FACT SHEET Leachate Recirculation in Municipal Solid Waste LandfillsWith Alternative Liners Incorporating GCLs INTRODUCTION This Fact Sheet describes the manufacturing, installation, testing, design and performance of geosynthetic clay liners, or GCLs. It focuses specifically on GCLs used as the lower component of a composite liner with a flexible membrane liner (or FML) placed above. [This Fact Sheet will refer to FMLs as geomembranes (i.e., GMs)]. The GCL component of the composite liner is considered to be an alternate to the regulated low permeability compacted soil (or compacted clay liner, i.e., CCL) per 40 CFR 25.8.28(a)(2). This regulation calls for the CCL to be at least 50cm(2-ft) thick with a permeability of 1 x 10-7 cm/sec, or less. The alternate, in this case GCL, must be equivalent, or superior, in its performance to the CCL. The geomembrane above the GCL or CCL is not at issue and remains as being up to 2.5mm(100 mils) thick. While GCLs are equally as appropriate in final cover systems, the complete focus of this fact sheet is GCL usage in base liner systems. MANUFACTURING GCLs are factory manufactured barrier materials using dry bentonite in powder or granular form. Bentonite is a geologically occurring clay mineral and is well recognized for its excellent sealing effects (Madsen and Nuesch, 1995). In the U. S., the bentonite used for GCLs is sodium based and is known to be the lowest permeability of any naturally occurring mineral. Its permeability ranges from 5 x 10-9 cm/sec to 5 x 10-10 cm/sec, which is 20 to 200 times lower than the soil used for a regulatory prescribed compacted clay liner (Daniel, et al., 1997). The bentonite is placed at approximately 4500 to 5000 g/m2 which results in a 7 to 10 mm thick layer between geotextiles, or on a geomembrane. For the geotextile-related GCLs, the composite can be needled together or stitch bonded. For the geomembrane-related GCLs, both thick and thin geomembranes have been used. The resulting cross-sections are shown in Figure 1 where GT refers to geotextile and GM to geomembrane. As manufactured, GCLs are typically 3.5 to 5.0 m (10 to 16 feet) wide and approximately 30 m (100 ft) in length. The currently available types are given in Figure 1.   Figure 1 - Typical GCL Cross-Sections   INSTALLATION A major advantage of GCLs over CCLs is their rapid installation. The photograph of Figure 2 illustrates the ease of placement. Adjacent GCLs rolls are installed with an overlap of approximately 220 mm (9 in.). The overlap is generally self-sealing, but can be augmented with additional bentonite in dry or paste form, see Figure 4. Due to the great attraction of water to bentonite (with subsequent swelling), the covering geomembrane must be placed before precipitation occurs, see Figure 3. Of course, this is the exact situation of an alternate base liner to which this Fact Sheet is directed, i.e., a GM/GCL composite liner. Figure 2 - A GCL Being placed on a Side Slope Figure 3 - Covering of a GCL by a Geomembrane thereby Forming a Composite iner, i.e., a GM/GCL Composite TESTING The manufacturing quality control (MQC) testing of GCLs is very well positioned insofar as ASTM test methods and standards are concerned. Table 1 illustrates that all components (bentonite, geotextiles and geomembranes) are addressed, as well as the finished product. Not only are properties and test methods covered in this standard, but also frequency of testing. Figure 4 - Different Overlap seam Methods Figure 5 - Simple GCL overlap   Table 1 - Various Tests and Testing Frequencies for the Manufacturing Quality Control of GCLs, per ASTM D5889 Test Designation Test Method Frequency of Testing Report Value ClayA Free swell   Fluid loss   D5890   D5891   One per truck or railcar but min, every 50 tonnes One per truck or railcar but min, every 50 tonnes   Minimum Average Minimum Average Geosynthetic Materials: Geotextile Mass per unit area   Grab tensile strength (MD and CD)     D5261   D4632     20,000 m2 (200,000 ft2)   20,000 m2 (200,000 ft2)     Typical and MARV MARV Geomembrane Mass per unit   Thickness Tensile strength at break and yield (MD and CD)   D5261   D5199 D638   20,000 m2 (200,000 ft2)   20,000 m2 (200,000 ft2) 20,000 m2 (200,000 ft2)   Typical and MARV MARV MARV Finished GCLB Clay mass per unit area (dried)C Clay Moisture Content Grab tensile strength (MD and CD)D Index FluxE   D5993 D4643 D4632 D5887   4,000 m2 (40,000 ft2) 4,000 m2 (40,000 ft2) 20,000 m2 (200,000 ft2) Once weekly with the last 20 values reportedG   MARV Average ValueF MARV Minimum value A The tests on the betonite are to be peformed on the as-received material before fabrication into the GCL product. B Components from finished GCL product should not be seperated and tested, because the production process may alter the properties of the components. C Dried betonite should be defined as 0% moisture content. D This test may not be applicable for geomembrane-based GCLs. E Certification letter from component manufacturer or QA from GCL manufacturer, or both. Certification letters must arrive and be checked before the components are used for the GCL production. F Only for information. G The last 20 values to be reported should end at the production date of supplied GCL. If the manufacturer has more production facilities or production lines, or both, the tests must be performed and reported for each line.   In addition to the previous MQC tests, which are often considered to be index tests, there are numerous performance tests which have been standardized by ASTM. These are presented in Table 2. Depending on the site-specific design process, some or all of these tests may be necessary. Table 2 - Selected Design (Performance) Tests for GCLs Property Test Method General Comments Flux ASTM D5887 always important should use site-specific stress and pressure conditions should have thickness measured at end of the test so as to calculate hydraulic conductivity (permeability coefficient) Direct Shear ASTM D6243 necessary for side slope designs generally the upper interface is of main concern sometimes internal strength is of concern sometimes the lower interface is of concern Creep Shear ASTM D6243-mod. difficult and costly test sometimes necessary with low factor-of-safety designs generally the upper interface is of concern sometimes internal strength is of concern rarely is the lower interface required Wide Width Tensile Strength and Elongation ASTM D4595 only necessary when tensile stresses are to be resisted (which is rare) possibly necessary when shear stresses are to be resisted Multi-Axial Tensile ASTM D5617-mod. only necessary for anticipated yielding subgrade situations Soil Compatibility, or Indentation ASTM D5818-mod. for subgrades with soil particles ≥ 12 mm (0.5 in.) Chemical Resistance ASTM D6141 for aggressive or reaction liquid permeants   DESIGN Having standardized test methods (both index and performance types) available, GCL design has proceeded in an organized manner as with all other engineering materials. In this regard, one counterpoints the alternative materials (GCLs) with the traditional materials (CCLs). Critical in this regard is to compare the flux that passes through the two materials under identical conditions. The example selected is to calculate the flow rate, or flux, through the two competing materials under 30 cm (12 in.) of hydraulic head. This value of head was selected because it is the maximum leachate head allowed on the base liner system beneath a municipal solid waste landfill.   While this flux comparison speaks well for GCLs over CCLs, there are additional issues to consider. They are presented in Table 3. It will be noted that all of these issues confirm the appropriateness of using GCLs as alternate barrier materials to CCLs. In addition to the contents of the table, see Note 1 with respect to freeze-thaw behavior, and Note 2 with respect to attenuation considerations.   Table 3 - Technical Equivalency Issues of GCLs versus CCLs Characteristic GCLs CCLs Materials Bentonite, adhesives, GTs and GMs Native soils, bentonite admixes Thickness 7 to 10 mm Typically 300 to 900 mm Hydraulic conductivity 5 x 10-9 to 5 x 10-10 cm/sec 1 x 10-7 cm/sec Construction deployment Rapid and simple installation Slow complicated construction Regarding manufacturing quality control (MQC) Factory manufacturing requires continuous MQC Naturally located clay mineral materials Regarding construction quality assurance (CQA) Relatively simple, straightforward, common-sense procedures Complex CQA procedures requiring detailed knowledge of clay soils and moisture/compaction relations Vulnerability to damage due to desiccation When dry, no concern; when wet, desiccation can occur but upon rewetting bentonite self heals CCLs are nearly saturated and can desiccate during construction, upon rewetting little self-healing occurs Availability of materials Materials easily shipped to any site Suitable materials not always available; may require expensive transportation Typical installed cost Approx. up to $ 10.00 per square meter for a large site Highly variable - estimated range: up to $50.00 per square meter Experience Began in 1986 and use is rapidly expanding Used successfully and unsuccessfully for many years   Note 1: Freeze-thaw cycling clearly favors GCLs over CCLs insofar as its potential susceptibility to damage is concerned. However, for base liner systems freeze-thaw issues are not of concern except during the installation period prior to covering to a frost-free depth. Note 2: Attenuation of pollutants passing through the covering geomembrane are better adsorbed by a thick CCL than by a thin GCL. If this is a major concern, the GCL can be augmented by an underlying layer of low permeability local soil, e.g., 1 x 10-4 to 1 x 10-5 cm/sec. This will provide attenuation and at a far more reasonable cost than a 1 x 10-7 cm/sec CCL. PERFORMANCE Since their inception in 1986, GCLs have been used in the upper base liner system of double liners with leak detection in a number of landfills. [There are 12 states requiring double liner systems for MSWLFs, Koerner, et al., 1998]. Having the underlying leak detection system as a witness drain allows for an assessment of the upper liner's performance. Fortunately, a major study has just been completed for the U. S. EPA which includes 91 landfills containing 287 single or multiple cells, Bonaparte, et al., 1999. Three different types of primary liners were involved (GM alone, GM/CCL and GM/GCL) and two types of leak detection materials (sand and geonet). Thus six combinations are available, see Table 4. Even further, data is available for three different stages during the life of the respective landfill cells (initial, active and post closure). The above data set has been plotted in Figure 6 (for the average flow rates) so as to give a graphic representation as to the effectiveness of the GM/GCL alternate barrier system. Note that the plotted data represents the average flow rates of 287 single or multiple cells monitored for up to 10-years. Readily seen is that the alternate GM/GCL outperforms the standard GM/CCL in all cases and at every life cycle stage. Clearly, the strong absorptive capability of the bentonite in the GCLs is having a significant influence in attenuating leakage through the covering geomembranes.   Figure 6 - Leakage rates for 287 landfill cells from CR 821448/01/0 final report to US. EPA, Bonaparte, et al. 1999   Table 4 - Leakage Rates from Leak Detection Systems of Double-Lined Landfillsfrom EPA Study CR-821448, by Bonaparte, et al., 1999 [All Flow Rates are in Gal/Acre-day (gpad)] Liner/LDS Type Type I (GM-Sand) Type II (GM-GN) Type III (GM/CCL-Sand) Life of Cycle Stage 1 2 3 1 2 3 1 2 3 Average Flow Minimum Flow Maximum Flow 41 0.81 229 18 0.0 158 6.8 0.02 26 10 0.51 40 11 0.15 38 ND ND ND 12 0.13 126 15 2.4 71 6.8 0.0 29 No. of "points" No. of landfills 30 11 32 11 8 4 7 4 11 6 ND ND 31 11 41 11 15 4 Liner/LDS Type Type IV (GM/CCL-GN) Type V (GM/GCL-Sand) Type VI (GM/GCL-GN) Life of Cycle Stage 1 2 3 1 2 3 1 2 3 Average Flow Minimum Flow Maximum Flow 18 0.0 74 8.9 0.0 54 7.0 0.0 14 14 0.0 104 2.38 0.0 30 0.03 0.0 0.10 0.70 0.0 3.6 0.28 0.0 1.0 ND ND ND No. of "points" No. of landfills 21 6 27 9 12 3 19 3 19 3 4 1 6 2 4 2 ND ND CONCLUSION This fact sheet has presented data and information indicating that geosynthetic clay liners (GCLs) are bona fide alternates to compacted clay liners (CCLs) used as the lower component of a composite base liner of a MSWLF. GCLs are factory manufactured materials challenged by an entire set of established ASTM standards, which also allows for a rational design methodology. By virtue of their ability to be rolled out directly upon a prepared soil subgrade or leak detection layer, their ease of installation is outstanding. All of these issues taken together lead to a very low installed cost in comparison to traditional CCLs. In 1991 when the original EPA regulation on MSWLF liners [i.e., 40 CFR 258.28 (a)(2)] was promulgated, GCLs were in their infancy. Today, GCLs have been shown to be superior in their barrier function to CCLs. The data of Figure 5 clearly attests to this situation. It is felt that if the technical background for Subtitle "D" regulations were developed today, the regulated cross section for MSWLF's would be a GM/GCL and not a GM/CCL! The alternate GM/GCL is clearly superior to the standard GM/CCL insofar as its barrier performance is concerned. In conclusion, it is recommended that GM/GCL liner composites be allowed as alternates to GM/GCL liner composites in the base liner system of MSWLFs that practice leachate recycling, i.e., bioreactor landfills. REFERENCES Bonaparte, R., Daniel, D. E. and Koerner, R. M., 1999, Assessment and Recommendations for Optimal Performance of Waste Containment Systems, Grant No. CR-821448, Final Report to Mr. D. A. Carson, U. S. EPA, ORD, Cincinnati, Ohio. Daniel, D. E., Bowders, J. J. and Gilbert, R. B., 1997, "Laboratory Hydraulic Conductivity Testing of GCLs in Flexible-Wall Permeameters," in Testing and Acceptance Criteria for Geosynthetic Clay Liners, L. W. Well, Ed., ASTM STP 1308, pp. 208-228. Koerner, J. R., Soong, T.-Y. and Koerner, R. M., 1998, A Survey of Solid Waste Landfill Liner and Cover Regulations, GRI Report #21, Folsom, PA, December 7, 1998, 211 pgs. Koerner, R. M., 1998, Designing with Geosynthetics, 4th Ed., Prentice Hall Publ. Co., Englewood Cliffs, NJ, 761 pgs. Madsen, F. and Nuesch, R., 1995, "Characteristics and Sealing Effects of Bentonites," Geosynthetic Clay Liners by R. M. Koerner, E. Gartung and H. Zanzinger Eds., A. A. Balkema, Rotterdam, pp. 31-50.