A laboratory and numerical investigation of solute transport in discontinuous fracture systems

Robinson, J. W. (James William) (1987) A laboratory and numerical investigation of solute transport in discontinuous fracture systems. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (8MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

The mixing of fluids at fracture intersections was examined, in the laboratory, using fourteen plexiglass models that simulated open fractures with no contact between the fracture walls. Twelve models contained two fully intersecting fractures. One model contained two intersecting but offset fractures (parallel flow model) and one fracture system model contained a total of eleven fractures in two sets of intersecting fractures, all with the same aperture. One set was composed of five parallel fractures and the other set was composed of six parallel fractures. The twelve fully intersecting fracture models were designed to investigate the effects, on mixing, of seven angles of intersection and three fracture apertures. Iodide solution of known concentration was injected into one fracture and distilled water into another (inlet ports). At each of the outlet ports the concentration of iodide and the discharge volume were measured. The ratio of the volumes of distilled water and iodide solution in each of the discharge fractures was compared to calculate the percent mixing at the fracture intersection. -- Testing, conducted at three hydraulic gradients, indicated that essentially no mixing occurred in the fully intersecting fracture models and only nominal mixing occurred in the parallel flow model. In general mixing was found to be dependent only upon the relative size of the inlet and outlet fractures. Testing, using the fracture system model, indicated a similar lack of mixing at six intersections through which the fluid moved. -- A two dimensional finite element model was written to simulate the transport of a conservative solute in a discontinuous, random, fracture system. Mixing at fracture intersections in the numerical model was based on the results of the physical model study. Hence no mixing was allowed to take place at the fracture intersections except that which was due to the differences in the apertures of the inlet and discharge fractures. Using this mixing algorithm the numerical model indicates that more longitudinal and less lateral dispersion takes place than when complete mixing at fracture intersections is assumed. In addition, more longitudinal transport takes place in discontinuous than in continuous fracture systems. These findings indicate that contaminants migrating through fractured media, where the fracture walls are not in contact, will not be dispersed and diluted to the extent that past numerical models have predicted and hence the contaminant will be discharged to the biosphere in much greater concentration than expected.

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