Characterizing fractured crystalline bedrock aquifers using hydrostructural domains in the Nashoba Terrane, eastern Massachusetts

Fractured crystalline bedrock aquifers are good sources of potable water in many parts of the world. However, siting of highly productive wells in these rock units remains a challenging and expensive task because fracture development at the regional scale is both heterogeneous and anisotropic. Using low cost field data to define units of rock that have similar lithologic and fracture characteristics can significantly reduce time and energy spent on determining areas with better than average aquifer productivity. These physical characteristics that impart a particular hydraulic character on rocks are used to delineate regions with similar hydrologic characteristics called hydrostructural domains (Mackie, 2002). Hydrostructural domains are delineated from fracture characterization data that were collected from 79 outcrops located in the Nashoba Terrane of eastern Massachusetts. Information collected and used to delineate the domains include 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. Discrete fracture networks are generated from the fracture characterization data to simulate groundwater flow in the region. Conductivity of particular units is evaluated and compared to results from existing pumping tests obtained from the US Geological Survey. Preliminary results indicate that there is great value in utilizing fracture characteristic data obtained from surface outcrops to predict subsurface groundwater flow characteristics of fractured bedrock aquifers. Water managers, developers and decision makers are eager to know which areas are the most promising for encountering highly conductive zones in the subsurface. Collecting extensive structural data from surface outcrops, although not as accurate as drilling wells, is a cheaper alternative that could provide at least a rough estimate of the hydraulic properties of fractured rocks leading to effective siting of new water wells. Hydrostructural domain maps may pinpoint specific areas that have a high potential for wells to encounter highly conductive zones and could therefore be a powerful tool in transferring information from one site to another without having to repeatedly undertake extensive site characterization.