Jonathan F. Sykes

Compositional simulation of groundwater contamination by organic compounds: 1. Model development and verification

A compositional simulator is developed for application to the analysis of contamination and remediation of groundwater systems. Simultaneous flow of three fluid phases (water, gas, and organic) is modeled. Interphase partitioning and transport of an arbitrary number of organic and inorganic components can be simulated. Phase densities are functions of pressure and phase composition. The model includes several numerical options, ranging from fully implicit with first-order upstream weighting to implicit in pressure, explicit in saturations and concentration with third-order upstream weighting. The model is verified to the extent possible with analytical solutions for simplified cases of multiphase flow and contaminant transport. The accuracy and efficiency of the various numerical options used in the model are illustrated.

Compositional simulation of groundwater contamination by organic compounds: 2. Model applications

The flow and transport of organic compounds in variably saturated porous media is investigated using a compositional simulator. The simulator incorporates a number of numerical options to maximize computational efficiency and accuracy. The effect of field scale heterogeneities on the movement of organic compounds is demonstrated. The influence of infiltrating wetting fronts on gas phase transport of volatile organic compounds is shown to be significant. The long-term fate of a subsurface spill of a three-component dense organic liquid is simulated. Three-dimensional simulations of soil vacuum extraction demonstrate the difficulty in removing dissolved organic compounds from the saturated zone with this process.

Use of groundwater lifetime expectancy for the performance assessment of a deep geologic waste repository: 1. Theory, illustrations, and implications

[1] Long-term solutions for the disposal of toxic wastes usually involve isolation of the wastes in a deep subsurface geologic environment. In i he case of spent nuclear fuel, if radionuclide leakage occurs from the engineered barrier, the geological medium represents the ultimate barrier that is relied upon to ensure safety. Consequently, an evaluation of radionuclide travel times from a repository to the biosp here is critically important in a performance assessment analysis. In this study, we deve op a travel time framework based on the concept of groundwater lifetime expectancy as a safety indicator. Lifetime expectancy characterizes the time that radionuclides will spend in the subsurface after their release from the repository and prior to discharging into the biosphere. The probability density function of lifetime expectancy is computed throughout the host rock by solving the backward-in-time solute transport adjoint equation subject to a properly posed set of boundary conditions. It can then be used to define opti mal repository locations. The risk associated with selected sites can be evaluated by simulating an appropriate contaminant release history. The utility of the method is illustrated by means of analytical and numerical examples, which focus on the effect of fra cture networks on the uncertainty of evaluated lifetime expectancy.

Head variance and macrodispersivity tensor in a semiconfined fractal porous medium

In this paper, the head variance and macroscopic dispersion are developed for one- and two-dimensional flows through a semiconfined aquifer with isotropic and anisotropic fractal hydraulic conductivity distribution. It is assumed that the spatial distribution of log hydraulic conductivity can be described as a fractional Brownian motion. The resulting head variance and macrodispersivity tensor, obtained from stochastic fluctuation equations of the steady flow and solute transport, are studied in terms of the hydraulic conductivity process and the leakage in a semiconfined aquifer bounded by a leaky layer above and an impervious stratum below. The impact of the fractal dimension of this process, the leakage factor, and the characteristic length scale on the macrodispersivity is investigated. The results show that the head variance and the transverse macrodispersivity decrease, while the longitudinal macrodispersivity slightly increases with increasing leakage factor. The head variance and longitudinal macrodispersion increase significantly, and the transverse macrodispersion increases slightly as the maximum length scale increases. The increasing fractal dimension generally reduces the head variance and longitudinal macrodispersion and enhances the transverse macrodispersion. The influence of statistical anisotropy of conductivity field on both the head variance and macrodispersivities is also investigated.

On the spatial-temporal averaging method for modeling transport in porous media

The spatial-temporal averaging procedure is considered with a nonhomogeneous distribution of elementary domains in the spatial-temporal space and the probabilistic interpretation of the ST-averaging is also given. Several averaging theorems and corollaries about the averages of spatial and temporal derivatives are presented and rigorously proved which allow elementary domain to vary in space and time. The macroscopic transport equation in the most general condition and the simplified macroscopic equation under the special form of distributions are developed which may be reduced to the classical macroscopic transport equation as the spatial-temporal average degenerates into the volume average.

Hydrogeologic Modelling in Support of a Proposed Deep Geologic Repository in Canada for Low and Intermediate Level Radioactive Waste

A Deep Geologic Repository (DGR) for Low and Intermediate Level radioactive waste has been proposed by Ontario Power Generation for the Bruce Nuclear Power Development site in Ontario, Canada. The DGR is to be constructed at a depth of about 680 m below ground surface within the argillaceous Ordovician limestone of the Cobourg Formation. This paper describes a regional-scale geologic conceptual model for the DGR site and analyzes flow system evolution using the FRAC3DVS-OPG flow and transport model. This provides a framework for the assembly and integration of site-specific geoscientific data that explains and illustrates the factors that influence the predicted long-term performance of the geosphere barrier. In the geologic framework of the Province of Ontario, the Bruce DGR is located at the eastern edge of the Michigan Basin. Borehole logs covering Southern Ontario combined with site specific data have been used to define the structural contours at the regional and site scale of the 31 sedimentary strata that may be present above the Precambrian crystalline basement rock. The regional-scale domain encompasses an 18.500km2 region extending from Lake Huron to Georgian Bay. The groundwater zone below the Devonian is characterized by units containing stagnant water having high concentrations of total dissolved solids that can exceed 300g/l. The computational sequence involves the calculation of steady-state density independent flow that is used as the initial condition for the determination of pseudo-equilibrium for a density dependent flow system that has an initial TDS distribution developed from observed data. Long-term simulations that consider future glaciation scenarios include the impact of ice thickness and permafrost. The selection of the performance measure used to evaluate a groundwater system is important. The traditional metric of average water particle travel time is inappropriate for geologic units such as the Ordovician where solute transport is diffusion dominant. The use of life expectancy and groundwater age is a more appropriate metric for such a system. The mean life expectancy for the DGR and base case parameters has been estimated to be in excess of 8 million years.Copyright

Assessing Alternative Scenarios for the Cause of Underpressures in the Ordovician Sediments along the Eastern Flank of the Michigan Basin

Geoscientific investigations for a proposed deep geologic repository at the Bruce Site, located on the eastern flank of the Michigan Basin, have identified unique and significant underpressured conditions. Along with the measurement of environmental tracer profiles (e.g., helium), this study aims to explore, through a series of numerical simulations, the nature of long-term phenomena responsible for the generation and preservation of formation underpressures. Three families of inverse numerical experiments for underpressure formation were examined by means of one-dimensional hydromechanically coupled models through the vertical hydrostratigraphic column: (i) uncertainty in glaciation scenarios; (ii) uncertainty in initial heads prior to glaciation; and (iii) uncertainty in the degree of hydraulic connectivity between the more permeable Guelph Formation at the Bruce Site and the applied glacial loading, for a total of 20 scenarios, assuming fully saturated conditions. Underpressured initial heads for the paleohydrogeologic simulations lead to lower calibrated vertical hydraulic conductivities. The robustness and resilience of the groundwater system to external perturbations are greater for the state where underpressured conditions predate the onset of glaciation and are better able to preserve the present day helium tracer profile in 260 Ma exhumation analyses.

The Hydrogeologic Environment for a Proposed Deep Geologic Repository in Canada for Low and Intermediate Level Radioactive Waste

A Deep Geologic Repository (DGR) for low and intermediate level radioactive waste has been proposed by Ontario Power Generation for the Bruce nuclear site in Ontario, Canada. As proposed the DGR would be constructed at a depth of about 680 m below ground surface within the argillaceous Ordovician limestone of the Cobourg Formation. This paper describes the hydrogeology of the DGR site developed through both site characterization studies and regional-scale numerical modelling analysis. The analysis provides a framework for the assembly and integration of the site-specific geoscientific data and examines the factors that influence the predicted long-term performance of the geosphere barrier. Flow system evolution was accomplished using both the density-dependent FRAC3DVS-OPG flow and transport model and the two-phase gas and water flow computational model TOUGH2-MP. In the geologic framework of the Province of Ontario, the DGR is located on the eastern flank of the Michigan Basin. Borehole logs covering Southern Ontario combined with site-specific data from 6 deep boreholes have been used to define the structural contours and hydrogeologic properties at the regional-scale of the modelled 31 sedimentary strata that may be partially present above the Precambrian crystalline basement rock. The regional-scale domain encompasses an approximately 18500km2 region extending from Lake Huron to Georgian Bay. The groundwater zone below the Devonian includes units containing stagnant water having high concentrations of total dissolved solids that can exceed 300g/L. The Ordovician sediments are significantly under-pressured. The horizontal hydraulic conductivity for the Cobourg limestone is estimated to be 2 × 10−14 m/s based on straddle-packer hydraulic tests. The low advective velocities in the Cobourg and other Ordovician units result in solute transport that is diffusion dominant with Peclet numbers less than 0.003 for a characteristic length of unity. Long-term simulations that consider future glaciation scenarios include the impact of ice thickness and permafrost. Solute transport in the Ordovician limestone and shale was diffusion dominant in all simulations. The Salina formations of the Upper Silurian prevented the deeper penetration of basal meltwater.Copyright

L 1 and L 2 Estimators in Groundwater Problems: Parameter Estimates and Covariances

This paper presents a comparison of a L 1 estimator and a L 2 estimator in solving groundwater parameter estimation problems. Posterior statistical inferences are investigated. The simulation model employed is a finite element model for steady state groundwater flow and solute transport in a two-dimensional vertically integrated aquifer system. The parameters considered are the hydraulic conductivities, the dispersivities, the porosities of the aquifer system, and the solute source concentration(s). The sensitivities of the state variables to the parameters are computed by using a sensitivity equation method. A few solutions to a hypothetical problem are presented to illustrate the L 1 and L 2 probabilistic characterizations of the various parameters.

Parameter identification for groundwater flow and solute transport simulation: a comparison of L1 and L2 estimators

Abstract This paper presents a comparison between an L 2 estimator and an L 1 estimator for solving parameter identification problems in groundwater flow and solute transport simulation. The L 1 estimator is formulated using a weighted L 1-norm as the error measure between the vector of the observed hydraulic heads and solute concentrations and the vector of the simulated heads and concentrations. The simulation model is comprised of a finite element formulation for steady-state groundwater flow and solute transport in a two-dimensional vertically averaged aquifer system. The gradients of the state variables with respect to the parameters are computed using a state sensitivity method. Posterior statistical inferences are conducted. The solutions to two sets of hypothetical groundwater parameter estimation problems illustrate that the L1 estimator is more robust than the L2 estimator in handling observation data containing certain outliers.