The predictable nature of the Paleozoic sedimentary sequence beneath the Bruce nuclear site in Southern Ontario, Canada

Document available in extended abstract form only. A key aspect of a Deep Geologic Repository (DGR) safety case is the ability to develop and communicate an understanding of the geologic stability and resilience to change at time frames relevant to demonstrating repository performance. As part of an on-going Environmental Assessment, Ontario Power Generation (OPG) recently completed site-specific investigations within an 850 m thick Paleozoic sedimentary sequence beneath the Bruce nuclear site for the proposed development of a DGR for Low and Intermediate Level Waste (L and ILW). As envisioned, the shaft-accessed DGR would be excavated at a nominal depth of 680 m within the low permeability Ordovician argillaceous limestone of the Cobourg Formation, which is overlain by more than 200 m of low permeability Ordovician shale. The geo-scientific investigations revealed a relatively undeformed and laterally continuous architecture within the sedimentary sequence at the repository scale (1.5 km2) and beyond. This paper explores the predictable nature of the sedimentary sequence that has contributed to increasing confidence in an understanding of the spatial distribution of groundwater system properties, deep groundwater system evolution and natural barrier performance. Multi-disciplinary geo-scientific investigations of the Bruce nuclear site were completed in 3 phases between 2006 and 2010. The sub-surface investigations included a deep drilling, coring and in-situ testing program and, the completion of a 19.7 km (9 lines) 2-D seismic reflection survey. The drilling program involved 6 (150 mm dia.) deep boreholes (4-vertical; 2 inclined) that were extended through the sedimentary sequence from 4 drill sites, arranged around the 0.3 km2 footprint of the proposed repository. The more than 3.8 km of rock core (77 mm dia.) retrieved have provided, in part, a strong basis to understand bedrock lithology and mineralogy, facies assemblages, structure, and oil and gas hydrocarbon occurrences within the sedimentary sequence underlying the Bruce nuclear site. This information, coupled with in-situ geophysical and hydraulic borehole testing, characterization of groundwater and matrix pore fluids and laboratory based petrophysical analyses, provides a unique opportunity to describe the sub-surface geologic conditions relevant to DGR implementation and safety. Core logging revealed that the stratigraphic sequence comprises 34 distinct bedrock formations/members/units, consistent with the known regional stratigraphic framework. These sedimentary formations dip 0.6 deg. (∼10 m/km) to the southwest with highly uniform thicknesses both at the site- and regional-scale, particularly the Ordovician formations, which vary on the order of metres. The occurrence of steeply-dipping faults originating within the underlying crystalline basement is not evident through surface outcrop fracture mapping, micro-seismic (M ≥ 1) monitoring, or intersection of hydrothermally dolomitized hydrocarbon reservoir systems. Potential fault structures, interpreted from the 2-D seismic survey, were drilled and cored by angled boreholes, which found no evidence for their existence. Formation-specific continuity is evidenced by the lateral traceability of chronostratigraphic marker beds, as well as, litho-facies and their vertical transitions, at decimetre scale through the sedimentary sequence. The correlation of hydraulic conductivities, porosities and diffusion coefficients within these near-horizontal lithostratigraphic units, regardless of which individual borehole dataset is examined, is a significant factor influencing confidence in sub-surface exploration. Effective horizontal hydraulic conductivities estimated by in-situ hydraulic testing yields uniformly low values (≤10-13 m/s) for the host Cobourg Formation and enclosing rock mass. In addition, there are evident trends, for example, between formation sheet silicate content (shale 50%; carbonate <20%), total porosity (shale 6-8%; carbonate 0.5-2%) and effective diffusion coefficient (range between 10-13 and 10-12 m2/s), which reveal a lithostratigraphic correlation with important parameters that contribute to governing mass transport. In summary, geo-scientific evidence gathered within the sedimentary sequence beneath the Bruce nuclear site illustrates a setting in which a high degree of lithostratigraphic continuity (e.g. the predictable nature of the Ordovician formations) increases confidence in the conceptualization of the spatial distribution of site properties and predictive estimates of far-field barrier performance. (authors)