Hydrogeochemical Characterization of Groundwaters and Porewaters Beneath the Bruce Nuclear Site: Evidence for Diffusion-dominated Transport in the Ordovician

Abstract

Investigations of the controls on solute transport in porewater and groundwater below the Bruce nuclear site involved laboratory-scale diffusion measurements, as well as measurements of natural tracer (δ¹⁸O, δ²H, ⁸⁷Sr/⁸⁶Sr, Cl and Br) distributions and isotopic compositions of CH₄, CO_2, and helium (³He/⁴He) in the porewaters and groundwaters. The analyses were performed on samples collected from drilled boreholes, DGR-1 through DGR-6, during site characterization activities and the results are summarized below.

With the exception of just a few samples from the Upper Silurian, the effective diffusion coefficient (D_e)values measured from DGR cores are all less than 10^-11 m²/s, which is approximately one order of magnitude lower than measured D_e values from international programs in sedimentary rock. The majority of the data are in the range 10^-13 < D_e < 10^-11 m²/s, with Lower Silurian and Upper Ordovician shale samples representing the higher end of this range because of their relatively high porosity (7 to 9%). The low porosity of the Middle Ordovician limestone (< 2%) results in low D_e values, clustering in the range 10^-13 < D_e < 10^-12 m²/s.

The δ¹⁸O, Cl, and Br profiles in the Middle Ordovician carbonates define trends of decreasing δ¹⁸O ith depth, and are interpreted to result from an extremely long period of diffusion-dominated transport (~300 Ma). Near the base of the Middle Ordovician carbonates the δ²H data display a slight enrichment, which could represent upward diffusion of deuterium-enriched water originating in the underlying Precambrian shield. A Precambrian shield influence on the Ordovician porewater chemistry is inferred from the measured ⁸⁷Sr/⁸⁶Sr ratios in the Middle Ordovician carbonates, which are elevated above values expected for porewater in equilibrium with carbonate rock.

The CH₄ isotope data indicate the presence of biogenic gas in the Cobourg Formation and in the overlying Ordovician shales, while gas of thermogenic origin is present in the Middle Ordovician carbonates below. The fact that methane of differing origins has not mixed by diffusion across the Cobourg limestone suggests that there is an effective barrier to vertical transport near the base of the formation. The presence of a vertical transport barrier at this depth is also supported by the persistence of large gradients in the isotopic composition of helium.

The Cambrian groundwater chemistry displays a distinct reversal in the natural tracer profiles relative to the overlying Ordovician carbonates. The reversal in the profiles is abrupt compared to the gradual decline in concentrations and isotopic compositions observed with depth through the Ordovician carbonates. Irrespective of the mechanism(s) responsible for the Cambrian fluid chemistry beneath the Bruce nuclear site, however, the fundamental hypothesis that solute migration with the Ordovician sediments is diffusion dominated is well supported by the data.