Enhanced contaminant retardation by novel modified calcium bentonite backfill in slurry trench cutoff walls

Abstract

This study aimed at assessing containment properties of slurry wall backfill containing sodium hexametaphosphate (SHMP) modified calcium bentonite to contaminated groundwater. Free swell index and flexible-wall hydraulic conductivity (k) tests were conducted to investigate swelling and hydraulic performances of this novel backfill. The relative concentration (RC) and cumulative mass ratio (CMR) methods were adopted to assess transport parameters including hydrodynamic dispersion coefficient (D) and the retardation factor (Rd) of calcium ion (Ca2+) through the backfills. The results showed negligible to moderate decrease in the swell index of the modified bentonite and the modified backfill when exposed to 5 mM and 1 M calcium chloride (CaCl2) solution. Permeation with CaCl2 solutions increased the short-term k of the backfill by a factor of 0.9 to 1.1 as compared to the tap water permeation. The D values varied from 1.44 × 10–10 m2/s to 2.89 × 10–10 m2/s regardless of the CaCl2 concentration, whereas Rd was vulnerable to CaCl2 concentration. The CMR method yielded a higher coefficient of determination than the RC method, but it showed no particular advantage in evaluating the transport parameters. The SHMP-modification delayed the breakthrough time of Ca2+ by a factor of 15 to 199 in a model slurry trench wall.