METAL BACKGROUND AND MOBILITY IN THE KENO HILL MINING DISTRICT, YUKON TERRITORY, CANADA

The Keno Hill mining district in the Yukon Territory was Canada’s second largest silver producer after Ontario’s Cobalt mining camp. A new mine is being developed and additional ones planned while remediation of old sites is in progress. To aid with developing practical sediment and water quality guidelines for permitting new and decommissioning old mines, a small lake located in the centre of the mining district and affected by drainage from the surrounding mines was comprehensively studied to shed light on the metal background and mobility in the mining district. Geochemical analyses coupled with Pb-210 age dating of a finely sliced sediment core (in 5 – 10 mm segments) show good correlation of sediment metal contents with past and recent mining and reclamation activities. Soils and sediments in the Keno Hill area are generally enriched in As, Cd, Zn and, to a lesser extent, Pb with average background concentrations of 329±19, 6.0±0.2, 785±58 and 44±1.5 μg/g, respectively. Mining-related activities have increased the average Cd and Zn contents in near-surface sediments by 1.6 and 100 times relative to their background values while the As and Pb concentrations do not change significantly. Integrating the sediment pore and surface water chemistry with historic monitoring data in the lake and the associated drainage, all of which are characterized by near-neutral to slightly basic pH, it becomes evident that Cd and Zn are readily mobilized while As, Cu and Pb are not. Redox cycling of Mn appears to control the downstream migration of dissolved Zn and some Cd, which could vary from up to 200 and 10 mg/L in an adit discharge to as low as 0.15 and <0.01 mg/L, respectively, near the confluence with the South McQuesten River 12 km downstream. Although progressive freezing during the winter months could increase the total dissolved solids content of the lake and creek waters by up to 40%, precipitation of secondary minerals apparently does not occur in the absence of a nucleating substrate. The roles of microbes and metal-organic matter interaction in attenuating aqueous contaminant transport in this drainage system underlain by discontinuous permafrost are yet to be explored.