Ivars Neretnieks

Flow Channeling in Heterogeneous Fractured Rocks

Experimental observations and theoretical studies over the last 10 years or so have demonstrated that flow channeling or preferred flow paths is a common phenomenon in fractured rocks. The reason it has come to the forefront of scientific investigation is the recent interest in predicting solute transport in geological media as part of safety assessment of geologic isolation of nuclear or toxic wastes. Solute transport is much more sensitive to medium heterogeneity than is temperature or pressure. In this paper, experimental observations of tracer transport over distances ranging from centimeters to hundreds of meters are reviewed, and theoretical efforts to explain or model these observations are summarized. Processes that may explain some of the experimental observations without the use of flow-channeling models are discussed. The paper concludes with a discussion of the implications of flow channeling on the practical problems related to contaminant transport in geologic systems.

Fluid flow and solute transport in a network of channels

Abstract A new model to describe flow and transort in fractured rocks is proposed. It is based on the concept of a network of channels. The individual channel members are given stochastically selected conductances and volumes. Flow-rate calculations have been performed. For large standard deviations in conductances, channeling becomes pronounced with most of the water flowing in a few paths. The effluent patterns and flow-rate distributions obtained in the simulations have been compared to three field measurements in drifts and tunnels of flow-rate distributions. Standard deviations of channel conductances were between 1.6 and ⪖2.4 in some cases. A particle-following technique was used to simulate solute transport in the network. Non-sorbing as well as sorbing solute transport can be simulated. By using a special technique, solutes that diffuse into the rock matrix can also be simulated.

Long-term processes in waste deposits

A conceptual model, which is a unitary and continuous description of the overall processes in waste deposits, has been developed. In the model the most important processes governing the long-term fate of organic matter in landfills and the transport and retention of toxic metals are included. With the model as a base, a number of scenarios with different levels of complexity have been defined and studied in order to carry out long-term assessments of the chemical evolution in waste deposits for industrial and municipal solid waste containing much organic matter and the leaching of toxic metals. The focus of the modelling has been to quantify the important processes occurring after the methane production phase has ceased, i.e. during the humic phase. The scenarios include the main mechanisms based on various transport processes as well as different landfill constructions, e.g. binding capacities of sulfides and humic substances. They also include transport mechanisms by which the reactant oxygen can intrude into a deposit, sorption capacities of hydrous ferric oxides, and pH-buffering reactions, etc. Scoping calculations have shown that the binding capacity of humic substances is sufficient to bind all toxic metals (Cd, Cr, Pb, Zn and Hg). In addition, the humics could also bind a smaller part of Ca, Fe and Al, provided much of the organic waste remain as humic substances. Sulfides on the other hand can bind approximately twice the amount of all toxic metals. The binding capacity of hydrous ferric oxides, which can be formed by oxidation reactions during the humic phase, is estimated to be three times the total content of metals that can sorb on hydrous ferric oxides. In the studied landfill the pH-buffering capacity, primarily represented by calcite, is estimated to be 1 mol/kg dry waste. Quantifications indicate that the alkalinity of the wastes is high enough to buffer the acidity produced by the oxidation of sulfides and by the degradation of organic matter, as well as that added by acid precipitation. Therefore, the main conclusion is that higher remobilisation rates of heavy metals due to lowering of pH are not expected for many thousands of years.

A Large‐Scale Flow and Tracer Experiment in Granite: 2. Results and Interpretation

Water and tracer flow has been monitored in a specially excavated drift in the Stripa mine. Several new experimental techniques and equipment were developed and used. The whole ceiling and the upper part of the walls were covered with 375 individual plastic sheets where the water flow into the drift could be collected. Eleven different tracers were injected at distances between 11 and 50 m from the ceiling of the drift. The flow rate and tracer monitoring was kept up for more than 2 years. The tracer breakthrough curves and flow rate distributions were used to study the flow paths, velocities, hydraulic conductivities, dispersivities and channeling effects in the rock. In a companion paper the experimental design and performed experiments are described. The present paper describes the interpretation of flow and tracer movement in the rock outside the drift. The tracer movement was measured by the more than 160 individual tracer curves. The tracer experiments have permitted the flow porosity and dispersion to be studied. The possible effects of channeling and the diffusion of tracers into stagnant waters in the rock matrix and in stagnant waters in the fractures have also been addressed.

Long-term fate of organics in waste deposits and its effect on metal release

The long-term chemical evolution in waste deposits and the release of toxic metals was investigated. The degradation of organic matter and hence the potential efflux of heavy metals in a long-term perspective was studied by defining some scenarios for waste deposits containing organic compounds, different longevity and functions of covers and different water and air intrusion rates. The scenarios were based on various transport processes as well as different landfill constructions. The rates of influx of oxygen into both saturated and partially saturated landfills have been estimated. Each scenario takes the form of a mathematical model. The starting point for all the studied cases is the humic phase, i.e. the phase after the methane production has stopped. Based on the different cases studied, it appeared that landfills where the waste is below the water table could have advantages over the other cases. Recognizing that this option is not accepted in most countries we, nevertheless, suggested it should be reevaluated. The main conclusion is that the degradation of humic matter and hence the release of toxic metals can be substantially decreased if potential build-up of hydraulic gradients are avoided and if the landfill is located below the water surface. A conceivable alternative construction would be to place it in a depression--either natural or artificial--and to construct it so that under normal conditions it would always be water-saturated.

A Large-Scale Flow and Tracer Experiment in Granite: 1. Experimental Design and Flow Distribution

This paper describes the Stripa three-dimensional experiment where water and tracer flow has been monitored in a specially excavated drift in the Stripa mine. Several new experimental techniques and equipment were developed and used. The experiment was performed in a specially excavated drift, 100 m long, at the 360 m level in granite. The whole ceiling and the upper part of the walls were covered with 375 individual plastic sheets where the water flow into the drift could be collected. Eleven different tracers were injected at distances between 11 and 50 m from the ceiling of the drift. The flow rate and tracer monitoring was kept up for more than 2 years. The tracer breakthrough curves and flow rate distributions were used to study the flow paths, velocities, hydraulic conductivities, dispersivities and channeling effects in the rock. This paper describes the experimental techniques, the fracture mapping, the tracer and flow rate measurements and the results of the flow rate measurements. The detailed observations made possible by the plastic sheeting technique have given some qualitative as well as quantitative information on flow rate distribution in fractured rock which previously has not been available. These observations may be of importance for assessing the transport of dissolved species such as radionuclides through fractured rock.

Analysis of some adsorption experiments with activated carbon

Abstract A simple method is proposed whereby the film transfer coefficient and coefficient of diffusion in the particles may be determined from finite bath adsorption experiments. The method also makes it possible to separate pore and surface diffusion. Under certain conditions it is also possible to determine the influence of particle phase concentration on the surface diffusivity. The method is based on models describing the instationary diffusion in the solids. Data from six different adsorption systems were analysed using this method. The adsorbed components were: phenol, paranitrophenol, parachlorophenol, bensoic acid, phenylacetic acid and 2-4-dichlorophenoxyacetic acid. In all systems surface diffusion was the determining transport mechanism in the particles. In the system phenol and phenylacetic acid the surface diffusion coefficient increased by about a factor 3 with an increase in surface concentration of about 40%. For parachlorophenol the increase was somewhat less. For the other systems there was no significant increase. The increase in diffusivity is explained by a decrease in bonding forces with increasing concentration.

Flow and nuclide transport in fractured media: the importance of the flow-wetted surface for radionuclide migration

Abstract Radionuclides which migrate from a repository for nuclear waste in crystalline rock are transported by the water in a very complex fracture network. One of the main retardation mechanisms is that of uptake by diffusion and sorption in the rock matrix. The uptake is strongly influenced by the size of the contact surface between the mobile water and the rock. It is also strongly influenced by the residence time distribution of the water in contact with these surfaces. In recent years, several large-scale field experiments and observations have been made that show strong channeling effects. A considerable fraction of the water may flow in preferential pathways without good mixing with the rest of the water. This leads to the question if the traditional advection-dispersion-based equations are sufficient to describe the transport. In this paper the advection-dispersion model is compared with a pure channeling model where the different channels have different flow-rates. In the channeling model no mixing between the waters in the channels takes place. In the advection-dispersion model there is very frequent mixing. These two models would seem to form the two extremes of the properties of fracture network or channel network models. A recently developed channel network model where the individual channel members form a three-dimensional network is also used to simulate tracer transport. The channel network model has stochastically varying conductivities of the different channel members. The channel network can be thought of as connecting the channels in the channeling model and allowing the waters to mix along the flow paths. In all models the magnitude of the flow-wetted surface has a very strong impact on the arrival times for sorbing species. The time to attain a certain concentration at a point is proportional to the magnitude of the flow-wetted surface squared. Considering that the flow-wetted surface in rock is one of the entities which is poorly known and is difficult to assess, this suggests that there is a need for more field data and for methods to assess this entity. The three models need different types of data. All need the magnitude of the flow-wetted surface. The channeling and channeling network models need the conductivity distribution of the channels and channel members respectively. These data can be obtained by hydraulic tests. The advection-dispersion model needs some measure of the dispersivity. This information could be obtained by tracer tests using nonsorbing tracers. It is shown in the paper that if the hydraulic system in the rock is best described by a channel network then the use of the advection-dispersion model can give ambiguous results when used to predict transport of sorbing tracers.

Adsorption in finite bath and countercurrent flow with systems having a nonlinear isotherm

Abstract Isothermal countercurrent, cocurrent and finite bath adsorption has been modelled mathematically. The model equations have been solved numerically by using the method of orthogonal collocation. The equilibrium between the solid and the fluid phase is assumed to be nonlinear and is described by either the Langmuir or the Freundlich type of equation. The transport mechanisms in the particles are assumed to be pore diffusion, solid diffusion or a combination thereof. The effect of film resistance is included. The above three adsorption cases are mathematically described by the same equations. Computed results are shown for various flowratios and parameters in the Langmuir and Freundlich equations. The results may be of help for evaluating coefficients of diffusion from finite bath experiments and for designing continuous countercurrent absorbers. The equations, their parameters and the diagrams showing the results are given in what is hoped to be a comprehensive way to facilitate the comparison of various mechanisms and isotherms.

Modelling of transport and reaction processes in a porous medium in an electrical field

A numerical model has been developed to describe the transport and reaction processes in a porous medium in an electrical field. The model discretizes the one-dimensional porous medium by a number of compartments. The governing equations are formulated by material balance over each compartment. The model describes transport processes of advection, dispersion, ionic migration and electroosmosis. Various reactions such as aqueous complexation, precipitation/dissolution, and electrochemical reactions are treated by kinetic approaches. For fast fluid-phase reactions such as complexation, local equilibrium assumption may be applied in the model, which reduces the number of primary components and hence the number of governing equations. The sorption processes occurring at the surfaces of the porous medium are at the moment treated only by a single-component linear isotherm. Numerical solution to the model gives concentration and electrical potential distributions in the porous medium at different times and gives as well the electrical current history. Modelling results for three sample cases are reported to demonstrate the application of the model. In the first sample case, the removal of copper from sand by an electrical field is simulated. The second case concerns an electrokinetic soil remediation process with cathode rinsing. The third case models the selective filtering function of the ion exchange membranes.