HR: 1340h
AN: H23G-1707    [Abstracts]
TI: Exploring the Depth and Nature of Flow Systems in Fractured Igneous and Metamorphic Bedrock Aquifers
AU: * Diggins, J P
EM: jdiggins@geo.umass.edu
AF: University Of Massachusetts-Amherst, Department of Geoscience Morrill Science Center 611 North Pleasant St, Amherst, MA 01003, United States
AU: Boutt, D F
EM: dboutt@geo.umass.edu
AF: University Of Massachusetts-Amherst, Department of Geoscience Morrill Science Center 611 North Pleasant St, Amherst, MA 01003, United States
AB: In rocks possessing highly discontinuous and heterogeneous flow paths, such as fractured igneous and metamorphic rocks, regional scale flow concepts are not as easily applied due to the lack of continuity in flow paths. It is traditionally assumed that flow in these bedrock systems decreases with depth due to a stress- induced decrease in permeability. However, recent hydrogeologic studies have suggested that deeper crustal materials are likely more porous then previously assumed and that critically stressed fractures are the most permeable. In the Northeast U.S. most bedrock consists of these rocks and is more frequently being relied upon as a source of drinking water for both private and public water supplies. It is of essential importance to understand the nature and depth of flow systems in these settings for determining the sustainability and vulnerability of water supplies. Previous workers have completed an extensive fracture mapping study of the Nashoba Terrane, a bedrock terrain located in the densely populated suburbs of Boston, MA, in an attempt to link the surface expression of fractures with the hydrogeologic properties of the subsurface. They suggest a series of hydrostructural domains, based on fracture orientation and density, which govern direction and quantity of flow. We present geophysical and hydrologic data collected in fifteen deep (> 100 meters) boreholes in this domain, in conjunction with a series of discrete fracture network (DFN) models that incorporate the field data, to test the structural domain hypothesis, and to quantify the depth and nature of flow in the terrain. DFN models are used to explore the relationship between flow and fracture orientation, employing a simple relationship between fracture aperture and depth. The field data suggests that most flow occurs in the upper 40 meters of shallow crust, but some flow is found deeper. Field data and DFN models suggest that these fractures are preferably oriented with respect to the assumed local stress field and support the hypothesis that even at shallow depths, permeable fractures are controlled by the local stress state.
DE: 1832 Groundwater transport
DE: 1835 Hydrogeophysics
DE: 1847 Modeling
DE: 1859 Rocks: physical properties
SC: Hydrology [H]
MN: 2007 Fall Meeting