Monday, June 23, 2008 : 1:30 p.m.

Characterization of the Fractured Crystalline Bedrock on the Southern Part of Manhattan, New York, Through Use of Advanced Borehole Geophysical Methods

Frederick Stumm and Anthony Chu, USGS, New York Water Science Center

Advanced borehole-geophysical methods were used to characterize the hydrogeology of fractured crystalline bedrock in 31 of 64 boreholes on the southern part of Manhattan Island, N.Y. in preparation of the construction of a new water tunnel.  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-megahertz 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).  Transmissivity of the bedrock boreholes ranged from 0.7 to 870 feet squared per day (0.07 to 81 meters squared per day).

Frederick Stumm, USGS, New York Water Science Center Frederick Stumm, Ph.D., is a research hydrologist with the U.S. Geological Survey, New York Water Science Center in Coram, NY (2045 Route 112, Coram, New York, 11727;(631-736-0783; fstumm@usgs.gov). He received a B.S. in geology from Hofstra University, an M.S. in geology from California State University at Fresno, and a Ph.D. in earth and environmental science from the City University of New York. His research interests include the application of surface and borehole geophysical methods to geotechnical and environmental problems, hydrogeologic framework mapping, and delineation of saltwater intrusion on Long Island and New York City.

Anthony Chu, USGS, New York Water Science Center Anthony Chu, is a hydrologist with the U.S. Geological Survey, New York Water Science Center in Coram, NY (2045 Route 112, Coram, New York, 11727;[631-736-0783]; achu@usgs.gov). He received a B.A. in earth and space sciences and an M.S. in hydrogeology from the State University of New York at Stony Brook. His research interests include water use on Long Island, surface and borehole geophysical methods, and delineation of saltwater intrusion on Long Island and New York City.


2008 NGWA Conference on Eastern Regional Ground Water Issues