Hydrogeologic-Framework Mapping - Long Island, New York

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Long Island is underlain by unconsolidated Holocene deposits, glacial deposits of Pleistocene age, and coastal-plain deposits  of Late Cretaceous age. These sediments consist of gravel, sand, silt, and clay underlain by crystalline bedrock of early Paleozoic age (fig. 1). The bedrock is relatively impermeable, and forms the base of the groundwater-flow system on Long Island. The geologic and hydrologic units underlying Long Island have been studied and characterized by the USGS since the early 1900s.  Much of this earlier work has been synthesized by Smolensky and others (1990); however, in this previous work, there continues to be uncertainty in areas of complex hydrogeology especially along the northern parts of Long Island where extensive glacial erosion and deposition has not been adequately mapped.

Approach

A synthesis of all existing lithologic data collected since the last framework compilation (Smolensky and others, 1990) will be augmented with a network of new observation wells that will be drilled in areas of complex hydrogeology to better define and reduce uncertainties in the hydrogeologic framework and to provide additional information for the numerical groundwater-flow model being developed as part of this study.  Up to 25 new wells will be drilled over the next three years to collect core samples, borehole-geophysical logs, water-quality samples, and groundwater levels.  In addition, surface-geophysical methods will be utilized to fill in data gaps where no wells exist.  These methods include passive-seismic and time-domain electromagnetic induction.

blue and brown colored map of Long Island with locations for 3 phases of wells

Locations of proposed  new drilling sites.(Public domain.)

 

3 illustrations of the hydrgeologic framework of the Long Island aquifer system

Figure 1 -  Hydrogeologic Framework of the Long Island Aquifer System - The figure above shows a hydrologic section of the Long Island aquifer system in western Nassau County, NY as delineated by Smolensky and others in 1990. This framework was developed using existing driller’s logs and well records. In 2004, the USGS installed outpost wells in the northernmost part of this hydrogeologic section and produced a much more detailed framework that indicates a more complex groundwater-flow system. Additional drilling and data collection for this study will be used to update our current understanding of the hydrogeologic framework of the Long Island aquifer system, and construct refined and higher-resolution hydrogeologic maps for most of Long Island (Public domain).

Drilling

The drilling program will consist of mud-rotary drilling and split-spoon core sampling. Most boreholes will be drilled through the unconsolidated sediments into bedrock, and core samples will be obtained at regular intervals for geologic analysis. Each drilled borehole will be cased with polyvinyl chloride (PVC) to allow for future groundwater sampling, groundwater-level measurements, and borehole-geophysical logging needed to assess long-term changes in aquifer conditions and saltwater-intrusion concerns.

Core Analysis

Core samples obtained during drilling will be analyzed for minerology, grain size, and color (fig. 2).  The core samples will be used to correlate borehole-geophysical logs and to determine contacts between hydrogeologic units.  The thickness of each hydrogeologic unit will be estimated at each observation well primarily from core data obtained during the drilling, from borehole-geophysical logs, driller’s logs, and through interpolation. Color descriptions of the core samples will be based on the standard Munsell color chart (Natural Resources Conservation Service).

 

long tube of claylike soil with plastic bag above it

Figure 2 -  Sediment Core Collected from a Well at Jamesport, New York: The image above shows a sediment-core sample collected during the drilling of monitoring well S-13428 in Jamesport, NY in September, 2017.  This sample was collected from a depth of 220 feet below land surface using the split-spoon coring method. Sediment-core samples provide information on the subsurface geology (Public domain).

Borehole Geophysics

Borehole-geophysical logs  will be collected from new and existing observation wells to provide information that cannot be obtained by drilling and sampling alone. The geophysical-logging systems that will be used in this study provide continuous-digital records that are dependent upon the physical properties of the sediment, the rock matrix, and the interstitial fluids. At each of the drilled boreholes, natural-gamma radiation (gamma), spontaneous potential (SP), single-point-resistance (SPR), and short-and long-normal resistivity (R) logs will be collected in mud-filled open boreholes before the casing is installed, then focused electromagnetic-induction (EM) logs will be obtained after installation of the polyvinyl chloride (PVC) casing. Key existing observation wells will also be logged for EM to determine if saltwater intrusion has changed over time at that location.

The types of geophysical logs being used for this study, and their descriptions and uses are shown below:

Natural-gamma radiation (gamma) logs —provide a record of the total gamma radiation detected in a borehole (Keys, 1990). Clays and fine-grained sediments tend to be more radioactive than the quartz sand that forms the bulk of the deposits on Long Island. Gamma logs commonly are used for lithologic and stratigraphic correlation.

Spontaneous-potential (SP) logs —provide a record of the electrical potential, or voltage, which develops at the contact between clay beds and sand aquifers within a borehole (Keys, 1990). SP logs are used to determine lithology, bed thickness, and salinity of formation water (Keys, 1990).

Single-point-resistance (SPR) logs —provide a measure of the resistance, in ohms, between an electrode in the borehole and an electrode at land surface.  SPR logs are used to obtain high-resolution lithologic information.

Normal-resistivity (R) logs—measure apparent resistivity in ohm-meters using two electrodes typically spaced 16 to 64 in. apart in the borehole, called short and long normal logs, respectively. R logs are used to interpret lithology and water salinity (Keys, 1990).

Focused electromagnetic-induction (EM) logs —measure formation conductivity using an electromagnetic emitter coil that induces current loops within the surrounding formation to generate a secondary electromagnetic field. The intensity of the secondary field received by the receiver coil is proportional to the formation conductivity (Keys, 1990; Serra, 1984; Keys and MacCary, 1971). EM logs are used in conjunction with gamma logs to distinguish between conductive fluids and conductive clays, and have been used on Long Island to delineate the freshwater-saltwater interface (Chu and Stumm, 1995; Stumm, 1993, 1994, 2001; Stumm and Lange, 1996; Stumm and others, 2002, 2004).

All borehole geophysical logs collected by the USGS are now available online. The USGS Geolog Locator can be used to obtain information on the logs that have been archived for Long Island.

Surface Geophysics (H/V Passive Seismic)

The H/V seismic method is considered a "passive" seismic method because it does not require an explosive charge or hammer blow to induce seismic noise (fig. 3). The H/V method measures three components of ambient seismic noise, which include micro tremors induced by wind, ocean waves, and anthropogenic (human) activity. The measured data are the vertical and two horizontal (north-south and east-west) components of the seismic noise.

The spectral ratio of the horizontal and vertical components of the measured ambient seismic noise is used to determine the fundamental seismic resonance at the site. Using this method depth to bedrock measurements will be collected throughout Long Island to fill in gaps between drilled boreholes.

gray cylindrical instrument on gray table on grass

Figure 3 - Three component seismometer The image above shows a three component seismometer used by the USGS for the collection of H/V data (Public domain).

Surface Geophysics (TDEM)

The time domain electromagnetic (TDEM) method is a surface-geophysical technique that uses a transmitter loop and receiver coil placed on the land surface, from which an electrical current is passed through the transmitter loop creating an electromagnetic field.  This field is switched on and off, which induces eddy currents in the subsurface producing a secondary electromagnetic field. The decay of this secondary electromagnetic field is then measured by the receiver coil.  The amplitude of the current starts to decay immediately, which induces more current to flow, but now at a greater depth from the transmitter loop.  The deeper current flow also decays, which then induces even deeper currents to flow.  Data are collected at the receiver coil, with progressively later times representing the electrical properties of the deeper layers in the subsurface.  These data are then processed and inverted to produce a one dimensional resistivity curve.