Characterizing fractured crystalline bedrock aquifers using discrete fracture networks in the Nashoba Terrane, Eastern Massachusetts

The use of fractured bedrock aquifers to meet private and public water supply needs is increasing in the northeast US. Sustaining and managing groundwater in these aquifers requires basic information on the physical characteristics of the ground water system and especially the hydraulic properties of the fractured bedrock aquifers. Determination of hydrologic properties of fractured bedrock aquifers is inherently difficult due to complexity associated with fracture networks in spatially heterogeneous crystalline rocks. Our goal is to evaluate the value of incorporating low-cost outcrop measurements of fracture populations and geologic information into discrete fracture network (DFN) models for predicting bulk fluid flow properties of the rock mass. We attempt to delineate hydrostructural domains (regions with similar hydrologic properties) based on DFN models under the assumption that tectonic and geologic factors impart a particular hydraulic character on the rocks. Hydrostructural domains are delineated from a combination of geologic mapping, fracture characterization data (the number and distribution of fracture sets, types of fractures present or absent, the degree of fracture development, fracture intensity/density, fracture connectivity and rock type), conceptual DFNs, and calibrated DFNs. Fracture data collected from 79 outcrops and geologic mapping of 8 quadrangles located in the Nashoba Terrane of eastern Massachusetts were used to first sub-divide domain based on perceived hydraulic properties of the rocks. Conceptual DFN models were subsequently built using grouped fracture properties of the original subdivided domains with the bulk conductivity tensor derived from DFN simulations as the response variable. Refinement of the original subdivided domains was based on the performance of hydrostructural domains in the DFN simulations. Finally, DFNs developed with detailed statistical information for outcrops located in the hydrostructural domains are compared with available borehole and well testing data. Results indicate that there is value in using fracture characteristic data and DFN models to approximate subsurface groundwater flow characteristics of fractured bedrock aquifers.