Apparent salt diffusion coefficients for soil–bentonite backfills

Abstract

Apparent diffusion coefficients, Da, were measured for two soil–bentonite (SB) backfills characteristic of those used in SB vertical cutoff walls for subsurface control of contaminant migration. The base soils for the backfills comprised either a natural lean clay or sand–bentonite mixtures. The base soils were mixed with 5% bentonite–water slurry to obtain a slump of 125 mm, resulting in total bentonite contents ranging from 4.76% to 7.31%. Values of Da for sodium chloride were measured using a recently developed dialysis-leaching test method. The Da values for the clay–bentonite and sand–bentonite backfills ranged from 2.5 × 10−10 to 5.3 × 10−10 m2/s and from 1.4 × 10−10 to 8.1 × 10−10 m2/s, respectively. Values of Da for both backfills increased with increasing average salt concentration in the specimen (Cave). Values of Da decreased by ≤50% with increasing backfill bentonite content. For all Cave values, the clay backfills exhibited lower Da than the sand–bentonite backfills, likely due to additional fines from the lean clay. Results of this study enhance understanding of solute diffusion through SB cutoff walls, as well as support future use of the dialysis-leaching test method to measure diffusion properties of SB backfills.