Abstract
From 1861 to the 1940s, mining and milling of gold in Nova Scotia generated more than 3,000,000 tonnes of tailings, many of which were discharged into wetland environments. These tailings contain high concentrations of As hosted in a range of primary and secondary (weathering-related) phases. We collected bulk samples of four main tailings endmembers (hardpan, oxic, wetland, and high Ca tailings) and exposed these to natural rainfall in field bins to investigate drainage chemistry under unsaturated conditions. These tests were designed to simulate exposure of freshly excavated tailings to surficial weathering during construction, remediation, or reprocessing activities. Leachate was sampled bi-weekly for two years and tailings were characterized before and after the field tests using geochemical (ICP-ES/MS), mineralogical, and static testing (ABA) methods. Acid-base accounting results suggest that three of the four tailings endmembers are potentially acid-generating and only one (high Ca tailings) was predicted to be acid-consuming. Leachate compositions were consistent with these predictions. Both the hardpan and wetland tailings produced acidic (pH 2-3.5), metal(loid)-rich waters. The oxic tailings generated mildly acidic (pH 4-5) leachate, whereas the high Ca tailings produced circumneutral (pH 7-8) drainage but still released high As concentrations. Arsenic and SO4 concentrations peaked in the late summer and early fall, reflecting sulphide oxidation during wetting and drying cycles in the warmer months. The relatively rapid onset of acidic conditions in leachate from the wetland tailings contrasts sharply with field measurements of in situ pore water pH in these saturated tailings, which were consistently circumneutral. These results suggest that sulphides in the wetland tailings are very reactive under unsaturated conditions and that historical tailings in wetlands should not be disturbed during mine site reclamation. Installation of a cover over the other tailings types may help slow sulphide oxidation but may also destabilize secondary As-bearing oxide minerals over time.