Numerical Evaluation of Vertical Cutoff Walls Comprising Zeolite-Amended Backfills for Enhanced Metals Containment

The potential for enhanced containment of two metals, potassium (K) and zinc (Zn), by soil-bentonite (SB) vertical cutoff walls comprising zeolite-amended backfills was evaluated on the basis of numerical transport simulations and previously measured adsorption data that exhibited nonlinear behavior over the range of concentrations of interest. The results indicate that the containment duration as reflected by the barrier flux breakthrough time increased for K with decreasing source concentration (Co), increasing content of zeolite amendment, and increasing adsorption capacity of the backfill. The results for Zn are similar to those for K except at lower Co (i.e.,100 and 1,000mg/L), where better performance occurred with the unamended backfill relative to that for the zeolite-amended backfills. This ostensibly counterintuitive result for Zn was attributed to the geochemical conditions existing in the adsorption tests, whereby adsorption of Zn was suppressed due to unfavorable competition with elevated concentrations of redissolved metals, primarily Na+, resulting from the zeolite amendment. Overall, the results of the study indicate that containment of metals may be enhanced from years to a century or more with zeolite-amended SB cutoff walls, but the magnitude of any enhanced containment is dependent on both the adsorption capacity and the adsorption behavior of the specific metal with the specific backfill.