WHAT CONTROLS THE DEPTH OF FLOW SYSTEMS IN NEW ENGLAND FRACTURED ROCK TERRAIN?

Regional groundwater flow in geologic systems characterized by flow through sedimentary derived porous media has long been understood in the context of Toth-based flow. In rocks possessing highly discontinuous and heterogeneous flow paths, such fractured igneous and metamorphic rocks, these concepts are not as easily applied due to the lack of continuity in flow paths. In the Northeast US 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.

In this paper we explore the controls on the depth of flow systems in section of this terrain in Eastern Massachusetts by combining 1) previously conducted surface based geologic mapping of fractures and faults, 2) geophysical and hydrologic data collected on over 20 deep (> 100 meters) boreholes, and 3) numerical modeling of the region using discrete fracture network based models that incorporate the field data. Analysis of the borehole geophysical data and qualitatitive information from well drillers logs suggest that high producing zones exist at depths greater than 300 meters. Ancillary geochemical data also suggest mixing of recent waters with some percentage of higher residence time fluids in these fractured rocks. Numerical modeling of the results supports a hypothesis that the depth of the flow systems are likely controlled by highly conductive fractures that quickly move recharge waters to the deeper producing zones. This “short-circuiting” of the flow system is likely to be characteristic of regional-scale flow systems in fractured rock terrain.