A Mass-Balance Tool for Monitoring Potential Dissolved Sulfur Oxidation Risks in Mining Impacted Waters

Sulfur oxidation intermediate compounds (SOIs) in mine tailings reservoir water are linked to adverse environmental impacts, such as acidity, toxicity, and oxygen consumption, and can lead to regulatory non-compliance. Their prediction and management has largely focused on a subset of SOI compounds referred to as thiosalts (SnOx2−), with thiosulfate (S2O32−) presumed to be the dominant thiosalt species, yet no published study to date has determined if SnOx2−and, specifically S2O32−, dominate SOIs in these waters. The objectives of this study were to: (1) determine sulfur mass balance across a range of mining impacted waters; (2) evaluate whether SnOx2−compounds dominate the SOI pool for these waters; and (3) compare current industry analytical methods for thiosalts determination with a new approach proposed here. From 2014 to 2018, 52 water samples were collected from four Canadian base metal mine water management systems in Ontario, Manitoba, and Newfoundland. These samples were characterized for total sulfur (TotS) as well as individual sulfur species (∑H2S, S0, SO32−, S2O32−, S3O62−, S4O62−, and SO42−). Thiosulfate was consistently found to represent only a minor component of TotS, with an average relative percentage of ≈ 4% across all samples. The reactive sulfur pool, (Sreact), defined here as all sulfur compounds capable of oxidation, was significant and variable, averaging 30 ± 25% of the S budget. A direct comparison of Sreact concentrations to thiosalts concentrations determined by either of the two currently available analytical methods (indirect acid titration method or direct S2O32− method by ion chromatography) indicated that Sreact was significantly higher (p < 0.05) for a selected suite of samples (n = 6) collected in 2018. These results indicate that currently available methods may underreport the concentrations of oxidizable sulfur compounds in tailings reservoir water caps and receiving environmental waters. Sreact provides a conservative, economically viable and directly comparable measurement to monitor potential dissolved sulfur oxidation risks in water discharged to receiving environments.