Long-term Biogeochemistry of Mine Tailings Amended with Organic Carbon for Water-quality Management Matthew B.J. Lindsay 1,* , David W. Blowes 1 , Peter D. Condon 2,1 and Carol J. Ptacek 1 1 University of Waterloo, Waterloo, ON, Canada, blowes@uwaterloo.ca, ptacek@uwaterloo.ca 2 Petros GeoConsulting, Fairbanks, AK, USA, condonpete@gmail.com * Present Address: University of British Columbia, Vancouver, BC, Canada, mlindsay@eos.ubc.ca Abstract Amendment of mine tailings with organic carbon (OC) was evaluated as a technique for water- quality management. The objective of this technique is to limit transport and discharge of sulfide-mineral oxidation products by inducing dissimilatory sulfate reduction (DSR) and metal- sulfide precipitation. Six experimental cells were constructed in the vadose zone of a sulfide- and carbonate-rich tailings deposit characterized by neutral drainage. Varied mixtures of peat, spent brewing grain (SBG) and municipal biosolids (MB) were incorporated into fresh mill tailings. The geochemistry, mineralogy and microbiology of the cells were monitored for six years. Sulfate-reducing conditions developed within three years in cells amended with multiple OC sources. Sulfate removal generally corresponded to alkalinity production, growth of sulfate- reducing bacteria, H2S production, 34 S-SO4 enrichment and Zn, Mn,and Tl removal. The addition of OC initially induced Fe and As mobilization; however, this process was most pronounced for cells that contained MB. Large increases in pore-water SO4 concentrations observed immediately below the tailings surface resulted from sulfide-mineral oxidation. Sustained sulfate removal was observed below the oxidation zone in tailings amended with peat+ SBG. Increased SO4 concentrations observed with other amendments were attributed to declining rates of DSR. Key Words:sulfate reduction, metal-sulfide precipitation, alkalinity production, passive in situ treatment Introduction Drainage from waste rock and tailings deposits can have negative impacts on groundwater and surface water quality at both active and decommissioned mines. This issue is of particular importanceat mines exploiting sulfide-mineral deposits, which are an important source of metals worldwide. The oxidative dissolution of sulfide minerals in mine wastedeposits posesthe primary risk to water quality (Blowes et al., 2003). This process generates H + and releases sulfate and metals to pore waters. The dissolution of carbonate minerals can maintain circumneutral pH conditions, however, weakly-hydrolyzing metals (i.e., Zn, Ni, Cu I , Fe II ) and oxyanion-forming metals (i.e., As, Mo, Sb, Se) may remain mobile under these conditions (Heikkinen et al., 2009; Lindsay et al., 2009a).Subsequent acidification due to ongoing sulfide- mineral oxidation and the depletion of acid neutralization capacity can lead to large increases in dissolved metal concentrations(Nordstrom et al., 2000; Moncur et al., 2005; Gunsinger et al., 2006). Unmitigated drainage from sulfide-rich mine waste deposits therefore can have widespread impacts on water quality that persist for decades to centuries (Moncur et al., 2006; Ruiz et al., 2008; Strosnider et al., 2011). Additionally, dissolved organic carbon (DOC), sulfoxyanions and metals contributed with residual mineral processing waters also influence water quality(Smuda et al., 2008; Lindsay et al., 2009a). Managing these potential impacts on water quality during active mining and following closure is fundamental to the environmental sustainability of metal production. The development of innovative techniques for managing water quality is essential for minimizingenvironmental impacts and financial liabilities associated with mining operations. Substantial