URANIUM MOBILITY IN GROUNDWATER AT HISTORICAL MINE SITES IN THE BANCROFT REGION OF ONTARIO, CANADA

Granitic pegmatite-hosted U-Th-REE deposits are widespread within the Grenville geological province of eastern Canada. Around Bancroft, Ontario, during a uranium exploration boom in the 1950s, four of these deposits entered production while numerous others saw underground development. The environmental legacy of past mining and renewed exploration activities in the area have raised public concerns about potential impacts to groundwater quality. To investigate these concerns, groundwater at ten historical mine properties was sampled in drill holes, flowing adits and flooded shafts. Uranium occurs almost entirely in the dissolved (< 0.45µm) phase at concentrations varying between 0.6 and 2579 µg/L. The Canadian standard for U in drinking water (20 µg/L) is exceeded in 70% of samples. By comparison, background concentrations in nearby un-mineralized granitic Precambrian bedrock range between 0.02 and 66 µg/L and only 5% of values exceed the standard. The mineralogy and solubility of U-bearing minerals are important primary controls on U concentrations. While uraninite and uranothorite are the main U-bearing phases, uranothorite may be more soluble because of its non-ideal composition and metamict structure. The highest U concentrations are observed in shallow, oxic groundwaters and speciation modeling indicates that over 98% of dissolved U is in the form of highly mobile uranyl-Ca-carbonate complexes that inhibit U adsorption. The lowest concentrations are found in reducing groundwaters in equilibrium with uraninite and coffinite. Activities of Ra-226 and Pb-210, decay products of U-238, are only weakly correlated with U concentrations although samples exceeding the standard for U generally exceed regulatory limits for Ra-226 (0.5 Bq/L) and Pb-210 (0.2 Bq/L) as well. In sub-oxic conditions Ra-226 is relatively more mobile than U whereas U and Pb-210 are more mobile in oxic groundwaters. Natural mitigation of U at former mine sites is observed when discharge drains to organic-rich wetlands exhibiting reducing conditions. Where mine adits drain to nearby lakes, U may pose an environmental risk because of the mobility of uranyl-Ca-carbonate complexes in oxic waters.