Delineation of the Fractured-Rock and Unconsolidated Overburden Ground-Water Flow Systems, on the Southern Part of Manhattan, New York, Through Use of Advanced Borehole-Geophysical Techniques

Advanced borehole-geophysical techniques were used to assess the geohydrology of crystalline bedrock in 31 of 64 boreholes on the southern part of Manhattan Island, N.Y. Ten wells were screened in the unconsolidated overburden (glacial aquifer) to determine water-table elevation, transmissivity, and chloride concentration. The borehole-logging techniques included natural gamma, single-point resistance, short-normal resistivity, mechanical and acoustic caliper, magnetic susceptibility, borehole-fluid temperature and resistivity, borehole-fluid specific conductance, dissolved oxygen, pH, redox, heat-pulse flowmeter (at selected boreholes), borehole deviation, acoustic and optical televiewer, and borehole radar (at selected boreholes). The boreholes penetrated gneiss, schist, and other crystalline bedrock that has an overall southwest to northwest-dipping foliation. Most of the fractures penetrated are nearly horizontal or have moderate- to high-angle northwest or eastward dip azimuths. Heat-pulse flowmeter logs obtained under pumping and nonpumping (ambient) conditions, together with other geophysical logs, indicate transmissive fracture zones in each borehole. 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. Fifty-three faults had mean orientation populations of N12°W, 66°W or N11°W, 70°E. Seventy-seven transmissive fractures delineated using the heat-pulse flowmeter had mean orientations of N11°E, 14°SE (majority) and N23°E, 57°NW (minority). The first potentiometric-surface and water-table maps were completed for southern Manhattan of the bedrock and glacial aquifer, respectively. Bedrock transmissivity ranged from 0.7 to 871 feet squared per day. Glacial aquifer transmissivity ranged from 2 to 93,000 feet squared per day. Chloride concentrations ranged from 25 to 17,800 milligrams per liter in the bedrock, and 28 to 15,250 milligrams per liter in the glacial aquifer. Three areas of saltwater intrusion in the bedrock and three wedges of saltwater intrusion in the glacial aquifer were delineated. The fractured-rock ground-water flow system within the study area is a highly interconnected continuum that responds as far as 3,200 feet away to hydraulic stresses