DECIPHERING THE SIGNATURES OF CONNECTIVITY AND CONDUCTANCE ON CHEMICAL TRANSPORT IN FRACTURE-CONTROLLED AQUIFERS

Regional geologic structures and fractures are the primary conduits of fluid movement and chemical transport in most fractured-rock aquifers. Typically, regional geologic features of significant conductance can be readily identified and incorporated into conceptual models of chemical transport. The pervasive fracturing from regional and local stress distributions, however, is equally important in controlling the fate of chemical constituents, but the effect of such fractures on chemical transport is much more difficult to discern. A numerical examination using three-dimensional discrete fracture network models is performed to evaluate chemical transport behavior across a range of observed fracture architectures. We find that chemical migration patterns are an amalgamation of several influences, including the fracture network geometry and transmissivity distribution, and their spatial correlation. Of particular importance is the degree of entropy that is present, which refers to the spatial correlation (connectedness) of high or low conductivity values. Our analysis indicates that when the spatial correlation and magnitude of length and/or conductance change so will the chemical migration patterns. In fact, transport behavior for two systems with identical fracture network geometries, but significantly different entropy, can be unrecognizable from one another. In this study we 1) reveal how observed transport patterns relate to various network characteristics, and 2) show that it is best to consider the conductivity and fracture network connectivity structures jointly as a means to understand transport behavior.