Long-Term Column Testing of Zeolite-Amended Backfills. II: Solute Transport Properties

Eight, long-term column tests (1.05–3.75 year) involving soil-bentonite (SB) backfills with 5 or 10% (by dry weight) of one of three types of zeolite amendment (clinoptilolite, chabazite-UB, chabazite-LB) and permeated with either 35 mM KCl or 20 mM ${\mathrm{ZnCl}}_2$ solutions were conducted to determine the effects of type and amount of zeolite amendment on the hydrodynamic dispersion coefficient, $D$, and the retardation factor, $R_d$, of ${\mathrm{Cl}}^{-}$, ${\mathrm{K}}^{+}$, and ${\mathrm{Zn}}^{2+}$. The superimposed effluent breakthrough curves for ${\mathrm{Cl}}^{-}$ were virtually coincidental indicating excellent reproducibility of all eight column specimens, and solute transport generally was dominated by diffusion relative to mechanical dispersion owing to low imposed seepage velocities ($1.60\times {10}^{-8}\mathrm{m}/\mathrm{s}\le v_s\le 6.88\times {10}^{-8}\mathrm{m}/\mathrm{s}$). Also, $D$ of ${\mathrm{Cl}}^{-}$ tended to be greater for the zeolite-amended backfills relative to the unamended backfill, which was attributed to the replacement of the coarse-grained sand with fine-grained zeolite amendments, leading to more tortuous pathways. The $R_d$ of ${\mathrm{Cl}}^{-}$ was essentially unity ($1.00\le R_d\le 1.03$) for seven of the eight column tests, suggesting the absence of any effective porosity. Retardation of ${\mathrm{K}}^{+}$ and ${\mathrm{Zn}}^{2+}$ correlated reasonably well with the measured cation exchange capacity of the backfills, as expected on the basis that cation exchange was the dominant attenuation (sorption) mechanism. Overall, as little as 5% zeolite amendment was effective in enhancing the sorption capacity ($R_d$) of the SB backfill by a factor ranging from 2.4 to 3.2 for ${\mathrm{K}}^{+}$ and 1.4 to 2.2 for ${\mathrm{Zn}}^{2+}$, whereas the backfill with 10% chabazite-UB amendment was 4.7 times more effective than the unamended backfill in sorbing ${\mathrm{K}}^{+}$. Thus, the column test results support the hypothesis that SB backfills for vertical cutoff walls that are amended with zeolite can significantly increase the sorption capacity for metals, thereby enhancing the containment function of the cutoff walls.