Lars Birgersson

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.

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.

Channeling experiments in crystalline fractured rocks

Abstract The channeling experiments were designed to study the transmissivity and aperture variations in fractures in crystalline rock at a depth comparable to high-level waste repository dephts. The experiments were performed in the Stripa experimental mine in mid-Sweden. Two types of experiments were designed. In the single-hole experiments a hole was drilled > 2 m into the plane of the fracture and injection flow-rates were measured in 5-cm sections using a specially designed injection packer. Photographs were also taken inside the hole along the fracture to determine the visible fracture aperture and to obtain other information such as fracture intersections and fracture infilling. In the double-hole experiment two parallel holes were drilled in the plane of the same fracture at a center distance of 1.95 m. Hydraulic tests and tracer tests were made between the two holes to obtain information on connections in the plane of the fracture and to obtain information on residence time distributions in different paths (channels).

A tracer migration experiment in a small fracture zone in granite

The water flow rate and tracer distribution has been studied in a fracture zone and averagely fractured rock in the Stripa underground research laboratory. The experimental site is located in granitic rock at a depth of 385 m below the ground surface. A 50-m-long drift with a diameter of 3 m was excavated. The drift was intersected by a 6-m-wide fracture zone. The upper part of the drift was covered by 150 plastic sheets in which water was collected. The water in the lower part of the drift was collected in sump holes. Different tracers were injected in seven locations at distances between 9.5 and 25 m from the drift. More than 50% of the water was found in one sampling area in the drift, and more than 90% in only eight sampling areas. The recovery of the tracers was also concentrated in a few sheets. Most of the flow preferentially takes place in a few paths. Two sets of tracers were used in tandem; one set of dyes which had previously been used in Stripa and one set of lanthanide-DTPA complexes. Although shown to be nonsorbing in long-time high-sensitivity laboratory tests, the dyes were found to have a somewhat smaller recovery than the metal tracers. This observation was used in an attempt to estimate the flow-wetted surface into which the dyes can diffuse and sorb. The flow porosity was estimated from the tracer residence times for all injection points. The flow in the intersected fracture zone is not like that in a porous medium; it is highly channeled.

Analysis of a long-term pumping and tracer test using the channel network model

Water flow and solute transport in a fractured medium are described by the Channel Network model, CHAN3D. In the field and in the model, most of the flow and transport take place in fracture zones. CHAN3D was used to simulate a long-term pumping and tracer test called LPT2. In the first simulations, no calibrations, conditioning or parameter adjustments were made to better fit the experimental results. Instead, as much as possible of the available field data were used. In six different paths the simulations predicted breakthrough that should be observable. However, only two of the performed tracer tests resulted in breakthrough of solutes. Some possible reasons for the scant recovery is discussed. In the analysis of LPT2, the impact of the flow rates at the injection positions, flow porosity and interaction with the rock matrix were studied.

Flow and Tracer Experiments in Crystalline Rocks: Results from Several Swedish in Situ Experiments

The purpose of the two investigations performed in the Stripa mine is to understand and describe water movement in sparsely fractured crystalline rock. The first experiment concerns water flow within a single fracture and sorbing tracers interaction with the rock matrix, such as sorption and diffusion into the matrix. The second experiment concerns water flow paths within a larger block of rock.

Diffusion in the Matrix of Granitic Rock. Field Test in the Stripa Mine

A migration experiment in the rock matrix is presented. The experiment has been carried out in “undisturbed” rock, that is in rock under its natural stress environment. Since the experiment was performed at the 360 m-level (in the Stripa mine), the rock was subject to nearly the same conditions as the rock surrounding a nuclear waste repository as proposed in the Swedish concept (KBS). The results show that all three tracers (Cr-EDTA, Uranine and 1-) have passed the zone disturbed by the presence of the injection hole and migrated some distance into “undisturbed” rock. These results indicate the existence of a connected micro fissure system in undisturbed rock, in which tracers (and therefore radionuclides) can migrate. Diffusivities obtained in this experiment are comparable to those obtained in laboratory experiments.

A Channeling Experiment to Study Flow and Transport in Natural Fractures

Model calculations show that channeling may have a detrimental effect on radionuclide transport because fast channels may carry some of the mass of the radionuclides considerably faster than the average flow would and may give this portion considerably less time to decay. Channeling further aggravates the retardation of the sorbing nuclides because less surface area is available for sorption in a channel within a fracture than if the whole fracture is exposed to the flowing water. An ongoing experiment in the Stripa mine aims at studying some aspects of channeling. A special packer of a new design is used. Preliminary results show that there is uneven flow in the fractures which indicates channeling.

Chemical Transport in Fractured Rock

Transport of dissolved species in fractured rock has become an area of special interest in recent years when deep lying crystalline rocks have become potential sites for repositories for nuclear waste. In Sweden, research was started in 1977 to investigate the flow and transport in low permeability crystalline rocks such as granites and gneisses.