Performance of polymer-enhanced bentonite–sand mixture for covering arsenic-rich gold mine tailings for up to 4 years

Experiments conducted over a 4 year period, on a polymer-enhanced bentonite–sand mixture (PEBSM) used as cover for gold mine tailings are reported. The effect on PEBSM hydraulic conductivity (k) of subgrade porewater chemistry, subgrade water content, and confining stress are investigated. Results show that the reduction in the mole fraction of bound Na+ (ESP) and corresponding increase in k of PEBSM with time was highly dependent on the ionic strength of the subgrade porewater. When the PEBSM was in direct contact with gold mine tailings with porewater having an ionic strength of 145 mmol/L, ESP decreased from 59% to 2% and k increased from 4 × 10−11 to 6.9 × 10−9 m/s. The ESP and k values of PEBSM over tailings with 11 mmol/L porewater were 21% and 6.9 × 10−11 m/s, respectively. A 0.15 m thick foundation layer between tailings and PEBSM layer significantly lowered the reduction in ESP and increase in k with time as did a reduction in the subgrade water content. There was no effect of changing confining stress from 15 to 7.5 kPa on k values of PEBSM. The PEBSM layer acted as a good barrier to the migration of arsenic from tailings upward towards cover soil above the PEBSM layer.