Rates of sulfate reduction and metal sulfide precipitation in a permeable reactive barrier

A full-scale reactive barrier, utilizing bacterially mediated SO4 reduction to promote metal sulfide precipitation and alkalinity generation, was installed in August 1995 at the Nickel Rim mine site near Sudbury, Ontario. Monitoring of groundwater chemistry over a 3-a period allows assessment of long-term reactive barrier performance. The overall rate of SO4 removal within the barrier declined with time by 30% from an initial rate of 58 to 40 mmol l(-1) a(-1) 38 months after installation. Over the same time, the rate of Fe removal declined by 50% from 38 to 18 mmol l(-1) a(-1). The degree of SO4 reduction and Fe sulfide precipitation within the barrier is both spatially and temporally variable. Spatial differences are primarily the result of different residence times due to hydraulic conductivity variations of the treatment material. Temporal variations are likely the result of a decline in organic C availability and reactivity over time and seasonal variations in the rate of SO4 reduction. Temperatures in the aquifer fluctuate from a low of 2 degreesC in the winter to a high of 16 degreesC in the summer and the rate of SO4 reduction in the summer is nearly twice as great as the winter rate. An effective activation energy (E-a) of 40 kJ mol(-1) can account for the temperature-induced changes. In Year 3, the barrier removed > 1000 mg/l SO4 and > 250 mg/l Fe, demonstrating the long-term viability of this remedial approach. (C) 2002 Published by Elsevier Science Ltd.