings concerning the prediction of barrier system performance are as follows:
Predicting the performance of covers: The HELP model does a good job predicting the volume of leachate, but it is not reliable for other uses.
Predicting the performance of liner systems: Accurate predictions of leaks through composite liners need to take into account holes in wrinkles and elevated leachate head.
Predicting gas transport through containment systems: A well-designed and well-constructed cover will not necessarily ensure that gas will not escape; the details of the leachate collection/removal system are important to make sure that gas does not bypass the containment system.
Predicting the performance of vertical barriers: While it is feasible to predict contaminant transport through vertical barrier walls around contaminated sites, the paucity of field data limits our ability to evaluate the accuracy of the predictions.
Predicting the performance of concrete barriers: The ability of computer models to predict long-term performance is largely unknown. Material coefficients that control transport and reaction in concrete must also be better characterized in models.
Predicting the “active” lifetime (contaminating life span) of waste: Landfills, especially some of the larger ones, are likely to require attention for centuries, not decades.
Predicting the overall performance of containment systems: Existing data suggest that modern containment systems are performing well and that predictive models are capable of predicting their performance. However, this positive finding is tempered by two facts: (1) there are relatively few field data that can be used to verify models, and (2) modern landfills have not been in existence long enough to allow an empirical assessment of long-term performance.