Eastern Section Meeting

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Hydrogeochemical Characterization of Porewater in Low Permeability Sediments

Abstract

The Nuclear Waste Management Organization (NWMO) has an extensive research and development program in partnership with universities and research institutes in Canada and abroad, with focus on the extraction of porewater from low-permeability crystalline and sedimentary rocks. With respect to sedimentary characterization, focus is on sedimentary basin environments, where porewater salinities typically are high and hydraulic conductivities low. Work to-date has involved the development, testing, implementation and validation of a variety of methods. Work performed at the Bruce Site, located on the eastern margin of the Michigan Basin in southwestern Ontario, has demonstrated that extracting porewater from the Ordovician carbonates and shales can be extremely difficult due to their low water contents (<7.5% porosity) and low formation hydraulic conductivities (<1.0E-11 m/s). A number of methods were attempted to extract porewaters from these low-permeability sediments, including advective displacement, ultracentrifugation, squeezing, isotopic diffusive exchange and vacuum-distillation. Of the methods employed, only a few were demonstrated applicable for these rocks, and only vacuum-distillation was consistently successful for all rock types (carbonate and shale) and at all porewater contents, allowing for the generation of 18O and 2H profiles with depth. In addition to isotopic characterization, crush and leach methods, as well as chromatography, have been utilized for ion analysis, allowing for the generation of natural solute tracer profiles with depth. Refinement of methods is an on-going process. Micro vacuum-distillation – an adaptation of vacuum-distillation – involves powdering of samples prior to a suite of geochemical analyses and attempts to minimize potential analytical artefacts. A promising new method has been under development for the last two years and involves using cellulosic paper sandwiched between sedimentary rock cores to extract porewater. The purpose of the NWMO's research program is to ensure high-quality method development and application in the context of porewater extraction and characterization in low permeability rock formations. Assessing solute origin, longevity and transport in deep groundwater systems is fundamental to ensuring understanding of both system evolution and long-term stability. Knowledge gained from the analysis of stable water isotopes, ion chemistry and dissolved gas concentrations/isotopes, allows for meaningful interpretations of porewater and solute residence time in deep aquiclude systems. This presentation briefly summarizes the work that has been done to enable the collection of high-quality data from high-salinity porewaters in the Michigan Basin of southwestern Ontario.