Delineation of the Hydrogeologic Framework and Saltwater-Freshwater Interface and Determination of Water-Supply Sustainability of Long Island, New York

Science Center Objects

Problem Long Island’s sole-source aquifer system, which includes the Lloyd, Magothy, Jameco, and upper glacial aquifers, supplies groundwater to over 2.8 million people. As a coastal aquifer system, it is susceptible to saltwater intrusion. Past pumpage and sewering (fig. 1) resulted in increased salinity in most aquifers in all counties (Buxton and Shernoff, 1999; Misut and others, 2004; Mis...

Problem
 
Long Island’s sole-source aquifer system, which includes the Lloyd, Magothy, Jameco, and upper glacial aquifers, supplies groundwater to over 2.8 million people. As a coastal aquifer system, it is susceptible to saltwater intrusion. Past pumpage and sewering (fig. 1) resulted in increased salinity in most aquifers in all counties (Buxton and Shernoff, 1999; Misut and others, 2004; Misut and Aphale, 2014). Simulation of drought has predicted increasing salinity in the lower part of the glacial aquifer of the North Fork of Suffolk County (Misut and others, 2004). In addition, simulation of future well pumpage in Queens County by the U.S. Geological Survey (USGS) has predicted increasing salinity in the Magothy aquifer in western Nassau County (Misut and Voss, 2007), and recent analysis of borehole geophysical logs indicates that saltwater intrusion of the Lloyd aquifer has occurred along the southern shores of Kings and Queens Counties and along the northern parts of Nassau County due to public-supply pumpage (Stumm, 1993; 2001; Stumm and Lange, 2002; Stumm and others 2004). Based on analysis of historic supply and industrial wells in Kings County, the western end of the Lloyd aquifer may also be intruded by saltwater.
 
In Suffolk County, saltwater was delineated in the upper glacial aquifer as freshwater lenses on the North Fork (Schubert and others, 2004). On the South Fork, freshwater was delineated in the upper glacial and Magothy aquifers (Nemickas and Kozalka, 1982). Significant pumping stresses during the summer in the Montauk area has caused public-supply wells to become subject to saltwater intrusion (Nemickas and Kozalka, 1982). Analysis of geophysical logs at a Lloyd aquifer test well (Q-3655) at John F. Kennedy International Airport from 1989 indicates that about 80 percent of the Lloyd aquifer was intruded with saltwater (fig. 2). Logs from recent geothermal- and industrial-well drilling in Queens County indicates significant differences in the hydrogeologic framework than that represented in existing groundwater-flow models. This has prompted questions about the accuracy of previously simulated groundwater pathlines, source-area delineations, and the location of the freshwater-saltwater interface and its landward movement on Long Island.
 
Currently, there is no USGS monitoring of groundwater levels, and no network of deep outpost wells to monitor saltwater intrusion in Kings and Queens Counties. It has been about 30 years since the position of the saltwater-freshwater interface in the Magothy aquifer was delineated using water-quality sampling in southeastern Nassau and southern Queens Counties (Terraciano, 1997). . Knowledge of groundwater-flow paths, source areas, and the movement of the freshwater-saltwater interface is essential for making informed planning decisions about public-supply strategies for western Long Island, including rebalancing of pumping, improvement in the operation of the recharge basin network, and plans for future contingencies related to climate change and New York City water-supply disruption.
 
Background
 
Water-supply wells on Long Island are affected by fate and transport of naturally occurring saltwater in surrounding coastal surface waters, and by human-derived contaminants entering the groundwater system from the land surface. An increased understanding of the fate and transport of contaminants and factors that affect them is useful for future planning purposes and to optimize water-supply operations. Groundwater models provide the framework in which to represent our knowledge of groundwater-flow systems, and they provide insights water-resources managers need to plan effectively plan for sustainable aquifer development ( Provost and others, 2009).
 
The USGS has worked in cooperation with the New York City Department of Environmental Protection to simulate the impacts of proposed alternative pumpage scenarios involving Kings and Queens County water-supply wells (Buxton and others, 1999; Buxton and .Smolensky, 1999; Kontis, 1999; Misut and Monti, 1999; Misut and Voss, 2007). These studies utilized coarsely discretized, steady-state groundwater-flow models with the freshwater-saltwater interface initially represented as fixed boundaries, and later represented as movable boundaries to predict changes in salinity at well-observation points. Due to a lack of data concerning offshore and deep-onshore parts of the groundwater system, however, there was and remains considerable uncertainty in of the simulation of proposed alternative water-supply scenarios. New information on the hydrogeologic framework, and hydrologic stresses,, will be incorporated into the proposed model. Effects present and projected future pumpage and climate change on the groundwater system will be simulated.
 
In 2015 the USGS National Water-Quality Assessment Program (NAWQA) began a regional groundwater-flow modeling project on Long Island. Objectives of the ongoing study include reworking the hydrogeologic framework, and producing an accurate, three-dimensional representation of the age of groundwater throughout the entire flow system, to be compared to and calibrated with age-dependent water-quality data. In addition to the federally funded USGS modeling study, New York State Department of Environmental Conservation has funded a local USGS study to produce a comprehensive set of groundwatershed delineations for all streams and estuaries on Long Island, based on the NAWQA study. Datasets resulting from these modeling efforts will be incorporated into the proposed Long Island model.
 
There are three main reasons why it is important at this juncture to develop a groundwater-flow and saltwater-intrusion model for Long Island. First, the loss of public supply wells due to saltwater intrusion calls into question the sustainability of water use at current levels. . Second, new data has recently become available from analysis of historic records and long-term trends, or is otherwise proposed here, to provide the needed data to fill gaps in or scientific knowledge. Third, technological improvements in groundwater-flow simulation methods have been made that will allow for more detailed and accurate model results. The following advancements will be included in the proposed model:
 
1. Increased computing capacity allows for finer model grids and time step sizes. Groundwater-flow model accuracy is related to discretization, and these technological improvements are an opportunity to improve previous groundwater models and our understanding of the cumulative effects of projected future pumpage on the region’s water resources. In the most recent USGS study of Kings and Queens Counties, model calibration was conducted on the basis of a present conditions steady state, with limited transient-state modeling of future scenarios. At present, it is technically feasible to conduct more refined model calibration on the basis of transient-state, historical conditions of observed water-level changes and salinity changes. During the course of the anticipated NAWQA modeling study, some of this calibration will be completed with emphasis on determining the age of groundwater; the remainder is proposed here with emphasis on accurately locating saltwater interfaces and predicting their future movement.
 
2. Improved USGS model codes allow for more accurate and robust mathematical solutions and representation of physical mechanisms. In the most recent USGS study of Kings and Queens Counties, the SUTRA code was applied. At present, the SEAWAT code is available. SUTRA and SEAWAT were recently compared in a USGS saltwater intrusion study of Manhasset Neck (Misut and Aphale, 2014). Groundwater-flow model accuracy is related to model coding, and the technological improvement of a new model code is an opportunity to further our understanding of the cumulative effects of projected future pumpage on the region’s water resources.
 
3. At present, an open source USGS graphical user interface is available for USGS model codes including SEAWAT and MODFLOW. This is an improvement over the proprietary user interfaces used in previous studies of Kings and Queens Counties because it allows for a broader base of the public to use these models. Furthermore, it increases the ease of use of models and facilitates model improvements, through greater capabilities for automated model construction. This technological improvement is an opportunity to further our understanding of the cumulative effects of projected future pumpage on the region’s water resources.
 
4. At present, the USGS has completed a project to integrate inverse modeling techniques into its open source graphical user interface, which facilitates model sensitivity analysis and parameter estimation. Groundwater-flow model accuracy is related to the completeness of model calibration exercises, and this technological improvement is an opportunity to improve previous groundwater models and our understanding of the effects of projected future pumpage on the region’s water resources.
 
Approach
 
Deep Lloyd and Magothy aquifer wells will be drilled at about twenty-four selected locations in the study area where no wells currently exist. The USGS will use these new wells, the current observation-well network in Nassau and Suffolk Counties, and other data sources to delineate the saltwater interface, map the hydrogeologic framework, and model the groundwater-flow system. The pre-existing network consists of shallow and deep wells in Nassau and Suffolk Counties, and some shallow wells in Kings and Queens Counties.
 
During and after completion of the newly drilled wells, the following work will be performed:
• collect split-spoon core samples and analyze core samples for hydrogeologic-unit mapping,
 
• extract and analyze filter-press samples for chloride concentration,
 
• collect geophysical logs including gamma, normal electrical resistivity, and electromagnetic (EM) induction conductivity and magnetic susceptibility logs from the network of wells to map the hydrogeologic framework, determine the thickness and estimate the chloride concentration of the saltwater interface, and map the location of the freshwater-saltwater interface in the aquifer system,
 
• sample the well network to determine the chloride concentration, and integrate these data with the geophysical data to delineate the location of the freshwater-saltwater interface, and
 
• monitor groundwater levels using periodic and continuous measurements, and re-log the outpost wells to collect EM conductivity logs periodically during the study and at peak pumping demand to determine the presence and rate of saltwater intrusion in the Magothy and Lloyd aquifers. Geophysical monitoring will also include currently existing observation wells in Inwood, Long Beach, Great Neck, Manhasset Neck and surrounding areas in Nassau County. In some areas, where observation wells are limited surface geophysical surveys such as time-domain electromagnetic and magnetotellurics can be used to fill in data gaps in some eastern Long Island areas.
 
Chloride breakthrough curves, locations of Magothy and Lloyd aquifer cones of depression, and geophysical logs from historic wells will be used to determine the locations of outpost well drilling. A preliminary model will be constructed by refining all the available USGS models including (Masterson and others, 2013; Misut and Aphale, 2014; Misut, 2014; Misut and Voss, 2007; and Misut and Monti, 1999). Field techniques will include water-level measurements, chloride-concentration sampling, core-sample analysis, and periodic (biannual) borehole geophysical logging using EM methods as described in Stumm (1993; 2001) and Stumm and others (2002).
 
The USGS will use the density-dependent solute-transport model SEAWAT (Langevin and others, 2007) to represent saltwater-interface dynamics, and the particle tracker MODPATH (Pollock, 1994) to delineate source areas. The finalized model will utilize an updated hydrogeologic framework and new delineations of the freshwater-saltwater-interface in the Magothy and Lloyd aquifers. The SEAWAT model will be calibrated to match field-measured groundwater levels and the newly delineated freshwater-saltwater interface locations, by varying parameter values, boundary conditions, and model discretization. The following scenarios will be simulated:
 
1. No change from present conditions and continuation of current trends in public-supply pumping and water use.
 
2. Change in groundwater recharge due to climate-change-related increases in air temperature, evapotranspiration, and precipitation intensity. Some of these factors may decrease groundwater recharge, leading to a decrease in groundwater availability, while others factors may increase groundwater recharge and availability, and perhaps cause flooding problems. The effect of the sum total of these factors is complex and unknown, but may be approached through computer simulation. These scenarios include simulation of a future reoccurrence of the drought characteristic of 1962-66.
 
3. Changes in pumpage to meet New York City water-supply demand including(a) hort-term and as-permitted (long term) reactivation of Queens County supply wells, (b) increased pumping of currently active Nassau County supply wells that have excess capacity and (or) reactivation of unused but permitted wells that may have high water-treatment burdensand (c) construction of upper glacial aquifer supply wells that may serve additional purposes such a groundwater-flood control or geothermal cooling. 
 
4. Agricultural pumpage and drought on the North and South Forks on eastern Long Island.
 
5. Sea level rise, saltwater intrusion, and depth to groundwater decrease. The middle and upper ranges of sea-level rise accepted by New York State will be simulated.
 
6. Diversion of water to stream headwaters through shallow-well pumpage, such as proposed for Massapequa Creek.
 
7. Redistributed pumping to mitigate severe local problems would be considered, and include identification of sites within Nassau County exhibiting potential for future supply-well development. 
 
8. Water conservation of decreased per capita water use and limitations on non-potable water pumpage such as domestic and/or golf course irrigation caps would be considered.
 
9. Increased recharge through re-engineering of recharge basins to maximize groundwater-system benefit would be considered.
 
A model maintenance website will be developed to serve the file archives of all USGS groundwater flow models of Long Island that are either used or developed during the course of this proposed study, including the NAWQA and groundwatershed model. The website will facilitate the use of models by water-resource planners and managers, and will be patterned after the USGS New Jersey model maintenance program ( http://nj.usgs.gov/special/mod_maint/archived_models.html). It will include maps of model domains and a table with links to the file archives and reports that describe the technical details of model development and application. The website will also be structured with placeholders for future activities related to updating models as new data becomes available after the completion of the proposed study.
 
Objectives
 
The primary objectives of this investigation are as follows:
 
• Due to a lack of deep Lloyd and Magothy aquifer observation wells in parts of the study area, a network of Lloyd and Magothy outpost wells will be installed. The approximate capital costs for this objective are shown in the attached budget, and will be administered separately through the Nassau County Department of Public Works (NCDPW) (for Kings, Queens, and Nassau County drilling) and the Suffolk County Water Authority (SCWA) for Suffolk County. Shallow to intermediate well drilling in Suffolk County may be augmented by utilizing newly acquired capabilities of the Suffolk County Department of Health Services (SCDHS). USGS cooperative drilling efforts in the recent past with the NCDPW, SCWA, and SCDHS have been very successful, and will provide the most cost effective and efficient means for installing these wells and collecting the required hydrogeologic, hydrologic, geophysical, and water quality data needed to properly delineate the hydrologic framework and saltwater-freshwater interface on Long Island.
 
• Complete offshore seismic-reflection survey surveys of northern Queens and Suffolk Counties. Offshore seismic-reflection surveys are a very rapid and cost effective tool to delineate major hydrogeologic features such as Pleistocene buried valleys and the Raritan Clay (Stumm, 1993; Stumm and Lange, 2002). Results of the seismic reflection surveys will be used to direct the drilling of new wells and delineate the extent of features.
 
• Produce a new hydrogeologic framework map for the major aquifers and confining units underlying Long Island. By integrating the seismic reflection survey, new borehole core samples, borehole geophysical logs, and pre-existing well drilling logs a higher resolution map that will build upon the last comprehensive hydrogeologic framework (Smolensky and others, 1990).
 
• Monitor groundwater levels, sample chloride concentrations, and collect an advanced suite of geophysical logs from the well network, which combines the current observation outpost well network with newly drilled wells as they become available.
 
• Determine the current location, thickness, and chloride concentration (used to assess salinity) of the freshwater-saltwater interface based on the new data and available historical information. Delineate a revised hydrogeologic framework in critical areas such as northern and central Long Island from new and available core samples and geophysical logs.
 
• Construct a groundwater-flow and interface model by combining pre-existing USGS models within the study area with new data as it becomes available. Preliminary simulations will be published as an extended abstract to help guide decisions about the location of new outpost-monitoring wells, interpretation of initial data, and management response to significant changes in stress such as reactivation of Queens County supply wells. 
 
• Construct detailed scenarios of pumpage and hydrologic conditions in Kings, Queens, Nassau, and Suffolk Counties.
 
• Finalize groundwater-flow models and evaluate freshwater-saltwater-interface movement, including delineation of groundwater-flow pathlines at supply-well capture zones susceptible to saltwater intrusion. The model grid shall be equally spaced and extend to all natural hydrologic boundaries of Long Island.
 
• Evaluate changes in groundwater pathlines as a result of scenarios at inland well sites, not susceptible to saltwater intrusion, but at risk of water-quality degradation from other sources of known contamination.
 
• Provide a model maintenance website (defined below) to facilitate the use of models by water resource planners and managers.
 
References
 
Buxton, H.T. and Shernoff, P.K., 1999, Ground-water resources of Kings and Queens Counties, Long Island, New York: US Geological Survey water-supply paper 2498, 113 p.
Buxton, H.T., and Smolensky, D.A., 1999, A quantitative analysis of the development of the ground-water flow system of Long Island, New York: U.S. Geological Survey Water-Resources Investigations Report 98-4069, 58 p.
Kontis, A., 1999, Simulation of Freshwater-Saltwater Interfaces in the Brooklyn-Queens Aquifer System, Long Island, New York: U.S. Geological Survey Water-Resources Investigations Report 98-4067, 26 p.
Langevin, C.D., Thorne, D.T., Jr., Dausman, A.M., Sukop, M.C., and Guo, W., 2007, SEAWAT version 4—A computer program for simulation of multi-species solute and heat transport: U.S. Geological Survey Techniques and Methods, book 6, chap. A22, 39 p.
Licata, A. 2014, Draft Scope of Work for Water for the Future: Upstate Water Supply Resiliency Project, NYC Environmental Protection Memorandum dated October 10, 2014, 6 p.
Masterson, J.P., Pope, J.P., Monti, Jack, Jr., Nardi, M.R., Finkelstein, J.S., and McCoy, K.J., 2013, Hydrogeology and hydrologic conditions of the Northern Atlantic Coastal Plain aquifer system from Long Island, New York, to North Carolina: U.S. Geological Survey Scientific Investigations Report 2013–5133, 76 p.
Misut, P.E., Schubert, C.E., Bova, R.G., and Colabufo, S.R., 2004, Simulated Effects of Pumping and Drought on Ground-Water Levels and the Freshwater-Saltwater Interface on the North Fork of Long Island, New York: U.S. Geological Survey Water-Resources Investigations Report 03-4184, 58 p.
Misut, P. E., Monti, Jack, Jr., 1999, Simulation of ground-water flow and pumpage in Kings and Queens Counties, Long Island, New York: U.S. Geological Survey Water-Resources Investigations Report 98-4071, v, 50 p.
Misut, P.E., and Voss, C.I., 2007, Freshwater-saltwater transition zone movement during aquifer storage and recovery cycles in Brooklyn and Queens, New York City, USA: Journal of Hydrology, v. 337, Issues 1-2, p. 87-103.
Misut, P.E., 2014, Simulation of zones of contribution to wells at site GM–38, Naval Weapons Industrial Reserve Plant, Bethpage, New York: U.S. Geological Survey Scientific Investigations Report 2014-5036, vii, 58 p.
Misut, P.E., and Aphale, O., 2014, Simulation of groundwater pathlines and freshwater/saltwater transition zone movement, Manhasset Neck, Nassau County, New York: U.S. Geological Survey Scientific Investigations Report 2013–5201, 71 p.
Provost, A.M., Reilly, T.E., Harbaugh, A.W., and Pollock, D.W., 2009, U.S. Geological Survey groundwater modeling software—Making sense of a complex natural resource: U.S. Geological Survey Fact Sheet 2009–3105, 4 p.
Nemickas, B, and Koszalka, E.J, 1982, Geohydrologic Appraisal of the Water Resources of the South Fork, Long Island, New York: U. S. Geological Survey Water Supply Paper 2073, 55p, 9 pl.
Pollock, D.W., 1994, User's Guide for MODPATH/MODPATH-PLOT, Version 3: A particle tracking post processing package for MODFLOW, the U.S. Geological Survey finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 94-464, 6 chapters.
Schubert, C.E., Hydrogeologic Framework of the North Fork and Surrounding Areas, Long island, New York: U. S. Geological Survey Water Resources Investigations Report 02-4284, 24 p., 4 pl.
Smolensky, D.A, Buxton, H.T., and Shernoff, P.K., 1990, Hydrologic Framework of Long island, New York: U.S. Geological Survey Hydrologic Atlas 709, 3 pl.
Stumm, F., 1993, Use of focused electromagnetic-induction borehole geophysics to delineate the saltwater-freshwater interface in Great Neck, Long island, New York, in Bell, R., Lepper, C., eds. Symposium on the Applications of Geophysics to Engineering and Environmental Problems, Vol. 2: Environmental and Engineering Society, proceedings, p. 513-525.
Stumm, F., 2001, Hydrogeology and extent of saltwater intrusion of the Great Neck peninsula, Great Neck, Long Island, New York: U.S. Geological Survey Water Resources Investigations Report, 99-4280, 41p.
Stumm, F., and Lange, A.D., 2002, Hydrogeology and extent of saltwater intrusion of the Manhasset Neck peninsula, Long Island, New York: U.S. Geological Survey Water Resources Investigations Report, 00-4193, 42 p.
Terracciano, S. A., 1997, Position of the freshwater/saltwater interface in southeastern Queens and southwestern Nassau counties, Long Island, New York, 1987-88: U.S. Geological Survey Open-File Report: 96-456, 17 p.

Project
Location by County

Suffolk
County, NY, Kings County, NY, Queens County, NY, Nassau County, NY