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IN-SERVICE HYDRAULIC CONDUCTIVITY OF GCLS IN LANDFILL COVERS - LABORATORY AND FIELD STUDIES
Citation:
MEER, S. R., C. H. BENSON, T. M. TOLAYMAT, AND D. A. CARSON. IN-SERVICE HYDRAULIC CONDUCTIVITY OF GCLS IN LANDFILL COVERS - LABORATORY AND FIELD STUDIES. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-05/148, 2005.
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Description:
Laboratory experiments using multi-species inorganic solutions (containing calcium and sodium) were conducted on specimens of a new geosynthetic clay liner (GCL) containing sodium bentonite to determine how cation exchange and desiccation affected the hydraulic conductivity. Calcium-for-sodium exchange decreases the ability fo the bentonite to swell during hydration. Cracks that form in the bentonite during desiccation do not completely close again during re-hydration once significant calcium-for-sodium exchange occurs in the bentonite. These cracks create preferential pathways for flow and increase the hydraulic conductivity of the GCL by several orders of magnitude. Results of free swell, hydraulic conductivity, and exchange complex analyses indicate that the ratio of monovalent to divalent cations (RMD)of the hydrating liquid controls whether cation exchange will occur in the bentonite. Samples of GCLs from four landfill covers were obtained and tested for water content, free swell, hydraulic conductivity, and composition of the exchange complex. Calcium-for-sodium exchange occurred in all of the exhumed GCLs, and bentonite from these samples had free swell similar to the free swell for calcium bentonite. Hydraulic conductivities of the exhumed GCLs varied over nearly 5 orders of magnitude and exhibited no relationship with cover soil thickness. Very high hydraulic conductivities were even obtained for a GCL that had been covered by a geomembrane. The hydraulic conductivity of the exhumed GCLs was strongly related to the gravimetric water content at the time of sampling. Controlled desiccation and re-hydration of exhumed GCLs that had low hydraulic conductivity (10-9 to 10-7 cm/s) resulted in increases in hydraulic conductivity of 1.5 to 4 orders of magnitude, even with overburden pressure simulating the field condition applied. These results confirm that desiccation or lack of hydration is a key factor controlling the hydraulic conductivity of GCLs once calcium-for-sodium exchange has occurred. Analysis of the exhumed GCLs, in conjunction with results reported in other field studies, suggest that calcium-for-sodium exchange in the bentonite in GCLs is almost certain to occur in most field applications. Moreover, increases in hydraulic conductivity are likely to occur unless the water content of the GCL can be maintained above 100%. Hydraulic conductivity data from the literature and from this study imply that commonly used protective measures are inadquate in many cases to prevent cation exchange and desiccation of GCLs in landfill covers.