Delineation of Ground Water Flow in Fractured Rock in the Southwestern Part of Manhattan, New York, Through Use of Advanced Borehole Geophysical Methods

Abstract

AbstractAdvanced borehole‐geophysical methods were used to assess the geohydrology of fractured crystalline bedrock at five test boreholes in southwestern Manhattan Island, New York, in preparation for construction of a third water tunnel for New York City. The boreholes penetrated gneiss and other crystalline bedrock that has an overall southwest to northwest dipping foliation with a 60° dip. Most of the fractures encountered are either nearly horizontal or have moderate northwest dip azimuths. Fracture indexes range from 0.25 to 0.44 fracture per foot (0.3 m) of borehole.Electromagnetic (EM) and heat‐pulse flowmeter logs obtained under ambient and pumping conditions, together with other geophysical logs, indicate transmissive fracture zones in each borehole. Pumping tests of each borehole indicated transmissivity ranges from <2 to 360 ft2/day (0.2 to 33 m2/day). Ground water appears to flow within an interconnected fracture network toward the south and west within the study area. No correlation was indicated between the fracture index and the total borehole transmissivity.