Ottawa, Canada
Ottawa, Canada

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Al T.A.,University of Ottawa | Clark I.D.,University of Ottawa | Kennell L.,Nuclear Waste Management Organization of Canada | Jensen M.,Nuclear Waste Management Organization of Canada | Raven K.G.,Geofirma Engineering
Chemical Geology | Year: 2015

Drill cores collected from an 860-m-thick Paleozoic (Cambrian to Devonian) sedimentary sequence of the Michigan Basin in southwest Ontario, Canada, were used to develop vertical profiles of natural tracers in porewater from the sedimentary sequence and build a conceptual model regarding processes and timing of solute migration. In this paper, the conceptual model is quantitatively assessed with numerical simulations of diffusion-dominated solute transport (Cl- and 18O). The conceptual model suggests that the tracer profiles result from mixing between a deep shield brine with isotopic characteristics consistent with water-rock interaction over a long period of geologic time, Ordovician seawater at intermediate depth, and an overlying hypersaline brine formed by evaporation of seawater in a restricted basin during the Silurian. Between the peak of basin consolidation at 300MaBP and the Pleistocene (2.6MaBP), salinity and isotopic concentration gradients are thought to have driven mixing, dominantly by diffusion. Water isotope data indicate that in the Pleistocene, melt water infiltrated to a maximum depth of 330m in the Silurian Salina Formation, likely in response to transient glacial loading and elevated sub-glacial hydraulic pressures. Also in the Pleistocene, over-pressured conditions in deep regions of the basin drove high-salinity brine eastward in the basal Cambrian aquifer, interrupting pre-existing tracer-diffusion profiles.A base case set of boundary and initial conditions provides a good fit between simulated and measured tracer profiles, but requires effective diffusion coefficients (De) in the Ordovician limestones that are lower than the average values obtained in laboratory measurements by as much as a factor of 10. The base-case values are still within the overall range of measured values. At the formation scale, relatively low in-situ De values are attributed to the intermittent occurrence of gas and liquid hydrocarbons in the Ordovician carbonate units and the effect of lithostatic confining pressure; neither of which are accounted for in the laboratory measurements but both would have the effect of decreasing De for aqueous solutes. The numerical model is most sensitive to the formation-specific De values and geologically reasonable variations in initial and boundary conditions have a relatively small influence on the results. Additional simulations indicate that hydraulic disturbance and ingress of meltwater during the Pleistocene can explain second-order features observed in the measured tracer profiles. This work provides supporting evidence for previous research indicating that the porewater residence time in the Ordovician sequence approaches the age of the rocks. © 2015 Elsevier B.V.

Avis J.,Geofirma Engineering | Suckling P.,Quintessa Ltd. | Calder N.,Geofirma Engineering | Walsh R.,Geofirma Engineering | And 2 more authors.
Nuclear Technology | Year: 2014

Deep geologic disposal of radioactive waste is being planned in a number of international programs. Within a deep geologic repository (DGR), gases can be generated by corrosion of metals and degradation of organics. Reactions, and thus gas generation rates, are dependent upon pressures, temperature, and the availability of water or water vapor within the repository. Furthermore, many reactions consume water. Consumption rates and repository state are not known a priori and are in general coupled processes. A numeric model of coupled gas generation and transport has been developed and implemented in the T2GGM code. T2GGM consists of a gas generation model (GGM), which calculates rates of gas generation and water consumption within the DGR due to corrosion and microbial degradation of the waste packages, integrated with the widely used two-phase-flow code TOUGH2, which models the subsequent two-phase transport of the water and gas through the repository and into the DGR shafts and geosphere. T2GGM has been applied to assess gas transport from a proposed low- and intermediate-level radioactive waste DGR and to study the impact of container corrosion in a hypothetical used fuel DGR.

Sengebush R.M.,Intera Inc. | Heagle D.J.,Geofirma Engineering | Jackson R.E.,Geofirma Engineering
Environmental and Engineering Geoscience | Year: 2015

Prior to World War II, the City of San Diego, California, extracted millions of gallons of high-quality groundwater daily from alluvial gravels in the lower San Diego River Valley that have since become contaminated with brackish water and hydrocarbons. The origin of this brackish groundwater and of the Quaternary sedimentary geology of the valley is interpreted through archived reports, journal articles, U.S. Geological Survey data, and samples from new city wells in the alluvial gravels. Eocene sediments were inundated by seawater during the last interglacial period (ca. 120 ka), when sea levels were ∼19 ft (6 m) higher than present levels. The brackish groundwater present in these Eocene sediments appears to be relict seawater from this inundation. We hypothesize that the city's pre-World War II well field-referred to herein as the Mission Valley Aquifer-was a buried channel gravel created following the Last Glacial Maximum of the Pleistocene Epoch (∼20 ka). As such, it would have been similar to other long (∼11 km, 7 mi) buried channel gravels along the southern Californian coast described in previous reports. We present evidence of groundwater freshening of the Eocene sedimentary rock that has led to increasing total dissolved solids in the Mission Valley Aquifer, which acts as a highpermeability drain for the valley. Freshening occurs as a Ca-HCO3 groundwater replaces a Na-Cl water, which we propose was derived from the marine inundation of 120 ka.

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