H43F-0414 1340h
DNAPL removal from a rough-walled single fracture with density-surfactant-motivated method
Remediation techniques to remove DNAPL in subsurface usually involve the delivery of the remedial fluids to the contamination source. However, the fractured rock environment makes effective delivery of remedial fluids very difficult to achieve. In particular, remedial fluid is rarely delivered to dead-end fractures or areas through which fluid hardly flows. Previous research shows density-surfactant-motivated method is an effective technology for the removal of DNAPL trapped in dead-end fractures, but it only considered drawing TCE out of the vertical dead-end fractures. In this study, experiments have been conducted to compare density-surfactant-motivated method with other remediation techniques and consequently to examine the applicability of the density-surfactant-motivated method to a rough-walled single fracture. A transparent glass replica was made by copying a natural granite with a fracture. Experimental apparatus was constructed, in which hydraulic gradient and the dipping angle of a fracture can be controlled. We have conducted the experiments under various conditions with a range of the bond and capillary numbers. In each experiment, a common residual DNAPL field was established, and then the mixture of dense fluid and surfactant-enhanced solution was injected into the glass fracture replica. Observations are made to compare the DNAPL residual distribution before and after the flushing of surfactant-enhanced solution.
H43F-0415 1340h
The effect of nonlinear groundwater flow on DNAPL migration in a rough-walled single fracture
Understanding the migration of DNAPL in water-saturated fractured rocks is important for DNAPL remediation design, because it affects the distribution or phase structures of DNAPL that determine the pressure-saturation-conductivity relation and the transport pattern of dissolved DANPL in fractured rocks. The migration characteristic of DNAPL in a single fracture needs to be thoroughly understood for the fracture network scale phase structure. Previous researches concerned with the DNAPL migration in two-dimensional fracture networks with a constant aperture show that ambient groundwater flow as well as aperture variation and network geometry influences DNAPL migration. However, rough walls in a single fracture make nonlinear flow even in low Re, which may control the phase structure of DNPAL in a single fracture. In this study, we present experimental results to consider the effect of nonlinear groundwater flow in a rough-walled single fracture on DNAPL migration. A glass replica of a granite sample containing a rough single fracture is made, which allows the visualization of DNAPL migration process, and experiments are conducted over a range of Re. Observations are compared to the results of DNAPL migration tests that are conducted in two parallel glass plates over the same range of Re to characterize the effect of nonlinear groundwater flow on the phase structure of DNAPL in a single fracture.
H43F-0416 1340h
INFLUENCE OF FRACTURE INTERSECTIONS UNDER UNSATURATED, LOW FLOW CONDITIONS
Recent experimental evidence suggests that the capillary heterogeneity associated with fracture intersections can act to impose temporal and spatial structure on network-scale flows. A simple intersection between orthogonal fractures, one horizontal and the other vertical, has been shown to integrate unsaturated flows. At low flows, the intersection forms a capillary barrier that accumulates water in a growing pool. Eventually the retaining meniscus snaps, discharging a pulse of water. Here, we develop a mechanistic explanation for this observed behavior and experimentally consider three perturbations to the geometry of the simple orthogonal intersection. Two of the perturbations also act as capillary barriers, while the third formed a capillary bridge across the intersection. At low flow, all of our experimental intersections imposed a temporal signal, with the nature of that signal dependent on intersection geometry and participation by the horizontal fractures in dynamic storage. At high flow, a continuous fluid tendril spanned the system from inlet to outlet with water pooled above the intersection caused by a narrow fluid connection that restricted flow across the intersection. Results from all experiments suggest that pulsation is critically sensitive to small variations in the geometry of fracture intersections, which can generate pulsation of flow across wide time periods and discharge volumes.
H43F-0417 1340h
Investigations of the Effective Diffusivity in the Oskarshamn Site Investigation Area
At present two sites in Sweden undergo site investigations for a deep geological repository for spent nuclear fuel. This paper present results concerning the effective diffusivity of the rock at repository depth in the Oskarshamn site investigation area. The geology of this site is dominated by granite, granodiorite and diorite. In a campaign, the effective diffusivity of the rock surrounding the 1000-meter deep boreholes KSH01A and KSH02 was investigated. The effective diffusivity is one of the most important parameters for radionuclide retention and strongly affects the performance of the geosphere as a natural barrier. The effective diffusivity was investigated in the laboratory on 83 samples, taken from the bore core of KSH01A and KSH02, by an electrical resistivity method. Formation factors ranging from $2.2 \times 10^{-7}$ to $9.0 \times 10^{-4}$ were obtained. The effective diffusivity was also investigated in-situ by formation factor logging using the electrical resistivity method. A total of 1867 in-situ formation factors were obtained that corresponded well with the formation factors obtained in the laboratory with a few exceptions. As such a great number of formation factors was obtained, formation factor distributions for specific rock types can be presented. The results suggest that the formation factor may be distributed according to the log-normal distribution. The results of this campaign show that the porosities and effective diffusivities used in the Swedish SR-97 safety assessment may have to be somewhat reassessed. The used of a single effective diffusivity value in a safety assessment could be questioned, as the obtained formation factors ranged over at least 3 orders of magnitude. In fact, at this site the traditional concept of the formation factor could be questioned, as the rock is very fractured and groundwater in fractions may greatly alter the storage capacity of the rock.
H43F-0418 1340h
Multiple approaches to studying pore connectivity in rock matrices
The current conceptual model of contaminant transport in unsaturated fractured rock has water moving through fractures, with migration of the entrained contaminants being retarded by diffusion into and within the rock matrix. The diffusion coefficient is implicitly assumed to have a constant, scale-invariant value. However, rocks whose pores are poorly interconnected are known to have anomalous transport properties that strongly impact long-term net diffusion. For example, rocks with low pore connectivity have a cross-over length below which diffusivity decreases with increasing distance, and above which diffusivity appears constant but with a diffusion coefficient less than that measured on samples smaller than the cross-over length. In order to examine the extent of pore connectivity in a variety of rock types from around the world and further investigate the cross-over lengths of several selected rocks, we developed several complementary experimental techniques: (1) imbibition tests wherein the water uptake was monitored over time, with the slope of log imbibed water mass versus log time indicative of the pore connectivity of the rock matrix; (2) tracer saturation tests by vacuum-saturating rock samples with a tracer solution, followed by micro-scale mapping to obtain tracer distribution with depth, which is related to pore connectivity; (3) gas diffusion experiments to measure diffusion coefficients for a rock at several different sample thicknesses to examine scaling effect; and (4) synchrotron microtomography to analyze the iso-concentration surfaces following tracer diffusion into a rock matrix. These experiments were also modeled using random walk methods on a 3-D lattice with different values of pore coordination. The results indicate that porespace in many rocks is close to the percolation threshold. Use of standard diffusion models for rock matrices with low pore connectivity can yield incorrectly calculated diffusivity values from measured data, and predict incorrect matrix diffusion and retardation coefficients for contaminants in fractured rock.
H43F-0419 1340h
Anaerobic Sulphate Reduction and Transport in a Rock Fracture Intersecting a Deposition Canister for Radioactive Waste
A model was developed describing the concentration gradients of sulphate and sulphide in a fracture intersecting a deposition hole for radioactive waste material. Sulphate is present in groundwaters around the $\ddot{A}$sp$\ddot{o}$ area in Sweden in concentrations ranging from 50-700 mg/l. The reduction of sulphate produces sulphide that may diffuse into the backfill material and corrode the copper canisters. The rate of sulphate reduction may also be increased, mediated by microorganisms. Anaerobic sulphate reduction mediated by microbes at present receives extensive attention in the literature, for example in the formation of large carbonate crusts in sea sediments with methane as the reducing agent: $CH_{4} + SO_{4}^{2-} \rightarrow HCO_{3}^{-} + HS^{-} + H_{2}O$ Methane exists in the $\ddot{A}$sp$\ddot{o}$ groundwaters in concentrations up to 1 mM. Hence, this or similar processes could be possible also in the anaerobic environment surrounding the repository and, in the long term, be a threat to the canisters. The microbial activity is most likely close to the copper canisters since corrosion of those is the main sink of the produced sulphide. However, the small pores in the bentonite clay are not large enough to allow a microbe population within the backfill material. Therefore, the model includes microbially mediated reduction of sulphate at the fracture opening facing the deposition hole. Transport of substrates to the microbes occurs by diffusion perpendicular to the potential flow lines in the fracture. The model domain consists of part of the fracture surrounding the deposition hole. The three-dimensional transport of sulphide within the backfill material is included implicitly. The aims of the model are to explore the transport processes of sulphate and sulphide in a fracture intersecting a deposition hole, and to evaluate the amount of sulphide that may be transported into the backfill material at different conditions.
H43F-0420 1340h
A Reaction-Transport Approach for Assessing Infiltration Rates in Unsaturated Fractured Rock From Stable Isotope Compositions
We use a reaction-transport model to assess the time scales and infiltration rates as indicated from stable isotope data from the proposed Yucca Mountain nuclear waste repository. These models simulate the response of stable isotope compositions in fracture and matrix water/vapor to various climate and infiltration conditions. The impact of changes in infiltration rate on stable isotope compositions is compared with solutes such as Cl and Sr. The stable isotope composition of water in the unsaturated zone is primarily related to the input composition, transport (downward percolation and upward transpiration), and evaporation. Evaporation increases the concentration of solutes in infiltrating waters and typically shifts stable isotope compositions to higher \delta$^{18}$O and \delta$^{2}$H values. However, these effects are minimized to some extent in fractured rock because waters may travel downward along fractures, quickly passing below the evaporation zone. The timescales over which the stable isotope compositions reflect infiltration conditions are affected by advection rates and fracture-matrix interaction. We use the principles of multiple interacting continua (dual permeability) to evaluate the effects of fracture-matrix interaction on stable isotope compositions in fractured rock. Periodic infiltration models that capture winter precipitation followed by summer evaporation suggest that disequilibrium between fracture and matrix waters can persist for decades to centuries depending on fracture spacing and the hydrological properties of the matrix. When considered in one-dimension, zero or negative infiltration leads to stable isotope profiles that resemble a diffusion profile, extending downward with time from the surface. A two-dimensional, mountain-scale model with variable topography indicates that these profiles may be disturbed by lateral migration from areas where infiltration rates are above zero. Based on these preliminary results, a change in the isotopic composition of infiltrating waters can take up to several thousand to tens of thousands of years to equilibrate with the matrix pore waters at the deeper levels of the repository. The simulation results suggest that samples and water isotope determinations from the upper 100 m will provide the most information about infiltration during the Holocene.
H43F-0421 1340h
Description of Contaminant Transport in a Single Fissure With a Lattice BGK Model
The transport of contaminants in fractured media is a complex phenomena that has an important impact on the environment. It has been studied for several authors giving rise to different models that have to be used with restrictive boundary conditions. This paper presents a lattice BGK model, evolution of the lattice Boltzmann model, as an alternative to overcome the limitations of the macroscopic approximations. The model has been developed for describing the contaminant transport in a single fissure in a rock matrix and considers the hydrodynamic dispersion along the fissure, the surface sorption onto the surface of the fracture walls and diffusion and sorption within the rock matrix as the main processes. The model is tested using the experimental breakthrough curves obtained for Moreno et al. (1985) and Neretnieks et al. (1982) for natural fissures in granite drill cores, showing a good agreement. According to the results of the sensitivity analysis, the mean tracer residence time is the parameter more influent in the shape of the breakthrough curves.
H43F-0422 1340h
Stochastic Upscaling of Transport and Retention in Two-Dimensional Discrete Fracture Networks.
Many natural phenomena are modelled as random processes despite being generally accepted as a complicated sequence of deterministic events. Tracer transport in discrete fracture networks is such a phenomena. Understanding the time evolution of a collection of tracer particles in groundwater flow is critical for the assessment of many environmental applications, such as subsurface containment of nuclear waste products. However, there is a difficulty in simulating flow in fractured rock at the geosphere scale due to the vast amount of fractures occuring at a multitude of scales. Efficient methods of upscaling are therefore necessary to predict tracer evolution through the geosphere barrier. We discuss the recently developed Markov-directed random walk (MDRW) method of upscaling tracer transport and retention in discrete fracture networks, and present results from stochastic two-dimensional flow simulations, along with MDRW calculations. We explore the asymptotic behaviour of the integrated random variables ($\tau$, $\beta$) which control advection and retention, and we consider how results of various scenarios affect the assumed relationship between $\tau$ and $\beta$, together with their implications to upscale modelling.