Drought and liming impacts of mine-impacted wetland sediments

Acid mine drainage (AMD) is a serious environmental problem at legacy and active mine sites around the world. Climate associated drought and rewetting events can increase the severity of AMD impacts through oxidation and release of stored metal(loid)s and acidity from contaminated sediments. The area surrounding Sudbury, Ontario, with its massive mining and smelting complexes, appears especially vulnerable to drought-driven effects. The impacts of drought-rewetting cycles in the heavily mine-impacted Frood subwatershed of Junction Creek, flowing through the centre of Sudbury and containing a large mine site and waste rock storage area was of particular interest, as drought and flood management strategies are being proposed. Laboratory drying and re-wetting experiments with sediment-cores collected from the Nickledale wetland in the Frood branch surprisingly showed no post-drought re-acidification (pH<6), with average pH among all drought treatments being 7.25 ± 0.20. However, highest concentrations of many metal(loid)s were observed in the longest (60-day) drought treatment, suggesting a drought effect may still occur. These included [Ni] (approx. 100x control), [Cu] (approx. 10x control), [Cd] (approx. 65x control) and [Co] (approx. 200x control), all depicting post-drought release from sediment. Pre-drought lime treatments showed reduced concentrations of many metal(loid)s in-solution following re-wetting. This included average [Cd], [Co], [Cu] and [Ni] observed to be significantly less in the lime-plus-30-day-drough treatment (L30d) compared to 30 day drought only treatment (30d) (RM ANOVA, p ≤ 0.007 [Cd]; p ≤ 0.004 [Co]; p = 0.022 [Cu]; p ≤ 0.009 [Ni]) (Figure 2a; Table A1 (supplementary)). This study suggests that the remedial work at the mine impacted site has reduced the vulnerability of this subwatershed to climate-driven impacts.