Hydrogeologic Modelling March 2011 Prepared by: J.F. Sykes, S.D. Normani, and Y. Yin NWMO DGR-TR-2011-16 Hydrogeologic Modelling March 2011 Prepared by: J.F. Sykes, S.D. Normani, and Y. Yin NWMO DGR-TR-2011-16 Hydrogeologic Modelling - ii - March 2011 THIS PAGE HAS BEEN LEFT BLANK INTENTIONALLY Hydrogeologic Modelling - iii - March 2011 Document History Title: Hydrogeologic Modelling Report Number: NWMO DGR-TR-2011-16 Revision: R000 Date: March 2011 AECOM Canada Ltd. Prepared by: J.F. Sykes (University of Waterloo), S.D. Normani (University of Waterloo), Y. Yin (University of Waterloo) Reviewed by: R.E.J. Leech Approved by: R.E.J. Leech Nuclear Waste Management Organization Reviewed by: M. Jensen Accepted by: M. Jensen Hydrogeologic Modelling - iv - March 2011 THIS PAGE HAS BEEN LEFT BLANK INTENTIONALLY Hydrogeologic Modelling - v - March 2011 EXECUTIVE SUMMARY A Deep Geologic Repository (DGR) for Low and Intermediate Level Waste (L&ILW) has been proposed by Ontario Power Generation (OPG) for the Bruce nuclear site in the Municipality of Kincardine, Ontario, Canada. This report presents hydrogeologic modelling and analyses that were completed as part of the Geosynthesis DGR work program. As envisioned, the proposed DGR would be constructed at a depth of about 680 m below ground surface within the argillaceous Ordovician limestone of the Cobourg Formation. The objectives of this report are to develop the regional-scale hydrogeological conceptual model for the DGR site, to undertake numerical modelling using the computational models FRAC3DVS-OPG and TOUGH2-MP and to support the safety case for the DGR. A primary focus of the numerical modelling study is the investigation of a DGR program hypothesis that solute transport in the Ordovician sediments is diffusion dominant. Within the geologic setting of southern Ontario, the Bruce nuclear site is positioned along the eastern flank of the Michigan Basin. Regional well logs from the Oil, Gas and Salt Resources (OGSR) Library in London, Ontario and data from deep boreholes of the DGR field study at the Bruce nuclear site were used to define the structural contours at the regional and site scale ofthe up to 31 bedrock units/formation/groups present above the Precambrian crystalline basement. The regional-scale domain encompasses an area of approximately 18,000 km2 and extends to the deepest points in Lake Huron and Georgian Bay. From a hydrogeologic perspective, the domain can be subdivided into three ground water systems at the Bruce nuclear site: a shallow zone characterized by the units of the Devonian and extending to the base of the Bass Islands Formation; an intermediate zone that extends from the base of the Bass Islands Formation to the Manitoulin Formation and includes the low permeability units of the Salina and the more permeable Niagaran Group; and a deep groundwater domain or zone that extends from the base of the Manitoulin to the Precambrian. A significant aquifer in the intermediate zone is the Niagaran Group which includes the Guelph, Goat Island, Gasport and Lions Head units. While these units can be differentiated in the DGR boreholes at the Bruce nuclear site, the OGSR well logs do not allow their identification. The Cambrian sandstone is the most significant aquiferin the deep zone at the DGR site. This formation is absent over the Algonquin Arch and outcrops west of Lake Michigan and north of Sault Sainte Marie, Ontario in a small band more than 300 km northwest of the DGR site. The conceptual model for the ground water system was developed using data from the DGR site characterization program. Hydraulic parameters for the model hydrostratigraphic units were defined using data from the DGR site boreholes and from lab analyses of cores. Boreholedata included hydraulic conductivities from straddle-packer hydraulic tests and pressure measurements from the Westbay MP38 and MP55 multi-level groundwater monitoring system. Data from this system indicate that units of the Salina and the Ordovician sediments are under-pressured relative to hydrostatic levels associated with ground surface at the DGR site. The Niagaran is slightly over-pressured while the Cambrian is significantly over-pressured. Core analyses yielded estimates of porosity, Young’s modulus, Poisson’s ratio, water saturations and gas saturations. Rock cores and opportunistic water samples were used to define the spatial distribution of the total dissolved solids concentration and water density. Layer dependent specific storage coefficients and one-dimensional loading efficiencies were calculated usingfield and laboratory data. The conceptual model of the Bruce DGR site required the development of constitutive models that relate the fluid density and viscosity to the fluid total dissolved solids concentration and water pressure. Hydrogeologic Modelling - vi - March 2011 The hydrogeological modelling strategy adopted for the proposed DGR at the Bruce nuclear site was to explore the processes and mechanisms relevant to groundwater system stability and the long-term performance of the multiple geologic barriers hosting and isolating the DGR. This study used four different numerical models and two different computational models to evaluate groundwater flow and solute transport. The modelling examined: regional-scale saturated density-dependent flow for a domain centred on the DGR; site-scale saturated density-dependent flow for a domain centred on the DGR; density-dependent flow for an approximately east-west cross section of the Michigan Basin; and, one-dimensional two-phase gas and water flow analyses of a stratigraphic column at the DGR. The regional-scale, site-scale, and basin cross-section modelling were accomplished using the computational model FRAC3DVS-OPG. To investigate the hypothesis that the under-pressures in the Ordovician sediments may indicate the presence of a gas phase, the two-phase air and water model TOUGH2-MP was used. The parameter perturbation and scenarios analyses of the hydrogeological modelling study involved a large number of simulations using the four numerical models. Important in the analyses of the hydrogeologic modelling study is the selection of the performance measure used to evaluate the system. The traditional metric of average water particle travel time is based solely on advective velocities. It is an inappropriate measure for geologic units where solute transport is controlled by diffusion. The use of lifetime expectancy and the related groundwater age is a more appropriate metric for such a system. Lifetime expectancy, a stochastic variable, can be estimated by determining the probability density function of the time required for water particles at a spatial position in a groundwater system to reach potential outflow points. The particles can migrate to the outflow points by both advection and hydrodynamic dispersion. The hydrodynamic dispersion includes both mechanical dispersion and diffusion. In this study only the first moment of the life expectancy is estimated, with the measure being expressed as the Mean Life Expectancy (MLE). Conservative tracers also are used to evaluate issues such as the diffusive dominance of solute transport in the low permeability units at the DGR site. Péclet numbers are used to assess whether solute transport is dominated by diffusion or advection. The Péclet number is a ratio of the product of the pore water velocity and a characteristic length to the effective diffusion coefficient. While literature indicates that the characteristic length used to estimate Péclet numbers should be to the order of the size of grains, conservative estimates are determined in this study by using a characteristic length of 1 metre. Energy gradients and groundwater velocities in the Michigan Basin are density-dependent and hence a fully-coupled transient flow and brine transport analysis is required for their estimation. The coupling of the flow equation and the brine transport equation results because both the water density and viscosity are dependent on the Total Dissolved Solids (TDS) concentration. A solution with a TDS concentration of zero will correspond to a water density of 1,000 kg/m3 at standard pressure and temperature. In comparison, water with a TDS concentration of 300 g/L will have a density of approximately 1,200 kg/m3. The study methodology determined a pseudo-equilibrium solution for density-dependent flow at one million years after the imposition of an initial total dissolved solids distribution in the regional domain. The environmental head profile from the assumed TDS concentrations and measured pressures at the DGR boreholes indicates that the Cambrian is over-pressured relative to the elevation of the surface while the Ordovician shale and limestone units are significantly under-pressured. An essential requirement of the abnormal pressures of the Cambrian is overlying extensive low hydraulic conductivity strata. The low pressures in the Ordovician may be the result of stress relief as a result of significant removal of mass through erosion, that was at a rate greater than that of water influx to these units from the over- and under-lying units with higher pressure; the pressure distribution is still evolving. The low pore fluid pressures also may indicate the presence Hydrogeologic Modelling - vii - March 2011 of a trapped non-wetting gas phase that would result in an effective hydraulic conductivity that is significantly less than the corresponding saturated hydraulic conductivities for the units. Other possible explanations