Mechanisms controlling acid neutralization and metal mobility within a Ni-rich tailings impoundment

Low-quality pore waters containing high concentrations of dissolved H⁺, SO₄, and metals have been generated in the East Tailings Management Area at Lynn Lake, Manitoba, as a result of sulfide-mineral oxidation. To assess the abundance, distribution, and solid-phase associations of S, Fe, and trace metals, the tailings pore water was analyzed, and investigations of the geochemical and mineralogical characteristics of the tailings solids were completed. The results were used to delineate the mechanisms that control acid neutralization, metal release, and metal attenuation. Migration of the low-pH conditions through the vadose zone is limited by acid-neutralization reactions, resulting in the development of distinct pore-water pH zones at depth; the neutralization reactions involve carbonate (pH ⩾ 5.7), Al-hydroxide (pH ⩾ 4.0), and aluminosilicate solids. As the zone of low-pH pore water expands, the pH will then be primarily controlled by less soluble solids, such as Fe(III) oxyhydroxides (pH < 3.5) and the relatively more recalcitrant aluminosilicates (pH ⩾ 1.3). Precipitation/dissolution reactions involving secondary Fe(III) oxyhydroxides and hydroxysulfates control the concentrations of dissolved Fe(III). Concentrations of dissolved SO₄ are principally controlled by the formation of gypsum and jarosite. Geochemical extractions indicate that the solid-phase concentrations of Ni, Co, and Zn are associated predominantly with reducible and acid-soluble fractions. The concentrations of dissolved trace metals are therefore primarily controlled by adsorption/complexation and (or) co-precipitation/dissolution reactions involving secondary Fe(III) oxyhydroxide and hydroxysulfate minerals. Concentrations of dissolved metals with relatively low mobility, such as Cu, are also controlled by the precipitation of discrete minerals. Because the major proportion of metals is sequestered through adsorption and (or) co-precipitation, the metals are susceptible to remobilization if low-pH or reducing conditions develop within the tailings.