Factors influencing groundwater inflows in a newly constructed cross-strike tunnel, eastern Massachusetts; 2, Fracture-supported coincident lineaments and subsurface structures

As part of the evaluation of the ability of lineaments to map subsurface structures, the coincident lineaments which intersect the tunnel (Curry et al., this volume), were evaluated to isolate those lineaments considered to be "fracture-supported". By definition, fracture-supported coincident lineaments are those which parallel nearby surface fracture sets, mapped faults, lithologic contacts, and/or primary ductile structures; features which may be influential to subsurface groundwater flow. Of the 37 coincident lineaments delineated on the three scales of imagery studied, approximately 70% are considered to be fracture-supported: 9 of the 13 on the 1:58,000 scale images, 10 of 14 on the 1:80,000, and 8 of 10 on the 1:250,000. However, the general lack of surface exposure precludes high confidence in the assignment of fracture-supported status to most lineaments. Large areas devoid of outcrops necessitated extrapolation of regional, surface fracture patterns (domains) to help define some fracture-supported coincident lineaments. There are two occurrences where fracture-supported coincident lineaments from all three scales overlap and are parallel. One occurrence successfully maps the zone of greatest fracture density and highest groundwater inflow (>560 l/min). The other occurrence maps an area of high fracture density and significant subsurface flow (95 l/min). In addition, one other high flow zone (>190 l/min) is mapped by a fracture-supported coincident lineament from the 1:80,000 scale imagery. However, many subsurface fractures and flow zones (<75 l/min) are not mapped by the coincident lineaments regardless of whether or not they are fracture-supported. When considering all fracture-supported coincident lineaments and parallel subsurface structures, the median flow (13,600 l/day) for the mapped structures is greater than the unmapped structures (6,800 liters/day). However, this difference is only significant at the 60% confidence level.Although the tunnel sections with the greatest fracture density and highest groundwater inflows are successfully mapped by fracture supported coincident lineaments, not all water-bearing zones are delineated.