Hydrogeology, geochemistry and microbiology of a reactive barrier for acid mine drainage

Abstract

A full-scale permeable reactive barrier was installed in August 1995 into an aquifer impacted by mine drainage waters at the Nickel Rim mine site, near Sudbury, Ontario. The reaction barrier (3.6 m x 15 m x 4 m) contains organic compost to promote bacterially mediated sulfate reduction and subsequent metal sulfide precipitation. Dramatic changes in concentrations of SO4 (decrease 2000-3000 mg/L), Fe (decrease of 270-1300 mg/L), trace metals (e.g., Ni decreases 30 mg/) and alkalinity (increase of 800-2700 mg/L) are observed. Populations of sulfate reducing bacteria, dissolved sulfide concentrations, and isotope 34S are elevated compared to the up-gradient aquifer. Solid phase analysis of the reactive mixture indicates the accumulation of Fe mono-sulfide precipitates.

The overall rate of SO4 and Fe removal declines with time from initial rates of 58 and 38 mmol L-1 a-1 respectively, to 40 and 18 mmol L-1 a-1 respectively, 38 months after installation, likely due to declining organic carbon reactivity. Heterogeneous flow, and resulting variation in residence times, produces spatially variable treatment and decreases barrier performance. The SO4 reduction rate varies seasonally by a factor of 2 which is attributed to seasonal shifts in groundwater temperature (3-16oC: an effective activation energy of Ea=10 kcal mol-1 can account for this change.

Enumeration of bacterial population in the Nickel Rim groundwater flow system indicate elevated populations of iron and sulfur oxidizing bacteria are restricted to zones of groundwater recharge and discharge. Sulfur oxidizers are highest in the tailings (1.27 x 10^3 MPN/g) where sulfide minerals are exposued to oxygen and iron oxidizers are highest (9.56 x 10^5 MPN/g) where effluent discharges to the surface. Comparatively low populations of oxidizing bacteria in the tailings reflect low rates of sulfide oxidation due to the high water content in the zone of active oxidation. An observed positive correlation between populations of sulfate reducing bacteria and sulfur oxidizing bacteria suggests interdependency.

Numerical flow modeling shows that heterogeneities in hydraulic conductivity within the aquifer will strongly affect thin barrier performance, while thicker barrier performance will be more strongly affected by variation in barrier hydraulic conductivity. More uniform flow can be attained utilizing thicker, homogeneous barriers.