Assessment of Municipal Solid-Waste Landfill Liner Performance

Abstract

The leachate collection system (LCS) and leak detection system (LDS) flow rate data from 240 cells (or a combination of cells) at 54 municipal solid-waste landfills (located in seven US states) with double-liner systems were analyzed to assess the performance of the primary liner system. The average LCS leachate collection rates for the study sites ranged from 380 L ha−1 day−1 (40.7 gal. acre−1 day−1) to 22,400 L ha−1 day−1 (2,390 gal. acre−1 day−1) on a sitewide basis, and the average LDS leachate collection rates ranged from 1.8 L ha−1 day−1 (0.2 gal. acre−1 day−1) to 577 L ha−1 day−1 (61.7 gal. acre−1 day−1) on a sitewide basis. Assuming all leachate generated is collected either by the LCS or LDS, the data suggest that the primary liner systems’ aggregated efficiency is over 98%. The collection efficiency at sites that used a composite liner (geomembrane underlain by a geosynthetic clay liner or a compacted clay liner) system was not statistically different from the sites that used only a geomembrane as the primary liner (geomembrane underlain by a permeable layer) (median of 99% for both types). Leakage rates were compared with those estimated from the equations used by the hydrologic evaluation of landfill performance (HELP) model. The comparison suggests that the equations used by the HELP model to estimate leakage through the liner overestimate the leakage rate through geomembrane primary liners but underestimate the leakage rate through composite primary liners based on the HELP-model-default defect size and suggested defect frequency. It is also possible that groundwater intrusion could contribute to a portion of the leachate collected from the LDS because leachate quality data collected from a few sites indicated the LCS leachate had a higher concentration of most constituents than the leachate collected from LDS. The findings from this investigation have practical relevance to landfill operators and researchers involved in this field of study. The results suggest that incorporating a geosynthetic clay liner (GCL) or compacted clay liner (CCL) between the LCS and LDS may not significantly impact the leakage rate through the primary liner, possibly because of the poor contact between the geomembrane and GCL due to the presence of wrinkles in the geomembrane. More data from additional sites should be evaluated before concluding that GCL underlying a geomembrane does not offer an additional benefit of minimizing leachate leakage through a geomembrane. Additionally, the inconsistency between the measured and predicted leakage rates from equations used in this field of study indicate that the models used by researchers and consultants to predict liner leakage may need to be revisited and updated to reflect the leachate leakage rates observed at MSW landfills. The authors have identified potential future research areas based on this study’s findings. A more in-depth analysis of the reasons for the inconsistency between the measured and predicted leakage rates and examining alternative equations should be considered for future research. Future research should consider collecting additional LCS, LDS, and groundwater quality data from sites with double-liner systems, which would help determine whether leachate from the LCS is the predominant component of the leachate collected from the LDS or if groundwater or another outside source of water is a larger contributor. Finally, future research should consider assessing the contact quality between the geomembrane and underlying GCL/CCL achieved with current construction practices and its impact on the leakage rate through the primary liner.