HR: 0800h
AN: H21A-0196    [Abstracts]
TI: Hydrogeologic Characterization of Fractured Crystalline Bedrock on the Southern Part of Manhattan, New York, Using Advanced Borehole Geophysical Methods
AU: * Stumm, F
EM: fstumm@usgs.gov
AF: U.S. Geological Survey New York Water Science Center, 2045 Route 112, Coram, NY 11792, United States
AU: Chu, A
EM: achu@usgs.gov
AF: U.S. Geological Survey New York Water Science Center, 2045 Route 112, Coram, NY 11792, United States
AU: Joesten, P K
EM: pjoesten@usgs.gov
AF: U.S. Geological Survey Office of Ground Water Branch of Geophysics, 11 Sherman Place, Unit 5015, Storrs, CT 06269, United States
AU: Lane, J W
EM: jwlane@usgs.gov
AF: U.S. Geological Survey Office of Ground Water Branch of Geophysics, 11 Sherman Place, Unit 5015, Storrs, CT 06269, United States
AB: ABSTRACT. Advanced borehole-geophysical methods were used to assess the hydrogeology of fractured crystalline bedrock in 31 of 64 boreholes on the southern part of Manhattan Island, N.Y. The majority of boreholes penetrated gneiss, schist, and other crystalline bedrock, and had an average depth of 591 ft (180 m) below land surface (BLS). In this study we use a combination of advanced and conventional borehole geophysical logs, and hydraulic measurements to characterize the fractured-rock ground-water flow system in southern Manhattan, N.Y. Borehole-geophysical logs collected in this study included natural gamma, single-point-resistance (SPR), short-normal resistivity (R), mechanical and acoustic caliper, magnetic susceptibility, borehole-fluid temperature and resistivity, specific conductance (SpC), dissolved oxygen (DO), pH, redox, heat-pulse flowmeter (at eight selected boreholes), borehole deviation, acoustic and optical televiewer (ATV and OTV), and directional borehole radar (at 23 selected boreholes). A new geophysical probe that collects multiple fluid parameters, included fluid- temperature, SpC, DO, pH, and redox logs; these were used to help delineate transmissive fractures in the boreholes. All boreholes penetrated moderately fractured bedrock that contained medium and large fractures. A total of 208 large fractures were delineated in the 31 boreholes logged with the OTV. Stereonet analysis of the large fractures indicates most are part of a subhorizontal population cluster with a mean orientation of N43 degrees E, 07 degrees SE and a smaller secondary population cluster dipping toward the northwest. A total of 53 faults were delineated with two major population clusters--one with a mean orientation of N12 degrees W, 66 degrees W and the other with a mean orientation of N11 degrees W, 70 degrees E. Foliation was fairly consistent throughout the study area with dip azimuths ranging from northwest to southwest and dip angles ranging from 30 to 70 degrees. A total of 59 bedrock boreholes had specific-capacity test data analyzed for total borehole transmissivity. The bedrock transmissivity ranged from 0.7 to 870 feet squared per day (0.07 to 81 meters squared per day) in the study area. The majority of boreholes (69 percent) had transmissivities less than 100 feet squared per day (9.3 meters squared per day). Heat-pulse flowmeter logging was completed at eight boreholes. A total of 77 transmissive fractures were delineated at the 8 boreholes. Stereonet analysis of these transmissive fractures indicates two population clusters of fractures--one with a mean orientation of N11 degrees E, 14 degrees SE and the other with a mean orientation of N23 degrees E, 57 degrees NW. These data suggest that the fractured-rock ground-water-flow system in southern Manhattan is interconnected, dominated by subhorizontal fractures, and fractures that dip moderately to the northwest.
DE: 1829 Groundwater hydrology
DE: 1835 Hydrogeophysics
DE: 1879 Watershed
SC: Hydrology [H]
MN: 2007 Fall Meeting