Geohydrologic assessment of fractured crystalline bedrock on the southern part of Manhattan, New York, through the use of advanced borehole geophysical methods

Abstract

Advanced borehole-geophysical methods were used to assess the geohydrology of fractured crystalline bedrock in 31 of 64 boreholes on the southern part of Manhattan Island, NY in preparation of the construction of a new water tunnel. The study area is located in a highly urbanized part of New York City. The boreholes penetrated gneiss, schist, and other crystalline bedrock that has an overall southwest-to northwest-dipping foliation. Most of the fractures intersected are nearly horizontal or have moderate- to high-angle northwest or eastward dip azimuths. Heat-pulse flowmeter logs obtained under nonpumping (ambient) and pumping conditions, together with other geophysical logs, delineated transmissive fracture zones in each borehole. Water-level and flowmeter data suggest the fractured-rock ground-water-flow system is interconnected. The 60 MHz directional borehole-radar logs delineated the location and orientation of several radar reflectors that did not intersect the projection of the borehole. A total of 53 faults intersected by the boreholes have mean orientation populations of N12°W, 66°W and N11°W, 70°E. A total of 77 transmissive fractures delineated using the heat-pulse flowmeter have mean orientations of N11°E, 14°SE (majority) and N23°E, 57°NW (minority). The transmissivity of the bedrock boreholes ranged from 0.7 to 870 feet squared (ft 2 ) per day (0.07 to 81 metres squared (m 2 ) per day).