Eastern Section Meeting

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Evolution of Environmental Tracer Profiles at the Bruce Nuclear Site

Abstract

Geoscientific research activities at the Bruce nuclear site, Ontario, included drilling and coring of six deep boreholes through an 840 m thick Paleozoic sedimentary sequence comprised of Devonian to Cambrian age carbonates, shales, evaporates and minor sandstones. A key element of the investigation was the derivation of a vertical profile of ground and porewater hydrogeochemical data for aqueous species (Cl, Br, δ18O, δ2H, 3He/He4, CH4) within the near horizontally layered sequence. This unique natural tracer data set, complete within an understanding of the sites’ physical hydrogeology and conceptual geologic history have provided the basis to develop a conceptual groundwater system model with which to explore fluid and solute migration within the low permeability sediments on time and space scales not previously reported. Geologic evidence indicates that the Ordovician sedimentary rocks in southwest Ontario were deposited in an environment with normal marine salinity and were overlain during the Silurian and Devonian by sedimentary rocks containing gypsum and halite. In the conceptual model, this sedimentary sequence defines the initial porewater chemistry as a two-layer system with porewater of normal marine salinity overlain by hypersaline brine. The sequence is dominated by low-permeability carbonates and shale, and diffusion is assumed to be the principal solute-transport process. This assumption is supported by 1-D numerical simulations that indicate a diffusion time scale on the order of ∼300 Ma. Isotopic evidence from CH4, δ13C-enriched CO2 and He with different 3He/4He ratios in the Ordovician shales and carbonates is internally consistent with a diffusion dominated transport regime. In its entirety, the evidence gathered indicates that the original porewater in the Ordovician rocks (400 m) at the eastern margin of the Michigan Basin has remain essentially immobile within the matrix, with solute concentrations modified by long-lived diffusive processes requiring on the order of 300 Mya to achieve observed conditions.