The purpose of column testing is to determine the transport and fate properties, typically the hydrodynamic dispersion coefficient (Dh) and retardation factor (Rd), of aqueous miscible chemical species (solutes) with respect to a given porous medium. This paper presents fundamental considerations required for column testing of engineered, low-permeability, clay-based barriers, such as compacted clay liners, geosynthetic clay liners, and soil-bentonite vertical cutoff walls, used to contain chemicals in a variety of solid and liquid waste containment and remediation applications. The presentation is limited to column tests conducted with a constant solute source concentration, and includes descriptions of the general procedures for conducting column tests as well as the expected behavior of the solute (effluent) breakthrough curve (BTC) based on several considerations. Significant information can be gained simply from the shape of the solute BTC. For example, a sigmoidal symmetric BTC implies advective-dominated solute transport with linear, instantaneous, and reversible sorption, whereas a sigmoidal asymmetric BTC can result from diffusion-dominated transport, nonlinear sorption, and/or nonequilibrium (kinetic) sorption. Also, because clay-based barriers are susceptible to hydraulic incompatibility when permeated with chemical solutions, significant hydraulic incompatibility can lead to a significant change in the seepage velocity (vs) under the constant hydraulic head (gradient) condition, which invalidates the use of analytical transport models to determine Dh and Rd via fitting of the measured solute BTCs, as well as the use of dimensionless time or pore volumes of flow. For this reason, constant flow hydraulic control is recommended for conducting column tests with engineered, low-permeability, clay-based barriers, because significant changes in hydraulic conductivity do not significantly affect vs. Finally, the potential significance of diffusion on solute transport through clay-based barriers can complicate interpretation of the existence of an effective porosity and determination of the correct, mass-based definition of Rd from the BTCs emanating from traditional concentration-based column tests. However, these issues are readily discerned when cumulative mass column testing is performed. Example results from both traditional, concentration-based column testing and alternative, cumulative mass column testing of clay-based barriers are provided to illustrate application of the concepts presented.

View full abstract

The removed soil generated from decontamination projects in Fukushima Prefecture is being transported to the interim storage facility (ISF) at present. In the ISF, modification agent such as water-absorbing polymer is used to improve handling of the removed soil during the separation process before storage. The expansion characteristic of the water-absorbing polymer agent might change the hydraulic property of the soil which will affect the leaching behavior. In addition, degradation of organic matter contained in the removed soil might influence the leachate quality as well. Thus, it is necessary to evaluate the environmental safety of the modified removed soil for safe storage in the ISF and its potential reuse in the future. In this study, two lysimeter tests on modified and unmodified removed soil was conducted to investigate its leachate quality. These tests lasted more than 400 days (liquid/solid ratios greater than 3.5). The highest 137Cs concentration in leachate from the unmodified soil was 6.6 Bq/L at 250 days, and the accumulated leaching ratio was 0.09%. In leachate from the modified soil (addition of 3% water-absorbing polymer agent powder), the highest 137Cs concentration was 3.3 Bq/L and the leaching ratio was 0.046%, a reduction by half. Leaching of 134Cs from both soils was extremely low. The soil modification resulted in accumulated leaching of total organic carbon that was 60 % higher and leaching of K+ and ammonia nitrogen (NH4-N) that was 35 % lower, respectively. No expansion of soil volume was observed after modification.

View full abstract

The paper discusses the results of an experimental research on hydraulic conductivity, sorption, diffusion and long-term performance of cement-bentonite mixtures in saline sulphate solutions as a function of their composition and curing time. The results show that permeation with solutions of K2SO4 can adversely affects the hydraulic performance of cement-bentonite mixtures depending on sulphate concentration and mixture composition. Choosing a very low permeable mixture at brief curing time is essential to limit adverse effects of interaction with SO42- on the hydraulic performance. Linear isotherms were found to model sorption of K+, with distribution coefficients resulted not to be significantly influenced with curing, allowing testing, usefully and safely, at brief curing. Values of the distribution coefficient in the range of those of cement paste were obtained. On the contrary, diffusion coefficients of K+ and of SO42- higher than those documented in the literature for cement pastes were found. In designing cut-off walls the results point out the importance of selecting a mixture with very low hydraulic conductivity values at brief curing and of considering its specific diffusion coefficients, as well as interaction and sorption capacity.

View full abstract

In assessing the environmental risk of contaminated soil and waste, it is crucial to understand the contaminants’ leaching characteristics. There are various leaching test methods such as single batch leaching test, serial batch leaching test, column water flow test, and tank leaching test. This paper outlines the test method and the results of the accuracy evaluation test of the up-flow percolation test.

View full abstract