Laboratory investigation of GCL performance for covering arsenic contaminated mine wastes

The performance of three geosynthetic clay liners (GCLs) above arsenic-bearing gold mine tailings is examined based on a series of laboratory column experiments conducted over a two year period. The GCLs examined had either untreated or polymer-enhanced sodium bentonite and had different carrier geotextiles (a woven geotextile, a scrim-reinforced nonwoven geotextile, and a woven geotextile laminated with a polypropylene film). After 24 months, the hydraulic conductivity (k) of GCL with untreated sodium bentonite increased by almost an order of magnitude from 3 × 10−11 m/s to 2 × 10−10 m/s with the swell index decreasing from 26 mL/2 g to 7-11 mL/2 g and the exchangeable sodium percentage (ESP) decreasing from 65% to 12-21%. Samples of this GCL exhumed after 9 months of moisture uptake and cation exchange and then subjected to 15 extreme wet-dry cycles experienced an increase in k up to 6 × 10−10 m/s. The GCL with polymer-enhanced bentonite maintained k less than 5 × 10−11 m/s after 24 months (virgin k = 1 × 10−11 m/s) and experienced less cation exchange (ESP decreased from 78% to 39-42%) than the GCL with untreated bentonite. For this GCL, 15 extreme wet-dry cycles gave k < 9 × 10−11 m/s provided that the soil stress was 15 kPa or higher (≥1.0 m of cover soil above the GCL). The GCL with polymer-enhanced bentonite and carrier geotextile laminated with a polypropylene geofilm demonstrated the best performance with k < 7 × 10−12 m/s even with exposure to the extreme wet-dry cycling. In all cases, the GCL prevented arsenic migration into the overlying cover soil over the two year period examined.