3
Monitoring Barrier Performance

Monitoring is an essential component of engineered barrier system design and operation. Preconstruction monitoring is required to develop a conceptual site model for barrier system design and analysis, to establish a baseline for evaluating the effectiveness of the engineered barrier system, and, in the case of a barrier system for preexisting contamination, to establish boundary conditions and geometric constraints for barrier system design. Postconstruction (long-term) monitoring is critical to ensure that barrier integrity is sound and that contaminants are not inadvertently released into the environment. Monitoring systems may observe both the physical conditions of the barrier and subgrade and the chemical environment surrounding the barriers. Information from monitoring of existing waste containment systems provides the basis for many of this report’s conclusions on the long-term performance of engineered barriers.

This chapter summarizes statutory requirements for monitoring barrier system performance and reviews techniques that can be used to monitor the integrity of engineered barrier systems and their components.

3.1
STATUTORY REQUIREMENTS FOR MONITORING

Statutory requirements for monitoring systems are prescribed in accordance with the regulatory classification of the waste. Thus, monitoring requirements depend on whether the waste contained by the barrier system is regulated under the Resource Conservation and Recovery Act (RCRA; Subtitles C and D); the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA); the Uranium Mill Tailings Remedial Action (UMTRA); the Low Level Waste Policy Act; or another regulatory program. A common element among almost all statutory monitoring programs is an initial 30-year postclosure monitoring period. Another commonality is that regulatory programs may be delegated to state governments and tribal authorities with regulatory programs that conform to the minimum federal requirements. Environmental Protection Agency (EPA)-approved states and tribal authorities may have monitoring requirements that exceed the federal minimum standards. In approved states, both state and local governments are generally involved in overseeing monitoring programs.

RCRA Subtitles C and D prescribe minimum standards for monitoring hazardous waste treatment, storage, and disposal facilities and municipal solid waste (MSW) landfills, respectively. These standards require owners and operators to monitor and maintain activities to preserve the integrity of the disposal system. These responsibilities are governed by closure and postclosure monitoring plans certified by the EPA regional administrator or the director of an approved state or tribal authority. Monitoring plans describe procedures for obtaining the data necessary to maintain the integrity of the final closure, to maintain the operating leachate collection and leak detection systems (and gas monitoring system, if applicable), and to monitor groundwater quality. Financial assurance requirements are based on “projected costs for an entire post-closure period of thirty years” (EPA, 2003b). At the end of the initial 30-year postclosure period, monitoring and maintenance may have to continue if the lead regulatory agency determines that the waste still poses a threat to human health or the environment. In addition, Subtitle D allows the director of an approved state or tribal authority to increase or decrease the 30-year postclosure period if the owner/operator demonstrates that a different period is needed to protect human health and safety. However, there are no financial assurance requirements for monitoring beyond 30 years.

Engineered barriers constructed under RCRA corrective actions are implemented either as part of a permit or through an order of consent. Postclosure monitoring requirements are similar to those for CERCLA sites (described below). Another class of RCRA engineered barriers are those constructed for “past practices units,” which are “closed” pre-RCRA facilities. These barriers are subject to separate monitoring requirements developed through a consent order.

CERCLA requires postclosure monitoring for 30 years or as long as the waste poses a threat to human health and



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Assessment of the Performance of Engineered Waste Containment Barriers 3 Monitoring Barrier Performance Monitoring is an essential component of engineered barrier system design and operation. Preconstruction monitoring is required to develop a conceptual site model for barrier system design and analysis, to establish a baseline for evaluating the effectiveness of the engineered barrier system, and, in the case of a barrier system for preexisting contamination, to establish boundary conditions and geometric constraints for barrier system design. Postconstruction (long-term) monitoring is critical to ensure that barrier integrity is sound and that contaminants are not inadvertently released into the environment. Monitoring systems may observe both the physical conditions of the barrier and subgrade and the chemical environment surrounding the barriers. Information from monitoring of existing waste containment systems provides the basis for many of this report’s conclusions on the long-term performance of engineered barriers. This chapter summarizes statutory requirements for monitoring barrier system performance and reviews techniques that can be used to monitor the integrity of engineered barrier systems and their components. 3.1 STATUTORY REQUIREMENTS FOR MONITORING Statutory requirements for monitoring systems are prescribed in accordance with the regulatory classification of the waste. Thus, monitoring requirements depend on whether the waste contained by the barrier system is regulated under the Resource Conservation and Recovery Act (RCRA; Subtitles C and D); the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA); the Uranium Mill Tailings Remedial Action (UMTRA); the Low Level Waste Policy Act; or another regulatory program. A common element among almost all statutory monitoring programs is an initial 30-year postclosure monitoring period. Another commonality is that regulatory programs may be delegated to state governments and tribal authorities with regulatory programs that conform to the minimum federal requirements. Environmental Protection Agency (EPA)-approved states and tribal authorities may have monitoring requirements that exceed the federal minimum standards. In approved states, both state and local governments are generally involved in overseeing monitoring programs. RCRA Subtitles C and D prescribe minimum standards for monitoring hazardous waste treatment, storage, and disposal facilities and municipal solid waste (MSW) landfills, respectively. These standards require owners and operators to monitor and maintain activities to preserve the integrity of the disposal system. These responsibilities are governed by closure and postclosure monitoring plans certified by the EPA regional administrator or the director of an approved state or tribal authority. Monitoring plans describe procedures for obtaining the data necessary to maintain the integrity of the final closure, to maintain the operating leachate collection and leak detection systems (and gas monitoring system, if applicable), and to monitor groundwater quality. Financial assurance requirements are based on “projected costs for an entire post-closure period of thirty years” (EPA, 2003b). At the end of the initial 30-year postclosure period, monitoring and maintenance may have to continue if the lead regulatory agency determines that the waste still poses a threat to human health or the environment. In addition, Subtitle D allows the director of an approved state or tribal authority to increase or decrease the 30-year postclosure period if the owner/operator demonstrates that a different period is needed to protect human health and safety. However, there are no financial assurance requirements for monitoring beyond 30 years. Engineered barriers constructed under RCRA corrective actions are implemented either as part of a permit or through an order of consent. Postclosure monitoring requirements are similar to those for CERCLA sites (described below). Another class of RCRA engineered barriers are those constructed for “past practices units,” which are “closed” pre-RCRA facilities. These barriers are subject to separate monitoring requirements developed through a consent order. CERCLA requires postclosure monitoring for 30 years or as long as the waste poses a threat to human health and

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Assessment of the Performance of Engineered Waste Containment Barriers the environment. CERCLA requirements include performance evaluations of the remedy, including monitoring systems, at 5-year intervals. The postclosure monitoring and evaluation periods can be modified if the closed CERCLA site is redeveloped for beneficial use following closure (e.g., development of a contaminated, or brownfield, site). In some cases, disposal cells are being built at these sites that comply with both CERCLA and RCRA regulations. Closed low-level radioactive waste sites are subject to a 30-year observation and maintenance period, which may be shortened or lengthened, based on site-specific conditions, as described in 40 CFR §61.29. The disposal site must be “designed, used, operated, and closed to achieve long-term stability” (10 CFR §61.44). When closed, the licensee is responsible for postoperational surveillance and must maintain a monitoring system based on the operating history of the site. UMTRA monitoring requirements appear to be tied to final ownership of the land, either the state or the Department of Energy (DOE). A remediated site may be transferred to a state subject to access by federal authorities or to tribes, or it may remain under DOE ownership. The Secretary of Interior takes ownership of removed radiation waste (42 USC §7914-7916). No RCRA facilities have reached a 30-year lifetime, so performance can only be judged on short and medium timescales. It is unknown if the regulatory requirements will be reduced or extended at the majority of sites, although indefinite maintenance and monitoring periods are anticipated for many sites. Given the discretion written into the regulations, these types of decisions will likely vary by state. 3.2 CONTAINMENT SITE MONITORING SYSTEMS Monitoring systems at waste containment sites may target a variety of media, including soil, groundwater, surface water, and air. Monitoring systems are often designed for the sole purpose of meeting the statutory requirements discussed above and are rarely designed to directly monitor barrier performance. Ideally, a monitoring system would do both. A well-conceived monitoring system is configured to provide information needed to assess barrier system performance and physical state (e.g., degradation), to provide information to assess the state of the waste mass to understand the progress of waste decomposition and stabilization, to monitor places where model scenarios predict contaminants are most likely to be released, to detect contaminant migration along unanticipated pathways, to provide early warning of a contaminant release and thus facilitate corrective action before migrating contaminants adversely impact human health and/or the environment, and to provide information to determine facility maintenance and rehabilitation needs. 3.2.1 Methods for Monitoring Monitoring system measurements may be made in situ or on samples recovered from monitoring wells or probes. Monitoring devices may take point, area, or volume measurements. Well points for groundwater, subsurface gas sampling probes, and piezometers for measuring hydraulic head are examples of point measurements. Area measurements include blanket drainage layers behind barriers (e.g., leak detection layers in double-liner systems, pan lysimeters beneath sumps) and some geophysical measurements (e.g., ground-penetrating radar, vertical seismic profiling, electrical resistivity tomography). Volume measurements, such as the electrical measurements of resistivity and conductivity (e.g., capacitance probes, time domain reflectometry) and other types of geophysical measurements (e.g., gamma and neutron probes), gauge the properties of a characteristic volume of soil. Table 3.1 identifies common monitoring methods for contaminant migration at waste containment sites. Geophysical techniques are included in the table and subsequent discussion, although their use in monitoring engineered barriers has been limited for reasons discussed below. Appendix B provides a more detailed list of typical metrics used in monitoring, how they are measured, and their use in monitoring containment system performance. 3.2.2 Saturated Zone Monitoring Systems Saturated zone (groundwater) monitoring systems are the most commonly employed method to evaluate barrier performance. Both the hydraulic potential (phreatic surface, hydraulic head) and the groundwater chemical composition in the pore water recovered from saturated soil beneath the phreatic surface are measured. Fixed groundwater monitoring systems include direct measurements made with wells, piezometers, or plate lysimeters, and indirect measurements made with electrical and other geophysical measurements. Groundwater monitoring sometimes includes one-time measurements made on samples recovered from push-in probes (e.g., cone penetrometers, hydropunch). Geophysical methods can be used to monitor groundwater, but they are rarely used in regulatory compliance monitoring systems because techniques have not yet been developed that provide sufficiently quantitative and reliable data. They may, however, be employed in evaluation monitoring programs or in investigations for the development of corrective action programs (e.g., Meju, 2006; Slater and Binley, 2006). Measurements of electrical conductivity or resistivity and electromagnetic potential are sometimes used to establish the extent of the saturated zone and they

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Assessment of the Performance of Engineered Waste Containment Barriers TABLE 3.1 Common Methods for Monitoring Water, Soil, and Air Media Method Measurement Advantages Disadvantages Saturated Zone       Wells Phreatic surface; water samples Simple; accurate determination of hydraulic head; allows sampling, which can be combined with laboratory analysis Invasive; risks cross-contamination Piezometers Hydraulic head or pressure; samples (some designs) Accurate determination of hydraulic head; allows sampling, which can be combined with laboratory analysis Invasive; risks cross-contamination Plate lysimeters Liquid flux; samples (some designs) Can allow sampling and combined laboratory analysis; long-term monitoring; applicable to relatively large volumes with low hydraulic conductivity Slow; cannot distinguish effects of liner consolidation from induced flow; may alter boundary conditions driving flow Geophysical: direct-current resistivity/ induced polarization, electromagnetic induction, transient electromagnetics, radio frequency magnetotellurics Moisture content; leak detection; changes in conductivity; chemical composition; hydrochemical parameters; temperature Non- or minimally invasive; applicable to large volumes/areas; can provide an indication of barrier integrity and performance; one- to four-dimensional and autonomous monitoring possible Nonuniqueness; natural ambiguity in using single technique but results much improved by using multiple techniques, affected by a variety of factors (e.g., mineralogy, grain size and its distribution, temperature); accuracy and resolution decrease with depth depending on survey geometry; results often mixed Self-potential Leak detection Noninvasive; applicable to large volumes/areas; can provide information on redox processes Source mechanism usually uncertain; interpretation mostly qualitative, although two- and three-dimensional inversion methods are now possible Push technology Soil stratigraphy; samples (some designs); hydraulic head, pressure, temperature; detection of some chemicals Rapid; inexpensive; nearly continuous profiling Invasive; risks cross-contamination Surface Water       Grab samples Laboratory characterization of chemical composition Easy; inexpensive; laboratory characterization possible Discreet samples; requires care to ensure representative sampling; sample degradation issues; sample transportation/chain of custody Vadose Zone       Tensiometers Soil suction Permanent installation; simple; robust Invasive; risks cross-contamination; vacuum gauge calibration; can develop air leaks; ceramic cup may plug; limited to −1 atm by cavitation Gas monitoring probes; borehole and well headspace monitoring; passive landfill vents Methane; oxygen; carbon dioxide; hydrocarbons; nonmethane organic compounds Permanent or temporary installation; simple; robust; can identify migration pathways Discrete samples; seasonally variable; does not include emissions from other sources (e.g., composting); not quantitative Flux box Methane; oxygen; carbon dioxide; hydrocarbons; nonmethane organic compounds Quantitative evaluation of gas transport out of covers Seasonably variable; may alter flow boundary conditions Lysimeters Liquid flux; samples (some designs) Can allow sampling and combined laboratory analysis; long-term monitoring; applicable to large volumes with low hydraulic conductivity Slow; cannot distinguish effects of liner consolidation from induced flow

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Assessment of the Performance of Engineered Waste Containment Barriers Method Measurement Advantages Disadvantages Electrical (TDR, capacitance gauges) and thermal probes Soil moisture content Water content; soil suction; liquid flux Invasive; variations in moisture content must be related to flux analytically; suction measurement requires site-specific calibration Neutron-neutron probe Soil moisture content Water content; soil suction; liquid flux Point measurement; requires site-specific calibration Nuclear magnetic resonance Soil moisture content; porosity   Difficult to implement in the field Electromagnetic induction (frequency and time domain, surface and borehole); electrical resistivity tomography (surface and borehole) Moisture content; unsaturated flow; changes in electrical conductivity; salinity Long-term spatial and temporal monitoring; applicable to large volumes/ areas; indication of barrier integrity; noninvasive; one- to four-dimensional and autonomous monitoring possible Globally averaged values; require contrasting properties (e.g., fluid resistivity) to detect change; accuracy and resolution decrease with depth (surface configuration); nonuniqueness of inversion methods and thus ambiguity in interpretation (however, this may be improved by use of multiple techniques) Ground-penetrating radar (surface and borehole) Volumetric water content Rapid; indication of barrier integrity; noninvasive; four-dimensional and autonomous monitoring possible Limited depth penetration in conductive subsurface media (surface configuration) Self potential Leak detection; fluid flow Inexpensive; noninvasive; spatial-temporal measurements possible Interpretation mostly qualitative, although two- and three-dimensional inversion methods are now possible Air       Gas sampling (discrete) Total VOCs; hydrogen sulfide; sulfur dioxide Easy; inexpensive; quantitative measurements Labor intensive; discreet samples; seasonally variable; does not include emissions from other sources (e.g., composting) Air quality monitoring Total hydrocarbons; particulates Can use continuously or semicontinuously; in situ quantitative with techniques such as Fourier transform infrared spectroscopy or ultraviolet spectroscopy; large area of measurement; laboratory quantitative with techniques such as flux chambers Does not include emissions from other sources (e.g., composting) NOTES: TDR = time domain reflectometry; VOCs = volatile organic compounds. SOURCE: McNeill (1980), O’Donnell et al. (1995), Pellerin et al. (1998), EPA (1998, 2003a, 2004), Daily and Ramirez (2000), DOE (2001), Bonaparte et al. (2002), Reedy and Scanlon (2002), Slater and Binley (2003, 2006), Haas et al. (2004), and Daniels et al. (2005). can also sometimes be related to concentrations of inorganic constituents and soil moisture content (Meju, 2006). Electromagnetic measurements made from the ground surface can use different frequencies to achieve different depths of penetration and/or sensitivities. Time-lapsed seismic reflection, ground-penetrating radar, or electrical resistivity data may be used to track water and gas movement in the subsurface. Acoustic monitoring can be used to locate leaks of significant size and other areas of concentrated subsurface flow. Fiber-optic sensors, which have been used to study dynamic hydrologic processes (Selker et al., 2006), offer the potential for monitoring temperature changes in landfills, although this technology may be too expensive for this application. Other geophysical techniques measure turbidity (measured optically) and photoluminescence, which can be correlated to the presence and concentration of certain organic chemicals. Finally, the emergence of autonomous monitoring systems enable time-lapsed imaging of dynamic processes.1 These areas may be fruitful for future research. 3.2.3 Vadose Zone Monitoring Systems Vadose zone monitoring systems measure hydraulic potential (soil suction), soil pore gas constituent concentrations, and the presence and chemical constituents of migrating 1 See, for example, <http://geophysics.inel.gov/h2/hermes/pages/login.php>.

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Assessment of the Performance of Engineered Waste Containment Barriers liquids above the phreatic surface. These techniques can also be used to monitor cover system performance and subsurface gas and leachate migration. Subsurface gas monitoring probes enable soil pore gas to be collected from a screened interval for subsequent compositional analysis. Head space (the space above the water surface in a well or borehole) monitoring can provide an indication of subsurface gas migration, either directly from the source (e.g., a landfill) or indirectly from off-gassing of groundwater contaminated by volatile and semivolatile organic compounds from the source. Atmospheric tracer tests using handheld instruments and flux chamber measurements can be used to determine the flux of emissions through the unsaturated soil in cover systems. In situ moisture measurements can be made using electrical probes, including capacitance probes, which directly measure the electrical capacitance of a representative volume of soil; time domain reflectometry probes, which indirectly measure soil capacitance; and thermal probes, which measure the rate at which heat is dissipated by the soil. Capacitance, time domain reflectometry, and thermal probes must be calibrated using site-specific soil and may be sensitive to accumulation of salts and other changes in soil and pore water chemistry. Shallow electromagnetic techniques and ground-penetrating radar may be used to estimate soil moisture content and to track the movement of fluids through the vadose zone. Lysimeters capture liquid migrating through the vadose zone and include suction lysimeters (sampling points from which migrating liquid is sucked from the soil), gypsum blocks (which absorb migrating liquid because of their affinity for water), and pan lysimeters (blanket drainage layers). Suction lysimeters and gypsum blocks capture relatively small samples of migrating liquid and can be bypassed by the migrating liquid if not properly configured. Pan lysimeters can capture large representative samples of migrating liquid for laboratory analysis. Pan lysimeters provide a direct measurement of percolation through the cover barrier element(s) subject to a combination of field and imposed boundary conditions. While some investigators maintain that pan lysimeter measurements are the best means available to assess cover system performance (Benson et al., 2001), other investigators have concerns about the impact of imposing a capillary break at the base of the cover and, for MSW landfills, the impact of obstructing heat and moisture flow from below on measured percolation. The net upward flow of heat and moisture in MSW landfills in arid and semiarid climates is discussed in Blight (2006), and the impact of this upward heat on lysimeter measurements is discussed in Kavazanjian et al. (2006a). Errors associated with lysimeters and other indirect methods to assess cover performance are discussed in Malusis and Benson (2006). Concerns about the impact of the capillary break at the base of pan lysimeters on the measured percolation are described in greater detail by Zornberg and McCartney (2005) and Kavazanjian et al. (2006a). Indirect monitoring of vadose zone flux in cover systems can be carried out using moisture content measurements (Kavazanjian et al., 2006a). The primary limitation of this approach is that percolation must be calculated from laboratory measurements of the soil water characteristic curve, which relates moisture content to soil suction. Gee and Hillel (1988) suggest that the uncertainty associated with percolation calculated in this manner can lead to large uncertainties in the calculated liquid flux. The vadose zone is also monitored for gas transport of contaminants from waste sites. Generally, fixed gas monitoring probes placed at designated intervals are used to periodically collect vadose zone samples, although one-time soil gas probes (e.g., geoprobes) may be used in some situations. An active vadose zone gas monitoring program for a closed hazardous waste landfill site in California is described in Box 3.1. 3.2.4 Air Quality Monitoring At sites where significant amounts of gas are generated, surface emissions sampling may be conducted using handheld instruments that measure the concentration of total volatile organic compounds or gases of concern (e.g., hydrogen sulfide, sulfur dioxide). A grid is generally laid out over the site, and sampling points within each grid square are chosen randomly. Where grid measurements are not feasible (e.g., on steep slopes), measurements taken with some overall minimum frequency and maximum spacing over a preestablished route may be employed. Air quality measurements may also be made at fixed sampling points to detect hydrocarbons, particulates, or other airborne substances. Other gas monitoring technologies include perfluorocarbon gas tracers and SEAtrace developed by Sandia National Laboratory, which measure the rate of migration of a gas tracer from the point of injection to a collection well (Sullivan et al., 1998). Gas tracers are injected on the inside of the barrier, and concentrations of perfluorocarbon gas tracers in the external monitoring wells are analyzed to determine whether there is a breach in the barrier (Pearlman, 1999). Finally, air quality monitoring is required at low-level nuclear waste disposal sites (e.g., see 40 CFR §192.02 for radon requirements). Tests for the presence of radioactive material in the air are conducted at both onsite and offsite locations. 3.2.5 Other Containment Monitoring Systems Other monitoring systems used at landfills and contaminated soil and groundwater sites include surface water monitoring, deformation monitoring, and radioisotope monitoring systems. Surface water monitoring typically involves manually capturing samples of surface water runoff and streamflow at designated times and locations. In deformation monitoring, surface settlement is measured by survey or by photogrammetry methods. Radioisotope monitoring may be

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Assessment of the Performance of Engineered Waste Containment Barriers BOX 3.1 Vadose Zone Monitoring for the McColl Superfund Site This case history describes a vadose zone monitoring program for a closed hazardous waste site. The McColl Superfund site, located in Fullerton, California, was an 8.8-ha hazardous waste disposal site with 12 unlined pits, or sumps, containing approximately 60,150 m3 of petroleum waste sludge generated from high-octane aviation fuel production. The waste sludge, disposed of during and just after Word War II, is highly acidic (pH < 1.0) and emits a strong, objectionable odor. During the 1950s and 1960s, three of the sumps were covered with diesel-oil-based drilling mud from petroleum production to control odors. In the late 1950s, six additional sumps were covered with soil. A golf course was subsequently developed over a portion of the site. Residential neighborhoods around the golf course followed in the 1960s. The site was initially brought to the attention of public health regulatory agencies as a result of odor and health complaints from nearby residents beginning in July 1978. By September 1982, EPA had added the site to its National Priorities List. Shortly after, the golf course owner abandoned a three-hole portion of the golf course due to waste “seeps” in the fairways. The McColl site was remediated between May 1996 and August 1998 under a CERCLA consent degree. The components of the barrier system installed as part of site remediation are shown in Figure 3.1 and include: a RCRA-compliant multilayer geomembrane-geosynthetic clay liner cap placed over the sumps to control infiltration and the release of hazardous emissions, a gas collection system beneath the cap connected to an activated carbon absorption gas treatment system, and a soil-bentonite vertical barrier placed around the sumps and tied into the cap barrier layer to control inward migration of groundwater and outward migration of gasses. FIGURE 3.1 (Left) Tie-in between the cap and vertical barrier in the area immediately adjacent to the homes, where a reinforced earth berm was required. (Right) Detail of the cap section over the Los Coyotes sumps (the golf course area), where a bio-intrusion barrier and geogrid reinforcement were incorporated into the cap. NOTE: HDPE = high density polyethylene. SOURCE: Collins et al. (1998). The primary function of the vertical barrier was to prevent lateral migration of volatile and semivolatile organic compounds and sulfur dioxide from the waste pits. The monitoring system for the site included 12 pairs of vadose zone gas monitoring probes on either side of the barrier wall. The monitoring program also included flow rate and exhaust air quality monitoring for the carbon absorption unit that was connected to the gas collection layer beneath the composite cap shown in Figure 3.1; 20 groundwater monitoring wells; survey monuments on top of the cap; and visual inspections of the cap, surface water drainage system, subsurface drainage system outlets, gas collection system vents, groundwater monitoring wells, and perimeter fence at regular intervals ranging from daily to semiannually. Additional inspections are performed after extreme events, such as significant earthquakes (defined on the basis of magnitude and distance from the site) and major rainstorms. The 12 sets of matched pairs of soil gas probes (pairs of probes set inside and outside the vertical barrier) were concentrated along the side of the site adjacent to residential development. Probes inside the barrier monitor soil pore gas pressure, and probes outside the barrier monitor the soil gas pore pressure and allow for sampling of the pore gas for analytical testing. The probe depths vary depending on the depth of the waste adjacent to the probes. The probes are monitored semiannually using handheld instruments for gas pressure, total volatile organic compounds, and sulfur dioxide. Differential gas pore pressure across the vertical barrier or detectable concentrations of volatile organic compounds or sulfur dioxide outside the wall trigger an evaluation monitoring program with likely additional sampling and testing. Since completion of the cap in November 1997, the evaluation monitoring program has not been triggered (i.e., there is no evidence of gas migration across the vertical barrier).

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Assessment of the Performance of Engineered Waste Containment Barriers conducted in soil, ground- and surface water, and air at low-level radioactive waste facilities. 3.3 MONITORING OF BARRIER COMPONENTS Engineered barrier components may be monitored for barrier integrity during and at the completion of construction (short-term monitoring) and for barrier performance after completion of construction (medium- and long-term monitoring). Monitoring of barrier integrity during and at the completion of construction generally involves direct measurement of barrier properties and performance. However, postconstruction monitoring of engineered barriers is usually based on monitoring of the environmental performance of the barrier system using the techniques discussed in Section 3.2. Direct monitoring of the integrity of barrier system elements in the medium and long term is relatively rare, although the use of embedded sensors, such as those being tested in the laboratory, pilot projects (Oh et al., 2003), and waste ponds (Frangos, 1997), may serve to facilitate more direct monitoring over the long term. Barrier components monitored in practice include compacted clay barriers, geomembrane barriers, leachate collection layers, vertical barriers, and geosynthetic clay liners. 3.3.1 Compacted Clay Barrier Monitoring Direct monitoring of the short-term performance of compacted clay earthen barriers is based primarily on hydraulic conductivity measurements using in situ hydraulic conductivity tests (e.g., Daniel, 1989; Trautwein and Boutwell, 1994), such as sealed double-ring infiltrometer tests. These tests are routinely performed on separate but smaller prototypes of the barriers, typically referred to as “test pads,” which are constructed and evaluated prior to construction of the full-scale barrier as part of construction quality assurance (CQA; see Appendix C). Test pads are constructed using the same equipment and materials as the full-scale barrier layer and typically have the same thickness but shorter widths and lengths. The test pad may sometimes be incorporated into the full-scale barrier layer. Other construction criteria typically evaluated for the test pad include soil compaction criteria, such as gravimetric water content and dry density, lift thickness, and number of passes with the compaction equipment. The results of these tests are used to develop specifications and quality assurance requirements for construction of the full-scale barrier system. The CQA requirements (e.g., compaction data, laboratory tests on specimens recovered using Shelby tubes) provide an indirect assessment of the short-term integrity of the full-scale barrier system. Direct postconstruction monitoring of compacted clay barrier layers is relatively rare. Indirect monitoring of barrier layers in cover systems often includes settlement monitoring because differential settlement is a major source of cracking and loss of integrity of clay barriers. Monitoring of gas concentrations at or near the ground surface also provides an indirect assessment of clay barrier integrity (i.e., of cracking) in covers of MSW landfills. Visual monitoring for cracks, ponded water after a storm (an indicator of nonuniform deformation), and distressed vegetation (and indicator of gas migration) may also provide an indirect assessment of clay barrier integrity in cover systems. Infrared and multispectral airborne and spaceborne monitoring of landfills where gas is being generated may also give an indirect assessment of cover barrier layer integrity, but these techniques have neither been investigated extensively nor employed in practice. 3.3.2 Geomembrane Barrier Monitoring Current landfill construction practice includes extensive short-term monitoring of geomembrane integrity. During geomembrane installation, it is common practice to continuously observe installation, to nondestructively test all seams between geomembrane panels, and to periodically remove seam samples for destructive laboratory testing as part of CQA activities (see Appendix C). Furthermore, electrical leak detection methods are being used with increasing frequency to detect defects in geomembranes. These surveys are conducted either immediately after seaming (for covers) or following placement of the overlying leachate collection layer (for liners). These measures generally provide a high degree of confidence in the short-term integrity of a properly designed and constructed geomembrane barrier system. Analysis of 10 years of postconstruction leak detection surveys showed that a typical defect frequency rate for geomembrane liners constructed using strict CQA procedures was approximately 0.5 defects per hectare (Hruby and Barrie, 2003). This very low defect frequency corresponds to extremely high integrity for the geomembrane. (Furthermore, defects detected in these surveys are generally exposed and repaired, reducing the final postconstruction defect frequency to a minimal value.) In contrast, the average defect rate for geomembranes constructed without strict CQA was approximately 16 defects per hectare (Hruby and Barrie, 2003), approximately 30 times greater than for geomembranes with strict CQA, demonstrating both the importance and effectiveness of modern CQA procedures for geomembrane construction. Direct postconstruction monitoring of geomembrane integrity is relatively rare. Because of limitations related to the thickness of soil or waste cover and the need for a conductive medium in the leak, electrical leak detection is generally useful as a CQA tool only for solid waste landfills. The primary measure of postconstruction integrity of geomembranes is measurement of the flow rate in the leak detection systems of double-liner systems. Measurements of volumetric seepage into the leak detection system provide a direct indication of the integrity (effectiveness) of the primary barrier system, which generally includes an overlying leachate collection layer and either a single geomembrane

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Assessment of the Performance of Engineered Waste Containment Barriers or a composite geomembrane/low-permeability soil barrier. In fact, the leak detection layer of a double-lined landfill provides continuous monitoring of advective flow through the primary liner system (assuming the leak detection layer is functioning properly). Leak detection system flows may contain not only advective leakage through the primary liner but also liquids from other sources, such as consolidation water from any compacted clay component of the primary liner and/or drainage of water that entered the system during construction (Bonaparte and Gross, 1990; Bonaparte et al., 2002). For landfills without double-liner systems, pan lysimeters are sometimes placed beneath the sump and in other strategic locations to monitor for leachate flow through the barrier system. Leak detection layers and pan lysimeters enable samples to be collected for analytical testing of leachate constituents. This information can be useful in subsequent groundwater and vadose zone monitoring because it helps establish which constituents to measure. Although leak detection layers and pan lysimeters are advantageous for monitoring large areas of barriers, data collection is slow and it is difficult, if not impossible, to distinguish the effects of liner consolidation from induced flow in systems where the pan lysimeter underlies a compacted clay liner. Nonetheless, leak detection systems provide an effective means of monitoring the integrity of the overlying liner system. An example of how the leakage rate can be measured and interpreted to assess performance is provided in Box 3.2. Temperature measurements are an essential requirement for evaluation of the potential long-term integrity of a geomembrane, since the service life of a geomembrane depends significantly on the temperature (Rowe, 2005). Furthermore, temperatures of multiple components can be used in heat and moisture transfer analysis. Time-temperature measurements are usually made on geosynthetic components of barrier systems or in the landfill mass itself for research purposes. Although temperatures have been measured in one landfill monitoring program (Rowe, 2005), routine monitoring of geomembrane temperatures is not yet a part of landfill engineering practice. 3.3.3 Leachate Collection Layer Monitoring The performance of leachate collection layers is monitored in a variety of ways. Leachate head may be monitored in a collection sump or, in some cases, directly on the liner to evaluate the efficiency of the leachate collection and removal system. Measurements of the volumetric seepage of leachate into the collection and removal system can also be used to assess the efficiency of the system. These measures are somewhat indirect, as the leachate head and leachate volume depend on the leachate generation rate as well as the collection system efficiency. However, excessive head within the leachate collection layer and/or significant decreases in the leachate collection rate over time without any apparent external cause can indicate clogging of the leachate collection layer. Other methods that have been used to evaluate the performance of leachate collection layers include dye tracer tests, pumping tests, and video surveys. Dye tracer tests and pumping tests provide some indication of leachate collection layer continuity but may not give an overall assessment of the condition of the collection system. Video surveys generally indicate only the condition of the collection line being surveyed. Chemical analysis of leachate samples is generally used to identify constituents of concern for vadose zone and groundwater monitoring beneath the liner. Although this approach identifies key organic or inorganic constituents, the potential for chemical transformation of leachate constituents between the leachate collection and removal system and the monitoring point must also be considered in establishing monitoring parameters. Chemical constituent concentrations in leachate may also be useful in evaluating the potential for diffusive transport across the liner and degradation of liner system components. The leachate collection layer monitoring system and other monitoring systems used at a low-level radioactive waste repository are described in Box 3.3. 3.3.4 Vertical Barrier Monitoring Vertical barriers for environmental protection commonly include slurry trenches, soil-mixed walls, and geomembranes dropped into trenches. Collection and extraction trenches may also be used as vertical barrier systems. Short-term monitoring of barrier effectiveness using a variety of CQA techniques, including sampling and testing of barrier materials and in situ testing of the barrier in slurry walls and soil cement walls (Appendix C), is standard practice. Although these methods generally do not ensure the same level of reliability as compacted clay layers or geomembranes, they can provide a high level of reliability for the short-term integrity of the as-constructed barrier. The long-term effectiveness of a vertical barrier is generally evaluated by monitoring the vadose and saturated zones down gradient of the barrier using the techniques described in Section 3.2. Physical sampling of soil-bentonite and soil-cement vertical barriers after construction is also possible but is generally done only after downstream monitoring indicates there may be a problem. The vertical barriers and monitoring systems for a large municipal solid waste landfill in California are described in Box 3.4. Although geophysical methods (e.g., electrical resistivity, electromagnetic, acoustic) offer the promise of cost-effective, noninvasive, postconstruction evaluation of flaws in vertical barriers, only a few case studies exist. In one study, three-dimensional surface electrical imaging was able to resolve the geometry of 0.4-m-high vertical walls emplaced at a depth of 0.6 m, but image quality was too poor to resolve walls emplaced at depths greater than 1 m (Chambers et

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Assessment of the Performance of Engineered Waste Containment Barriers BOX 3.2 Case History on Monitoring of a Double-Liner System This case history illustrates how measurements from a leak detection system can be used to assess performance of the liner system. An example set of flow data from collection systems in a double-lined landfill in Lake Charles, Louisiana, is shown in Figure 3.2. The landfill cell extends below grade with side slopes graded at 2.5 horizontal to 1 vertical and a bottom sloped at 2.5 percent toward a sump area (Gilbert, 1993). The primary liner consists of a single high-density polyethylene geomembrane on the side slopes and a composite liner with a high-density polyethylene geomembrane overlying a compacted clay liner on the bottom. The cell covers an area of 1.3 ha, with the side slopes covering 36 percent and the bottom covering 64 percent of the area, respectively. The average annual rainfall is 1,530 mm, or 55,000 l/day over the total area of the landfill cell. The data in Figure 3.2 were obtained by measuring the volume of fluid pumped from the primary and the secondary collection system sumps on a monthly basis. FIGURE 3.2 Field data from leachate collection and leak detection systems in a double-lined landfill. Waste placement began at time zero. SOURCE: Created from data provided by Chemical Waste Management, Inc. The flow rate into the secondary collection system was initially high as the compacted clay liner in the primary liner system was consolidated from waste emplacement. Once the below-grade portion of the landfill cell was completed (21 months after the cell was opened), consolidation slowed and the flow rates into the primary and secondary collection systems became better correlated with each other. This correlation allows estimation of the leakage through the primary liner, because as the flow rate into the primary collection system increases, the average hydraulic head in that system also increases. As a consequence, the leakage rate across the primary barrier layers is expected to increase. The data show that the flow rate through the primary liner is about 100 times smaller than the rate of flow into the overlying primary leachate collection system. Furthermore, the average leakage rate is less than 100 lphd (liter/ha/day) for the landfill cell, which is 1.3 ha in area. While illustrating that small leakage rates through a liner can be achieved, this case history also demonstrates that the measured flow rate into the leak detection system should be considered carefully in assessing how the liner is actually performing. First, the leak detection system pumping data for the first 21 months during waste filling included consolidation flow as well as leakage. Second, reporting the data as a “leakage rate” per unit area is misleading in this case because the primary liner consists of a single geomembrane on the side slopes and a composite liner on the floor. The majority of the leakage is likely coming from the single geomembrane on the side slopes, which makes up less than one-half of the total area of the liner. Furthermore, the leachate head driving leakage can be significantly smaller on the side slopes versus the floor, and the leachate head may change with time even for a constant leachate generation rate due to clogging (see Box 4.4). Finally, the accuracy of the flowmeter system used to measure the volume of flow from both the primary and secondary sumps was flagged as questionable by the operator of the landfill during a 12-month period starting in month 25; the flowmeters were replaced in month 37 (Figure 3.2). Therefore, flow rates into leak detection systems provide at best an indirect measure of the performance of the entire primary liner system, including the drainage and barrier layers.

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Assessment of the Performance of Engineered Waste Containment Barriers al., 2002). Electrical resistance tomography has been used to image the full-scale test emplacement of both thin-wall grout barriers and thick-wall polymer barriers (i.e., an approximately 1-m-thick wall constructed by injecting colloidal silica into fine- and coarse-grained sands; see Box 3.5; Daily and Ramirez, 2000). The thick-wall polymer barrier was readily imaged, whereas the thin-wall grout barrier could be resolved only by measuring the difference in resistivity before and after emplacement. This study also demonstrated that electrical resistance tomography can be an effective technique for assessing the performance of barriers. Ground-penetrating radar surveys have been conducted to verify the integrity of thin-wall barriers (Pellerin et al., 1998) and the emplacement, location, and continuity of subsurface barriers (Rumer and Mitchell, 1995). Other geophysical techniques, such as seismic and acoustic methods, have been used to detect subsurface features such as a mud slurry trench, cement monoliths, and grout materials and to monitor the emplacement of a viscous liquid barrier and thin diaphragm wall (Pearlman, 1999). However, with the exception of electrical leak detection in geomembranes, the use of geophysical methods to assess barrier integrity has not generally found its way into routine engineering practice. Consequently, few data exist on the long-term integrity of vertical barrier elements. Among the factors limiting the successful use of geophysics are small contrasts in the physical properties being measured, different geometries of the targets from those for which geophysical methods have been developed and used successfully in the past, lack of information about geophysical-barrier property relationships, instrument degradation over long time periods, and high costs. In addition, engineers and regulators may be unwilling to try what many believe to be unproven technologies. Improvements in several areas may help facilitate wider BOX 3.3 Description of a Low-Level Radioactive Waste Landfill Monitoring System The Fernald, Ohio, onsite disposal facility, completed in September 2006, represents a state-of-the-practice disposal facility for mixed wastes, including construction and demolition debris and low-level radioactive waste. The disposal facility covers 32 ha and contains 1.9 million m3 of waste. The facility includes eight discrete cells underlain by a 1.8-m-thick double composite liner system and capped with a 2.9-m-thick composite final cover system (Figure 3.3). The final cover system includes a 0.9-m bio-intrusion layer and is designed to limit the release of radon-222 to the environment. FIGURE 3.3 Fernald facility liner and cover cross sections. SOURCE: Geosyntec Consultants.

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Assessment of the Performance of Engineered Waste Containment Barriers DOE design criteria called for the containment system to maintain its integrity for longer than 200 years and up to 1,000 years to the extent achievable. The cover system is designed with a low profile for “geomorphologic conformity” with the surrounding terrain (Figure 3.4). FIGURE 3.4 Fernald onsite disposal facility (OSDF). SOURCE: Geosyntec Consultants. Short-term monitoring of the facility called for comprehensive CQA during containment system construction, including sealed double-ring infiltrometer testing for the compacted clay layers and electrical leak detection surveys following geomembrane installation. Medium- and long-term monitoring includes settlement monitoring of the cap, monitoring of the cap drainage layer, and independent monitoring of the leachate collection and leak detection systems for each of the eight cells. The monitoring system (Figure 3.5) also includes eight horizontal monitoring wells in the glacial till underlying the facility (one horizontal well beneath each unit). The leachate collection systems all include auxiliary removal pipe systems and all drainage systems include pipe cleanouts. During the construction period, liquids collected from the leachate collection and leak detection systems are transferred to an industrial wastewater treatment plant and scanned for radioactivity prior to discharge. In the postclosure period, the drainage layers in the liner and cover systems all drain by gravity to a constructed wetland (no pumping is required). The monitoring plan also calls for periodic inspection of the cap to detect and repair any erosional gullies and to remove any deep-rooted vegetation that could potentially penetrate the barrier layers in the cover. FIGURE 3.5 Fernald facility bottom liner system monitoring. NOTE: WWTP = wastewater treatment plant. SOURCE: Geosyntec Consultants.

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Assessment of the Performance of Engineered Waste Containment Barriers BOX 3.4 Case History on the Puente Hills Landfill Canyons 1 and 3 Vertical Barriers This case study illustrates the effective use of vertical barrier walls and an extraction system to control offsite migration of organic contamination. The Puente Hills Sanitary Landfill, owned and operated by the County Sanitation Districts of Los Angeles County (the Districts), is the largest operating municipal solid waste landfill in the United States, receiving approximately 12,000 metric tonnes of municipal solid waste per day, 6 days per week. The oldest disposal area of the Puente Hills Landfill facility (the Main Canyon area) is not underlain by an engineered liner system and contains over 150 m of municipal solid waste. While the bedrock underlying the Main Canyon has relatively low permeability, it is overlain by several alluvium-filled tributary canyons. Because groundwater monitoring indicated that these tributary canyons provided a conduit for transport of leachate-impacted groundwater offsite, the Districts constructed a system of vertical barriers and extraction wells to control leachate migration through the alluvium in the canyons. The vertical barriers were constructed of bentonite and Portland cement supplemented with fly ash to create a very low permeability barrier material. Figure 3.6 shows the Canyon 3 barrier and monitoring system. Four extraction wells were placed in Canyon 3 on the landfill side of the cement-bentonite barrier wall and pumped to maintain an inward gradient (i.e., to draw water in the alluvium outside the landfill back toward the landfill and barrier). Monitoring wells on the opposite side of the barrier wall from the landfill monitor groundwater quality in both the alluvium and the low-permeability bedrock. FIGURE 3.6 Cross sections through the vertical barrier systems at Canyons 3 and 1 at the Puente Hills landfill facility. SOURCE: County Sanitation Districts of Los Angeles County. A similar strategy, but with 17 extraction wells, was employed in Canyon 1. A cross section through Canyon 1 in an area where the dipping bedrock includes more pervious strata interspersed among the low-permeability layers is shown in Figure 3.6. In this part of Canyon 1, the monitoring wells are placed in the pervious bedrock units outside the barrier wall. Figure 3.7 shows the concentration of volatile organic compounds with time in the monitoring wells closest to, but outside, the vertical barriers in Canyons 3 and 1. The steady decrease in volatile organic compounds with time in both of these wells demonstrates the effectiveness of the vertical barrier and extraction well system in controlling the offsite migration of groundwater impacted by landfill leachate through the tributary canyons. FIGURE 3.7 Volatile organic concentrations (VOCs) in groundwater outside the vertical barrier systems for Canyons 3 and 1 at the Puente Hills landfill facility. SOURCE: County Sanitation Districts of Los Angeles County.

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Assessment of the Performance of Engineered Waste Containment Barriers BOX 3.5 Case History on Geophysical Imaging of Engineered Hydraulic Barriers This case history illustrates the potential of using geophysical imaging techniques during emplacement of two prototype engineered barriers: a thin-wall grout barrier and a thick-wall polymer barrier. Electrical resistance tomography (ERT) was used to image the full-scale test emplacement of a thin-wall grout barrier installed by high-pressure jetting at Dover Air Force Base and a thick-wall polymer barrier installed by low-permeation injection at Brookhaven National Laboratory (Daily and Ramirez, 2000). ERT uses currents injected into the ground to image the electrical structure of the subsurface. A plan view of the thin-wall grout barrier wall installation is shown in Figure 3.8a. The test site is underlain by medium to fine sands with gravel, silt, and clay lenses over 6 to9mofmarine clays with thin laminations of silt. Electrical measurements were obtained before and after the installations. By comparing the two images it is possible to remove the heterogeneity of the natural background. The conductivity contrast between the emplaced materials, which are more conductive, and the native soils, which are more resistive, made the barriers excellent targets for electrical imaging. The electrical resistivity of the sands varied between 300 and 600 Ohm-m and the resistivity of the clay was <50 Ohm-m. FIGURE 3.8 (a) Plan view of the thin-wall barrier showing locations of the ERT wells. The dashed lines indicate the hole pairs used to acquire the ERT data. (b) Three-dimensional image block of electrical conductivity before barrier emplacement. (c) Difference image block between the baseline and post emplacement image block with all boreholes and surface anomalies removed. (d) Plan view of the horizontal section of barrier at 7.5 m depth. SOURCE: Daily and Ramirez (2000). The study found that the thin-wall grout barrier could only be imaged by obtaining the difference in resistivity between the background image obtained before installation and the image obtained after installation (Figure 3.8b-d). However, the viscous liquid barrier was electrically conductive compared to the background, so it could be imaged without having to subtract the background electrical resistivity structure (Figure 3.9).

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Assessment of the Performance of Engineered Waste Containment Barriers FIGURE 3.9 (a) Plan and schematic views of the viscous liquid barrier. (b) ERT two-dimensional section of the viscous liquid barrier. Left image is the baseline image taken on June 19; middle image is the difference taken on July 31 after the primary row was injected; right image is the difference taken on September 19 after all three rows were injected. SOURCE: Daily and Ramirez (2000). ERT was also used to investigate the performance of another thin-wall barrier (Daily and Ramirez, 2000). Saltwater was used as an electrical tracer to determine the presence and location of any leakage flow paths. The authors were able to image electrical conductivity changes associated with defects in the wall or the joining of two walls. The authors drew the following conclusions from these case studies. First, ERT can successfully image the location, size, and shape of subsurface barrier structures. Large defects can also be imaged with ERT, although resolution might be insufficient for imaging smaller defects. Second, ERT can be used to assess the performance of a barrier using gas tracers or water flooding. Third, ERT results can be substantially improved by obtaining differences between background resistivity structure before and after barrier emplacement. Fourth, the performance of ERT depends on the survey design, especially placement of the electrode wells. The best results come from placing electrodes in boreholes, rather than only on the surface.

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Assessment of the Performance of Engineered Waste Containment Barriers and more successful uses of geophysics for monitoring of waste barrier systems, such as (1) better integration of geophysics into the design of monitoring plans; (2) improved methods for data collection, processing, and interpretation; (3) development of new computer codes; and (4) new instrumentation (Majer, 2006). For example, geophysical imaging of vertical barriers may be more effective if a background dataset is obtained prior to barrier emplacement (Slater and Binley, 2006). A background dataset would also be useful in liner monitoring systems. Any changes in subsurface physical properties caused by leaks could be detected through comparison of monitoring data and background data. 3.3.5 Geosynthetic Clay Liner Barrier Monitoring Because geosynthetic clay liners (GCLs) are manufactured and thus undergo manufacturing quality assurance, their end-of-construction reliability tends to be significantly higher than that of compacted clay layers and is probably similar to that of geomembranes. GCLs are often used beneath relatively shallow depths (e.g., less than 1 m) of soil in cover systems. Because of serious performance concerns about GCLs buried under shallow depths of soil covers, GCLs have been exhumed and tested after several years of service to evaluate their integrity in the early medium term (e.g., Mansour, 2001; Henken-Mellies et al., 2002). However, this type of examination has been conducted only for research purposes and not as a routine part of barrier system monitoring. Recently, exposure of the GCLs in several composite liner systems employing the GCL as the low-permeability soil layer beneath the geomembrane has shown that GCL seams can separate as a result of environmental effects (Thiel and Richardson, 2005). The GCL seams in these cases were generally exposed because of other performance concerns (e.g., during repair of mechanical defects to the overlying geomembrane). The accidental discovery of GCL seam separation indicates the value of direct monitoring of barrier components. Most barrier system components are hidden from view after construction, and thus component defects will not be identified until performance problems appear elsewhere in the system. Geophysical techniques may be capable of detecting flaws in caps after construction (Slater and Binley, 2006). Moisture fluxes from cracks in the cap can be detected with electromagnetic induction methods and ground-penetrating radar (e.g., Ward and Gee, 2001). Caps could thus be monitored through long-term continuous measurements of near-surface moisture conditions (Meju, 2006). The development and deployment of permanent autonomous monitoring systems would facilitate such monitoring and also reduce labor costs. 3.4 MONITORING FREQUENCY AND REPORTING The frequency of monitoring measurements and reporting is generally established in the monitoring plan for a waste containment facility. Fixed probes (e.g., groundwater monitoring wells) are typically monitored quarterly or semi-annually, but they can be monitored monthly or more frequently if warranted. Surface gas emission measurements and air quality measurements are often made at monthly intervals. Sometimes daily or even hourly measurements are warranted, as in time domain reflectometry measurements of soil moisture in evapotranspirative caps, where modeling tries to capture daily fluctuations in near surface water content caused by fluctuations in temperature, solar radiation, barometric pressure, and precipitation. However, measurements of this frequency are rarely required to assess barrier performance or to monitor environmental protection because subsurface contaminant migration patterns rarely change rapidly over scales in excess of tens of meters. Where electronic data acquisition systems are employed, data may be recorded at closely spaced intervals but may not be looked at in detail unless a problem arises. An example of electronic data collection and dissemination is the system installed in Fernald, Ohio. A final cover monitoring program developed for the Fernald Closure Project relies on a data logger to collect pore water pressure, drainage layer temperature, soil water content, soil water potential, and soil temperature above the geomembrane (Benson et al., 2003). The data are collected hourly and uploaded to a publicly accessible Webbased data management system. Monitoring data may be reported only at quarterly or annual intervals, even if the data are captured more frequently. Quarterly reports are often simply data reports, and data interpretation is provided in annual or even multiyear summary reports. Some monitoring programs, however, have trigger levels, which, if exceeded, require immediate reporting and interpretation. 3.5 CONCLUSIONS Performance of a containment barrier system is defined by how well the system and its components work over time. A good barrier system is one that meets or exceeds its design specifications. While well-designed engineered barrier systems generally function adequately immediately after construction, long-term performance depends on the long-term integrity of the system components, as well as proper operation and maintenance of the system before and after closure. It is important to recognize that the consequences of an engineered barrier system failure may have environmental and financial costs that far exceed the incremental cost of a facility monitoring program designed to detect potential problems before they occur. Consequently, it is critical to monitor the performance of engineered barrier systems with a variety of techniques and in a variety of media (surface water, groundwater, air, and soil). For landfills, one of the most effective ways to both monitor performance and minimize the impact of a failure of a primary barrier system is to have a double-lined system, as is now required in a number

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Assessment of the Performance of Engineered Waste Containment Barriers of states. Confidence in the long-term proper performance of waste containment systems can be gained only if the proper monitoring protocols are implemented. Indirect monitoring of engineered barrier performance by monitoring for contaminant migration downstream of the waste containment system is commonplace, as it is mandated by regulations. Direct monitoring of barrier system component integrity is generally limited to an end-of-construction assessment of the component. Modern construction quality assurance procedures generally provide a high level of reliability for barrier component integrity in the short term. However, there has been little long-term direct monitoring of the integrity of individual barrier system components. The primary liner in a double-liner system is perhaps the only type of engineered barrier system in which postconstruction integrity is routinely monitored directly. Liquids collected in the leak detection layer between the primary and secondary liners provide a direct assessment of the performance of the primary liner system. The absence of direct postconstruction monitoring of barrier integrity for other types of systems may be attributed to a variety of factors, including difficulty in directly monitoring barrier integrity, particularly for barrier systems overlain by tens to hundreds of meters of waste or soil; a philosophy that it is the overall performance of a waste containment system, not the integrity of individual elements of the system, that is important; and the reluctance of designers, owners, operators, and regulators to monitor something they may not be able to remedy or that would be exceedingly costly to address. Of these factors, the technological limitation is perhaps the easiest to overcome, particularly for caps and many near-surface vertical barriers. A variety of techniques can be used to monitor the postconstruction integrity of caps. Since a cap generally has relatively shallow soil cover, exhumation and recovery of samples of cap material and tests for degradation of their properties are feasible in most cases. While this has been done for short-term or early medium-term monitoring of geosynthetic clay liners, no long-term evaluations of buried cover system elements of this type have been conducted to the committee’s knowledge. In situ moisture content monitoring of soil layers in caps above, in, and below the barrier system can also provide an indication of cap performance. Furthermore, electrical surveys and leak detection surveys could be employed with geosynthetic (geomembrane and perhaps geosynthetic clay liner) caps if a wire grid is placed below the barrier layer during construction, and other geophysical monitoring techniques (e.g., electromagnetic surveys) could be used to assess changes in the physical properties of the cap over time. Temperatures can be monitored in caps and bottom liner systems to determine the service environments for soil and geosynthetic barrier components. Geophysical techniques also offer promise for cost-effective long-term monitoring of vertical barriers. Electrical resistivity surveys and electromagnetic surveys offer the potential to detect gross defects that concentrate flow in vertical barriers. Tomographic imaging and seismic velocity surveys have the potential to detect changes in physical properties over time that may suggest barrier degradation. Inferred changes in barrier properties could be evaluated by in situ testing of the barrier or by physical sampling and laboratory testing when warranted. Airborne and satellite-based remote monitoring techniques may offer the potential for cost-effective indirect monitoring of cap and vertical barrier effectiveness. Multi-spectral imaging can indicate water content and temperature changes in near-surface soils, as well as distress and other changes in vegetation, each of which may indicate barrier performance problems. Interferometric synthetic aperture radar, light detection and ranging, and other airborne/satellite techniques can resolve centimeter-scale deformations caused by local or global instability or barrier performance problems. Autonomous monitoring systems could detect moisture fluxes from cracks in caps. However, these techniques have not yet been demonstrated as useful tools for evaluating containment barrier effectiveness.