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The bentonite and ocean-floor clay used in hydraulic barriers for on-land and ocean waste disposal sites, respectively, act as the last defense against environmental pollution, and therefore must have high chemical resistivity and long-term stability. The primary factors that determine the hydraulic conductivity of bentonite and ocean-floor clay are the polarity and functional groups of their organic compounds as well as their concentration and hydration characteristics in solution. However, it is not known which of the organic compounds reduce the hydraulic barrier performance of clay liners, and the extent of degradation has not been quantified. Recently, high concentrations of 1,4-dioxane and bisphenol A have been detected in some waste leachates. Although these low-polarity compounds hinder the hydration reaction of clay minerals and increase the hydraulic conductivity of clay liners, the magnitude of their impact in the limited concentrations found in leachates remains unclear. In this study, long-term hydraulic conductivity tests are conducted on a bentonite liner and ocean-floor clay samples, where the permeation liquid contains 1,4-dioxane or bisphenol A. The hydraulic barrier performance is evaluated based on the hydraulic conductivity and the distribution coefficient.

This study investigated the soil adsorption characteristics against gaseous mercury. A method for the adsorption tests using gaseous mercury was established, and the distribution coefficients for silica sand, decomposed granite soil, mordenite, and calcium bentonite were evaluated. Glass vials with polytetrafluoroethylene (PTFE) plugs were suitable for the adsorption tests as gaseous mercury did not adsorb onto the glass vials, indicating that the effects of the gaseous mercury adsorbed onto the glass vials are negligible. Although the PTFE plug could not completely seal a vial containing gaseous mercury, the leakage could be corrected using the damping time constant. The soil adsorption tests revealed changes in the gaseous mercury concentrations, which were fitted to the Langmuir nonequilibrium adsorption model. Assuming gaseous mercury did not leak from the vial, numerical simulations were conducted using the fitted parameters of the maximum adsorption amount, adsorption rate, and desorption rate. The evaluated distribution coefficients at equilibrium conditions were 56.3 mL/g for silica sand, 2070 mL/g for decomposed granite soil, 3490 mL/g for mordenite, and 7140 mL/g for calcium bentonite.

Previous studies have shown that within a month benzene easily passes through geomembranes (PVC, LDPE, and HDPE geomembranes). This study uses diffusive permeation tests to clarify the barrier performance of geomembranes against bisphenol A and 1,4-dioxane, which have been detected in some Japanese waste leachates. Although dissolved bisphenol A can be adsorbed on the geomembranes, it does not pass through them during the testing period (at least 400 days are required for it to completely pass through 0.5-mm-thick PVC geomembranes). In addition, dissolved 1,4-dioxane is not adsorbed on any geomembranes and hardly passes through any of them (at least five years are required for it to completely pass through 0.5-mm-thick PVC geomembranes). Thus, geomembranes exhibit a good barrier performance against these two organic compounds, especially 1,4-dioxane.

When geosynthetic clay liners (GCLs) are applied as bottom liners at waste containment facilities, they are naturally prehydrated by absorbing moisture in the underlying base layers. In order to evaluate the effects of cations contained in waste leachates, this study investigated the effects of the water content distribution of the GCLs prehydrated with actual soils on their hydraulic conductivities against CaCl2solutions. The “prehydration tests”, which were conducted prior to the hydraulic conductivity tests, showed that the water content distribution of the prehydrated GCLs depends on the properties of the GCLs and the base layers. In particular, drastic differences between GCLs with powdered bentonite and GCLs with granular bentonite were observed in the prehydration water content and its distribution. Prehydrated GCLs with powdered bentonite had a higher water content and a more homogenous distribution than those with granular bentonite. The hydraulic conductivity tests showed that most of the prehydrated GCLs exhibit a low hydraulic conductivity of k≍1.0×10-8cm/s against CaCl2solutions with 0.1-0.5 M. However, GCLs with granular bentonite may be difficult to homogeneously prehydrate and exhibit an unstable hydraulic conductivity, which varies from k=2.9×10-9cm/s to k=1.5×10-6cm/s. The homogeneity of the water content distribution has been considered an important factor to obtain a required barrier performance under prehydration conditions, which are naturally generated in actual sites.

To investigate systematically the effects of electrolytic solutions on the barrier performance of geosynthetic clay liners (GCLs), a long-term hydraulic conductivity test for 3 years at longest was conducted on a nonprehydrated GCL permeated with inorganic chemical solutions. The hydraulic conductivity test for waste leachates was also conducted. The results of the test show that the hydraulic conductivity of GCLs significantly correlates with the swelling capacity of bentonite contained in GCLs. GCLs have excellent barrier performance of k<1.0×10-8cm/s when the free swell is larger than 15 mL/2 g-solid regardless of the type and concentration of the permeant solution. In addition, when the results of the hydraulic conductivity test with chemical inorganic solutions were compared to those with waste leachates, the hydraulic conductivity of GCL permeated with chemical solution was almost the same within the electric conductivity of 0-25 S/m as that permeated with waste leachate having similar electric conductivity. The hydraulic conductivity of GCLs to be used in landfill bottom liners can be estimated by the hydraulic conductivity values obtained from the experiment using chemical solutions having the similar electric conductivity values, if the chemical solution had the electric conductivity within=25 S/m.

To evaluate the effects of a clay layer on the performance of pump-and-treat remediation, an advection-dispersion analysis assuming non-sorption was calculated by changing five parametric variables, namely, the length of the clay layer, the thickness of the clay layer, the hydraulic conductivity of the clay layer, the effective molecular diffusion coefficient of the contaminants, and the contamination duration. The simulation procedure consisted of firstly contaminating the analysis domain, and then cleaning up this contaminated domain by a pump-and-treat system in order to consider strictly the contamination conditions of a clay layer at a real contaminated site. From the calculation results, the pump-and-treat remediation was found to be effective if at least one of the following two conditions was satisfied, namely, the hydraulic conductivity of the clay layer was greater than 5.0×10-5cm/s or the thickness of the clay layer was less than 3 m. In addition, the contamination duration was an important parameter in evaluating the applicability of this pump-and-treat remediation to contaminated sites. For cases in which the contamination duration was short, the contamination conditions of the clay layer easily became heterogeneous such that the remediation duration was much longer than the contamination duration. It is necessary to note that contaminated sites may not always be remedied in a short period of time, even if the sites are discovered early.