Variations in U concentrations and isotope signatures in two Canadian lakes impacted by U mining: A combination of anthropogenic and biogeochemical processes

Temporal and vertical variations in uranium (U) concentrations and U isotope (delta U-238, parts per thousand) signatures were examined in sediment cores collected seven times over a one year period, from two lakes in Ontario, Canada, which are contaminated with U by historical mining activities. Bow Lake is holomictic, experiencing seasonal anoxia, while the sediments of meromictic Bentley Lake are permanently anoxic. Average annual peak concentrations of U in Bow Lake subsurface sediments were approximately 300 mu g L-1 and 600 mu g g(-1) in porewater and bulk sediments, respectively. Similar ranges of concentrations (900 mu g L-1 and 600 mu g g(-1), respectively) were observed in Bentley Lake sediments. The exceedingly high levels of U observed in the porewaters of both lakes, as well as the seasonal variability in U levels, challenge the traditional paradigm regarding U chemistry, i.e., that reduced U(IV) should be insoluble under anoxic conditions. The average annual delta U-238 parts per thousand values at the sediment-water interface of both lakes were similar (i.e., 0.47 +/- 0.09 parts per thousand and 0.50 +/- 0.16 parts per thousand, relative to IRMM-184). The deep sediments in both Bentley Lake and Bow Lake record U isotope composition with a typical fractionation of 0.6 parts per thousand relative to the surface water, confirming authigenic U accumulation, i.e., negligible contribution of particulate material from the tailings. Also, the delta U-238 values in porewater have an average offset of ca. -0.1 parts per thousand relative to bulk sediments in anoxic zones and are reversed in the oxic sediment layer.