The Diavik Waste Rock Project: Geochemical and microbiological characterization of drainage from low-sulfide waste rock: Active zone field experiments

• Effluent from waste rock with low S content and equally low C content was studied.
• High concentrations of SO42−, H+, and dissolved metals were recorded after 1 year.
• S- and Fe-oxidizing microorganisms were detected.
• Annual temperature fluctuations influenced sulfide oxidation rates.
• A very modest sulfide mass resulted in the generation of AMD.

A field experiment was conducted at the Diavik Diamond Mine to evaluate the potential for acid generation and metal release from the active layer of unsaturated waste-rock piles in the Arctic. Four active zone lysimeters (AZLs) were constructed in 2006 using high-density polyethylene tanks filled with, and surrounded by, waste rock to evaluate the geochemical and biogeochemical characteristics in the upper 2 m of waste-rock stockpiles. Two AZLs were filled with waste rock containing an average of 0.014 wt.% S predominantly as pyrrhotite (Type I AZLs) and two AZLs were filled with waste rock containing an average of 0.035 wt.% S (Type III AZLs). The geochemistry and microbiology of the effluent from the AZLs was monitored for 4 years. The effluent from the Type III AZLs was characterized by low pH (<3.5) with high concentrations of SO42− and dissolved metals. An increase in SO42−, Fe, and most dissolved metals coincided to the decrease in pH to 3.0 and the depletion of alkalinity. Maximum concentrations of SO42−, Al, Zn, Ni, Co, and Cu were observed in 2010, the final year of the dataset analyzed here. The effluent from the Type I AZLs maintained neutral pH (∼7.5) and lower concentrations of SO42− (<2000 mg L−1), Fe (<1.4 mg L−1), and other dissolved metals. The examination of effluent from the upper 2 m of waste rock indicated the active freeze-thaw layer underwent extensive weathering during the first 4 years of rock placement. Populations of Fe- and S-oxidizing bacteria in the AZLs were determined by most probable number methods. Enumerations for the Type III AZLs indicated an abundance of acidophilic S-oxidizing microorganisms, whereas neutrophilic S-oxidizing microorganisms were most abundant in the Type I AZLs. Iron-oxidizing microorganisms were generally less abundant than S-oxidizing microorganisms, but were detected in low-pH effluent with high dissolved metals in one of the Type III AZLs. The results of this experiment demonstrate that, in spite of the very low sulfide content of the waste rock and the cold temperatures prevalent at the Diavik site, colonization by S- and Fe-oxidizing bacteria and the microbially mediated oxidation of pyrrhotite can result in the generation of acidic drainage. Furthermore, although waste rock with a sulfide content of 0.035 wt.% S generated acidic drainage within the first year following construction, the pH of the effluent from the waste rock containing 0.014 wt.% S remained neutral, demonstrating the need for careful planning and storage of waste rock at the Diavik site.