ESTIMATING BULK PERMEABILITY OF FRACTURED ROCK AQUIFERS USING DETAILED OUTCROP DATA AND DISCRETE FRACTURE NETWORK MODELING

The use of fractured-bedrock aquifers to meet private, public and commercial water supply needs is increasing in the northeast United States. Sustaining and managing ground water in these fractured-bedrock aquifers requires basic information on the physical characteristics of the ground water system and especially the fluid flow properties of the fractured-bedrock aquifer. Determination of these properties in fractured-bedrock aquifers is inherently difficult due to complexity associated with fracture networks in spatially heterogeneous crystalline rocks. The cost-benefit ratio of traditional site investigations, such as drilling and logging of boreholes, is typically small in these types of geologic settings. Our goal is to evaluate the value of incorporating low-cost outcrop measurements of fracture populations into discrete fracture network (DFN) models for predicting bulk fluid flow properties of the rock mass.

The basic tenet behind DFN approaches is that the orientation, connection, and properties of the fracture networks are important for predicting bulk-scale hydrologic behavior. This study uses statistical data from 79 outcrops to parameterize a DFN model using FRACMAN, a 3-D transport code developed by Golder Associates. Outcrop data used to develop the DFN includes fracture size, spacing, orientation, nature of terminations, and degree of openness. The nature of the fractures is modeled by using several statistical analyses including the Enhanced Baecher distribution and P₁₀ density calculations. Governing equations of fluid flow are solved within the fracture network to determine the bulk permeability and storage of the rock mass

Derived outcrop fluid flow properties are compared with other existing data collected with traditional means such as aquifer pumping tests. The fracture models provide an order of magnitude determination of bulk permeability in the region studied. Use of DFN models in this system is useful for understanding the general flow of groundwater in this crystalline bedrock fracture network, and could be as useful in other similar geologic settings. If proven reliable, this method is a cost-efficient tool for providing order of magnitude estimates of the fluid flow properties of fractured rocks.