High-carbon wood ash biochar for restoration of metal mine tailings
J.M. Williams . 2024.
Abstract
Canada's history of unregulated mining activities has left a legacy of orphaned, barren mine
tailings areas exposed and unmanaged, posing structural and chemical risks, while modern
operational mines contribute an ever-increasing volume of tailings stored on site. Re-establishing
vegetation to ensure that tailings structures are self-sustaining, coherent with surrounding
ecology, and chemically secure remains a critical challenge. Bottom ash from bioenergy facilities
is currently landfilled in Canada, yet represents a potential nutrient-rich, alkaline soil
amendment. Bottom ash can contain high levels of chemically recalcitrant charcoal residues, and
thus qualify as a type of biochar for use on soils; its broad availability and low cost makes it
appealing for tailings restoration. This thesis investigates recycling of high-carbon wood ash
biochar (HCWAB) for emulating wildfire residues on disturbed mining land to promote
ecosystem recovery. Through multi-year field studies on both historic (exposed) and modern
(sand-capped) metal mine tailings in Canada's boreal forest, I found that: (1) low to moderate
dosage (3-13 t/ha) applications of HCWAB can be highly beneficial for encouraging volunteer
vegetation, but that wood ash impurities can result in deleterious effects at high dosages; (2)
volunteer plant species composition displays site- and dosage-specific responses to HCWAB
additions; however, greatest species richness is observed at intermediate dosages; (3) the survival
and growth of native saplings and transplanted wild trees ("wildings") peaks at mid-range
HCWAB dosages; and (4) tree tissue concentrations and substrate availability of potentially toxic
metals from tailings and HCWAB remain below levels of toxicity concern across both sites;
however, metals concentrations increase with high dosages of HCWAB (30 t/ha) at the historic,
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more initially contaminated site. Higher HCWAB application dosages at the sand-capped site are
also associated with increased herbaceous volunteer cover and planted tree performance is
negatively correlated with vegetation cover, consistent with a resource competition effect.
Methodologically, I introduce laser-ablation ICP-MS for conifer tissue analysis, making possible
trace metal estimates for small shoot extension growth samples typical of northern boreal forests.
In sum, HCWAB holds great potential for restoration of both historic, exposed tailings and
modern, sand-capped tailings; future field operational trials are advocated, focusing on strategies
to refine site-specific dosages, reduce competition effects, and explore possible co-amendment
strategies.
