Microbiology of a multi-layer biosolid/desulfurized tailings cover on a mill tailings impoundment.

The Strathcona Waste Water Treatment System (SWWTS; Sudbury, ON, Canada) has received mill tailings from Ni/Cu ore processing from 1970 to present. Demonstration-scale, multi-layer cover systems were installed on selected tailings deposition cells at the SWWTS. The cover systems are comprised of an upper layer of organic carbon-rich material, composed of a layer biosolids fertilizer along with composted municipal food and yard waste, then a layer of desulfurized, fine-grained tailings. Organic carbon components used in these covers promote microbial communities that consume O2, thus decreasing sulfide oxidation rates in the underlying tailings. The aim of this study was to investigate the microbiology of the cover systems and the underlying tailings, using a combination of culture-dependent (most probable number) and culture-independent (16S rRNA gene amplicon sequencing) techniques, and assess the impact of the organic component of the cover system four to six years after implementation. Most tailings samples were characterized by circumneutral bulk pH and low concentrations of dissolved metals. The presence of the organic cover resulted in elevated counts of sulfate-reducers (by two orders of magnitude, compared to control samples) immediately below the organic cover, as well as an increased abundance of heterotrophic species (∼108 cells g-1) at greater depth (∼4 m) in the tailings profile. Mineral-oxidizing microorganisms were also present in the tailings, with neutrophilic sulfur-oxidizers dominating the samples (mean ∼106 cells g-1). Relative abundances of sulfur- and/or iron-oxidizers determined by sequencing ranged from 0.5 to 18.3% of total reads (mean ∼5.6% in amended tailings) and indicated the presence of local microenvironments with ongoing sulfide oxidation. This work provides a detailed characterization of the microbiology of a tailings impoundment with an organic cover, highlighting the opportunities associated with monitoring microbial processes in such remediation systems.

Native plants facilitate vegetation succession on amended and unamended mine tailings.

Facilitating the establishment of native pioneer plant species on mine tailings with inherent metal and/or acid tolerance is important to speed up natural succession at minimal cost, especially in remote areas where phytoremediation can be labor intensive. We investigated vegetation community dynamics after ∼48 years of succession along two legacy Ni-Cu mine tailings and waste rock deposits in the Sudbury Basin, Ontario, Canada with and without various site amendments (i.e. liming and fertilization) and planting. Metal/acid tolerant pioneer plants (Betula papyrifera, Populus tremuloides, Pohlia nutans) appeared to facilitate the establishment of less tolerant species. Conifers and nitrogen-fixers less tolerant to site conditions were planted at the fully amended (limed, fertilized, planted) mine tailings site in the 1970s, but conifers were not propagating at the site or facilitating understory succession. The planted nitrogen-fixing leguminous species Lotus corniculatus was, however, associated with increased diversity. These findings have implications for long-term reclamation strategies in acidic mine waste deposits utilizing native species, as primary colonizing tree species are only recently emerging as candidates for phytoremediation. Novelty statement The potential for native species to act as facilitators for vegetation colonization has rarely been investigated on tailings, despite wide use in remediation of less toxic sites. This study provides a retrospective of over 40 years of plant growth following initial treatment of toxic tailings. We observed that regardless of tailings geochemical conditions, acid/metal tolerant pioneer plants were facilitating ecological succession on acidic Ni-Cu mine tailings sites.

Blended pulp mill, forest humus and mine residual material Technosols for mine reclamation: A growth-chamber study to explore the role of physiochemical properties of substrates and microbial inoculation on plant growth.

A growth chamber trial was conducted to investigate the effects of blends of pulp and paper mill residuals and forest humus on soil properties, microbial communities and germination rate and biomass production of annual ryegrass (Lolium multiflorum) in both acid-producing and neutral to mildly alkaline mine tailings in a mine reclamation context. The organic residual amendments improved the nutritional status of the tailings substrates, and increased pH in acid-generating tailings, leading to higher germination rates and improved plant growth. A trace addition (<0.02% of sludge by dry weight) of natural forest floor material as a microbial inoculum to the sludge could increase plant biomass up to four-fold. The effects of sludge application on bioavailability of metals were variable, with the concentration of soluble copper (Cu) and nickel (Ni) increasing in some of the substrates following organic amendments. Addition of paper mill residuals to mine tailings modified the microbial communities observed in the oligotrophic tailings with the majority of DNA sequences in the sludge amended substrates being found to be closely related to heterotrophic bacterial species rather than the chemolithotrophic communities that dominate tailings environments.