CONTAMINANT TRANSPORT IN FRACTURED ROCKS WITH SIGNIFICANT MATRIX PERMEABILITY, USING NATURAL FRACTURE GEOMETRIES

Some results from numerical models of flow and contaminant transport i n fractured permeable rocks, where fractures are more conductive than rock matrix, are described. The 2D flow field in the fractured and per meable rock matrix is calculated using a finite difference, 'conductan ce mesh' method, and the contaminant transport is simulated by particl e tracking methods using an advection-biased, random walk technique. T he model is applied to simulated and naturally occurring fracture patt erns. The simulated pattern is an en echelon array of unconnected frac tures, as an example of st common, naturally occurring fracture geomet ry. Two natural fracture patterns are used: one of unconnected, sub-pa rallel fractures and one with oblique fracture sets which is well conn ected. Commonly occurring matrix permeability and fracture aperture va lues are chosen. The simulations show that the presence of fractures c reates complex and heterogeneous flow fields and contaminant distribut ion in the permeable rock matrix. The modelling results have shown tha t some effects are non-intuitive and therefore difficult to foresee wi thout the help of a model. With respect to contaminant transport rates and plume heterogeneity, it was found that fracture connectivity (cru cial when the matrix is impermeable) can play a secondary role to frac ture orientation and density. Connected fracture systems can produce s mooth break-through curves of contaminants summed over, for example, a bore-hole length, whereas in detail the contaminant plume is spatiall y highly heterogeneous. Close to a constant-pressure boundary (e.g. an extraction bore-hole), flow and contaminants can be channelled by fra ctures. Thus observations at a bore-hole may suggest that contaminants are largely confined to the fracture system, when, in fact, significa nt contamination resides in the matrix. (C) 1997 Elsevier Science B.V.