Diffusion through soil-bentonite backfill from a constructed vertical cutoff wall

Abstract

Diffusion coefficients were measured for soil-bentonite (SB) backfill sampled from a constructed vertical cutoff wall in Pennsylvania, USA. The backfill consisted of lean clay (the base soil) combined with bentonite-water slurry containing 5–6% sodium bentonite, resulting in a total bentonite content of ~1%. Ten dialysis leaching tests (DLTs) were conducted to measure apparent diffusion coefficients (Da) for chloride (Cl−) diffusion through the lean clay-bentonite backfill. In addition, model sand-bentonite backfills (total bentonite content = 4.7%) were prepared in the laboratory, and eight DLTs were performed for comparison with the field-sampled backfill and the diffusion literature. Both backfill types were tested with monovalent and divalent salt solutions (KCl and CaCl2), over a wide range of concentrations (60–350 mM). Despite the difference in bentonite content, values of Da were similar for the two backfills. Changes in Da with increasing average Cl− concentration were relatively minor, increasing by less than half an order of magnitude over the wide range of salt solutions, likely due to the low percentages of bentonite (<5%) in both backfills. Thus, for SB backfills with bentonite contents <5%, Da appears to be relatively insensitive to backfill composition, salt type and concentration.