Solute exchange at the fracture-matrix wall involving significant buoyancy effects - an analog experiment
Abstract
Contaminant transport in heterogeneous fractured aquifers occurs mostly through the networks of intersecting fractures. The physical mechanisms of solute transport in a single fracture with impermeable walls are well identified: advection, micro-dispersion (including molecular diffusion) Taylor-Aris dispersion, roughness dispersion, and aperture-variation dispersion. The description of the mass transfert coefficient between the region of high permeability (the fracture) and that of low permeability (the surrounding matrix), when the permeability of the latter cannot be neglected, is in contrast poorly understood. We address here solute transport through a synthetic fracture with a porous wall placed under the fracture, and for which buoyancy effects significantly promote solute exchange at the porous wall. We have developed an analog experimental setup in which the planar horizontal fracture is 1 m long, 5 cm wide and its mean aperture is 5 mm. It is bounded by either two smooth parallel Plexiglass plates (impermeable walls configuration), or by one such plate and a porous medium consisting of 1 mm glass beads ("semi-permeable" configuration). A permanent laminar water flow is forced through the fracture at controlled mean velocity (~ 1mm/s). The flow conditions inside the experimental fracture have been characterized using three-dimensional finite volume numerical simulations of the flow. A dye (patent blue) injection system simulates a point source of contaminant along the center plane of the experimental fracture. The tracer plume is tracked using a visualization system based on (i) lasers illuminating a series of vertical linear optical sensor arrays, and (ii) 4 cameras positioned side by side and providing a composite image of the fracture length, viewed from the side. The two measurement systems yield consistent quantitative temporal descriptions of the tracer concentration, integrated over the fracture width and at several positions along the fracture length. We check the conservation of the total dye quantity in the impermeable walls configuration, and investigate the impermeable-porous configuration. Mass transfer between the fracture and the bounding porous matrix is measured at different volumetric flows and for various concentrations of the injected dye, and this in different geometries (roughness) of the fracture-matrix interface. Our goal is to extract descriptive laws of the solute exchange at the fracture wall, that will be integrated in future numerical models.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.H41F0944M
- Keywords:
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- 1832 Groundwater transport;
- 1859 Rocks: physical properties;
- 5104 Fracture and flow;
- 5139 Transport properties