Effect of Tailings Mineralogy and Infiltration Water Chemistry on Arsenic Release From Historic Gold Mine Tailings

Abstract

The existence of small-scale gold mining in Nova Scotia between 1868 and 1942 has resulted in many high arsenic (As) tailings areas in the province, some of which are near rural/urban areas and are used for recreational activities such as dirt bike racing and all-terrain vehicle (ATV) riding. Because of the natural association of As with gold ore in the Meguma Terrane, processing of ore has resulted in As-rich mine waste that contains up to 2500 times more As than the Canadian soil quality guideline of 12 mg/kg. These high As concentrations in combination with the recreational use of these sites creates a risk of human exposure. The objective of this work was to investigate the effects of different cover options that might be used to mitigate the risk of human exposure. Four tailings samples were selected to represent the geochemical variability from two tailings areas: Montague gold mines and Goldenville. These samples were characterized and subjected to 29 weeks of column testing, in which each sample was leached with three different input solutions including synthetic rainwater (to simulate uncovered tailings exposed to natural acid rain), synthetic rainwater equilibrated with calcium carbonate (to simulate rainwater percolation through a crushed limestone cover), and a dilute organic acid solution (to simulate a vegetative cover). Results of acid base accounting (ABA) tests indicate that samples have the potential to generate acid in the future (ratio of neutralization potential to acid potential is less than 2), though surface water at the sites is currently circum-neutral. Acidic paste pH values (2.9) from a sample of As-rich hardpan indicate that a small volume of tailings at Montague are currently generating acid. Results of column testing indicate that the cover types simulated by the input solutions had less of an effect on the out-flowing leachate chemistry than did the small volume of secondary As phases in each sample (scorodite, yukonite, hydrous ferric arsenate and hydrous ferric oxides). For the majority of sample types, columns leached with an organic acid solution reported higher leachate As concentrations than were reported from columns leached with either the rainwater or carbonate-rainwater solutions.