MODELING STRUCTURAL CONTROLS ON GROUNDWATER FLOW AND SOLUTE TRANSPORT IN FRACTURED CONSOLIDATED ROCK AQUIFERS

Groundwater-transmitting planar discontinuities in many bedrock formations consist primarily of non-random networks of genetically-related joints and bedding plane partings rather than the random systems of generic fractures assumed in certain fractured aquifer models. A method is presented by which groundwater flow direction and volume within each set of pervasive, sub-parallel planar discontinuities is defined by a calculated flow index, based on discontinuity orientation and spacing with respect to the average rock mass hydraulic gradient and average fracture aperture (for a simple cubic law approximation of Poiseuille hydraulic conductivity). Flow indices for principle discontinuity sets are superimposed by vector summation to obtain the direction and rate of the average flow through the fractured rock mass using only structural data for the formation. The anisotropic nature of the hydraulic conductivity tensor results in a calculated flow direction that is typically not aligned with the hydraulic gradient. The structurally based method described produces testable transport and dispersion predictions without the need for costly aquifer testing to determine a priori the directional components of the hydraulic conductivity ellipse. A case study is presented of a jointed formation in which groundwater is calculated to flow in a direction offset from that of the local hydraulic gradient; a direction confirmed with subsequent water quality monitoring. The model has been applied successfully to other sites with a variety of rock types and is currently being coded for computer based applications.