Swelling and hydraulic conductivity of bentonites permeated with landfill leachates

This study investigates and discusses the free swell and hydraulic conductivities of bentonites which were gathered from the local companies in Turkey. Total of 26 hydraulic conductivity tests were carried out along the study 12 of which were permeated with deionized (DIW) and tap water (TW) and the rest of them were permeated with landfill leachates (LLs). The free swell volumes of bentonites decreased when the pore fluid type was changed from water to LL. The recorded final swell volumes were within the range of 14.5-27.0 mL/2g in water, whereas 5.0-19.5 mL/2g in LLs. The hydraulic conductivity tests were performed on artificially prepared geosynthetic clay liners (AP-GCLs) which were prepared in the laboratory. The hydraulic conductivities of AP-GCLs were within the range of 5.2×10−12 to 3.0×10−11 m/s when TW and DIW were used as the permeant. The results also showed that the hydraulic conductivities permeated with LLs were almost the same as with those permeated with DIW or TW. The hydraulic conductivities of AP-GCLs to LLs were within the range of 2.3×10−12 to 2.0×10−11 m/s. Electrical conductivity and pH measurements were also conducted on influents and effluents along the test duration. The effluent to influent ratio of electrical conductivity was generally less than 1.0 which indicates the continuation of cation exchange process between bentonites and LLs. This conclusion was further approved with the ICP analysis conducted on influent and effluent samples. The exchangeable Ca2+ concentrations in the effluents were still less than those in the influents when the tests were terminated. In contrast, Na+ concentrations in effluent were greater than those of influent, suggesting the fully replacement of cations were not completed at the time of termination. The uncompleted cation exchange process was physically observed by performing free swell tests on post-test samples. The free swell values of bentonites were in between characteristics of the values obtained in water and LLs. It was expected that the hydraulic conductivity of AP-GCLs would increase when LLs are used as the permeant. However, comparable results were measured as with those of water. This is possibly due to the greater effective stress applied during the study which masked the negative influence of cation exchange on the hydraulic conductivity of the AP-GCLs.