ASSESSING THE CUMULATIVE EFFECTS OF RESOURCE DEVELOPMENT ON A MINING-IMPACTED WATERSHED, COBALT MINING CAMP, ONTARIO

Increasing demand for critical minerals has led to renewed exploration and development in many historical mining districts, posing unique challenges for assessing and regulating environmental impacts from new mining activities. The goal of this study is to determine the pre-mining and present-day geochemical baseline conditions in lake sediments downstream of the Cobalt Mining Camp in Ontario, and to develop new methods for distinguishing the environmental effects of future mining from complex background conditions. From 1904 to the mid-1930s, and intermittently until 1989, more than 420 million ounces of Ag were mined from polymetallic (Ag-Co-Ni-Bi-As-Cu-Sb) vein deposits in the Cobalt region, resulting in 18 unconfined tailings disposal areas and widespread contamination of the local environment. Today, the region is experiencing a new exploration boom focused on Co for rechargeable batteries. Samples of tailings, surface water, and lake sediments were collected from 2019 to 2022 to evaluate the concentrations and mineral hosts for key elements of concern. Geochemical analyses using ICP-ES/MS following aqua regia digestion show that the tailings contain high concentrations of many metal(loid)s, including As (338 to 32,400 mg/kg; median 1520 mg/kg), Co (97 to 2790 mg/kg; median 482 mg/kg), and Hg (0.06 to 11.1 mg/kg; median 0.83 mg/kg). Several decades after the cessation of mining, weathering and erosion of these mine wastes continue to release high concentrations of metal(loid)s, especially As (up to 540 µg/L; <0.45 µm), to surface waters, which travel downstream via Farr Creek into Lake Temiskaming. Analyses of metal(loid) profiles in dated sediment cores from nine locations in the lake demonstrate that historical mining impacts extend at least 3 km south from the mouth of Farr Creek. Metal(loid) concentrations in these cores increase sharply after the onset of mining in Cobalt and remain elevated throughout most of the 20^th century before decreasing to a new geochemical baseline in the near-surface sediments. Results from this study will provide decision makers with a better understanding of the cumulative effects of mining and other human activities in the Cobalt region over time and help to guide environmental assessments for future resource development projects.