Column percolation tests may be suitable for prediction of chemical leaching from soil and soil materials. However, compared with batch leaching tests, they are time-consuming. It is therefore important to investigate ways to shorten the tests without affecting the quality of results. In this study, we evaluate the feasibility of decreasing testing time by increasing flow rate and decreasing equilibration time compared to the conditions specified in ISO/TS 21268-3, with equilibration periods of 48h and flow rate of 12mL/h. We tested three equilibration periods (0, 12-16, and 48h) and two flow rates (12 and 36mL/h) on four different soils and compared the inorganic constituent releases. For soils A and D, we observed similar values for all conditions except for the 0h-36mL/h case. For soil B, we observed no appreciable differences between the tested conditions, while for soil C there were no consistent trends probably due to the difference in ongoing oxidation reactions between soil samples. These results suggest that column percolation tests can be shortened from 20 to 30days to 7-9days by decreasing the equilibration time to 12-16h and increasing the flow rate to 36mL/h for inorganic substances.

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Up-flow column percolation tests are used at laboratory scale to assess the leaching behavior of hazardous substance from contaminated soils in a specific condition as a function of time. Monitoring the quality of these test results inter or within laboratory is crucial, especially if used for Environment-related legal policy or for routine testing purposes. We tested three different sandy loam type soils (Soils I, II and III) to determine the reproducibility (variability inter laboratory) of test results and to evaluate the difference in the test results within laboratory. Up-flow column percolation tests were performed following the procedure described in the ISO/TS 21268–3. This procedure consists of percolating solution (calcium chloride 1 mM) from bottom to top at a flow rate of 12 mL/h through softly compacted soil contained in a column of 5 cm diameter and 30 ± 5 cm height. Eluate samples were collected at liquid-to-solid ratio of 0.1, 0.2, 0.5, 1, 2, 5 and 10 L/kg and analyzed for quantification of the target elements (Cu, As, Se, Cl, Ca, F, Mg, DOC and B in this research). For Soil I, 17 institutions in Japan joined this validation test. The up-flow column experiments were conducted in duplicate, after 48 h of equilibration time and at a flow rate of 12 mL/h. Column percolation test results from Soils II and III were used to evaluate the difference in test results from the experiments conducted in duplicate in a single laboratory, after 16 h of equilibration time and at a flow rate of 36 mL/h. Overall results showed good reproducibility (expressed in terms of the coefficient of variation, CV, calculated by dividing the standard deviation by the mean), as the CV was lower than 30% in more than 90% of the test results associated with Soil I. Moreover, low variability (expressed in terms of difference between the two test results divided by the mean) was observed in the test results related to Soils II and III, with a variability lower than 30% in more than 88% of the cases for Soil II and in more than 96% of the cases for Soil III. We also discussed the possible factors that affect the reproducibility and variability in the test results from the up-flow column percolation tests. The low variability inter and within laboratory obtained in this research indicates that the ISO/TS 21268–3 can be successfully upgraded to a fully validated ISO standard.

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Longterm barrier performance of geosynthetic clay liners (GCLs) when exposed to acid rock drainage (ARD), which is one of the most severe and expensive environmental problems facing the mining and some construction operations, was evaluated. Free swelling, sorption, and a ninemonth hydraulic conductivity tests on a needle punched GCL against an artificial ARD (pH = 3) that contained Al, Fe, Cu, Zn, As, and Pb, were conducted. Free swelling tests showed that a high metal concentration and/or a low pH negatively impacted on osmotic swelling. Sorption test results provided information about the competition among metals, and the Nabentonite capacity to sorb single metals and metalloids. Ninemonth hydraulic conductivity tests demonstrated that pH, EC and permeability changes over time, due to metal sorption/release and precipitation (physical clogging). The hydraulic conductivity remained low during the test duration and was approximately five times lower when GCL was prehydrated with water before ARD permeation (1.1x10 10 m/s) compared to the case in which prehydration and permeation were done using ARD (5.0x10 10 m/s). In each case, effluents were evaluated and breakthrough curves were constructed to get information about the GCL attenuation capacity toward metals present in ARD. Considering that bentonite (or GCLs) has the potential to retain heavy metals present in solution, showed relatively low hydraulic conductivity under even extreme conditions, and is available in many parts of the world, GCLs seem to be one possible solution for ARD mitigation.

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Evaluating the factors that affect hydraulic conductivity and metal retention in geosynthetic clay liners (GCLs) becomes crucial in order to decide whether or not GCLs can be used as a barrier material in rock containments with potential for acid rock drainage (ARD) generation. This study presents a state-of-the-art review of the factors that impact the hydraulic conductivity and heavy metal retention on GCLs: effect of the type of bentonite inside GCLs, prehydration, confining pressure, pH, metal concentration, and type of metal ion in ARDs. ARDs affect the hydraulic performance of GCLs because of their characteristic low pH (usually lower than pH 3) and presence of different heavy metals and concentrations (especially the presence of divalent and trivalent metals). The retention of metals present in ARD through GCLs are governed by the pH (higher retention at pH higher than 3.5), the metals that are present in the ARD as well as the buffering capacity of the bentonite (determined by the cation exchange capacity of bentonite). Ion exchange constitutes the main mechanism for metal uptake, but other mechanisms also take place. Secondary sorption on ferrihydrite or gypsum (formed through precipitation of metals present in ARD) of metalloids such as As, which cannot be sorbed by bentonite directly, may also occur.