Arsenic fate in the historic Cobalt Mining Camp, Ontario: Geochemical, mineralogical, and vegetation-related controls on metal(loid) mobility.

Abstract

This study investigates the controls on metal(loid) mobility from mine tailings and pond sediments near Cobalt, Ontario with a focus on arsenic (As), which is present at high concentrations throughout this former mining district. Mineralogical and geochemical analyses were performed on tailings from four abandoned mill sites and nearby pond sediments. The mass of readily soluble metal(loid)s in the tailings and sediments was evaluated using shake flask experiments. Horsetails (Equisetum spp.) growing on these tailings were collected for chemical characterization, backscattered electron imaging, and synchrotron-based chemical analysis. Results indicate that mineralogical hosts of As vary between depositional environments. Authigenic, oxidized phases (e.g., erythrite and an iron- and calcium-bearing arsenate) sequester As in near-surface environments. Ore minerals (e.g., cobaltite and safflorite) and authigenic reduced phases (e.g., realgar) host As in submerged tailings and anoxic pond sediments. Short-term exposure of tailings and sediments to oxidized, ultrapure water releases As from the ore minerals and reduced phases (1.31-325 mg L^-1 As; median = 19.1 mg L^-1 As). Horsetails growing on these tailings sequester As via formation of an oxidized iron-bearing plaque on the outside of the plant roots and shoots. This study demonstrates that As mobility in mine-impacted environments is controlled by geochemical reactions, mineral dissolution and precipitation, and vegetation growth. These processes must be considered when developing long-term management decisions for legacy mine sites.