Comprehensive Delineation of Groundwater Source Areas and Times-of-travel to Long Island Streams and Estuaries

Science Center Objects

Problem The discharge of freshwater and associated loading of nutrients and other dissolved constituents from the Long Island aquifer system to surrounding estuaries and their tributaries are increasingly recognized as critical factors in the health of these ecosystems. However, further work is needed to scientifically characterize these factors and present them to the public in an appropriat...

Problem
The discharge of freshwater and associated loading of nutrients and other dissolved constituents from the Long Island aquifer system to surrounding estuaries and their tributaries are increasingly recognized as critical factors in the health of these ecosystems. However, further work is needed to scientifically characterize these factors and present them to the public in an appropriate manner. Many organizations have undertaken assessments of this discharge and loading for discrete groundwater source areas and (or) receiving surface waters, applying a variety of techniques and assumptions. In part, this is because there is no delineation of recharge areas to the island’s groundwater-fed streams and estuaries that is comprehensive, finely scaled, and accurate. In addition, the full results of previous county-level assessments of these ground-water recharge areas—the delineation methods, non-proprietary versions of a previously developed regional model and its input and output, and a graphical user interface to set up and run the model —are not widely available to other interested parties and the general public. A comprehensive and fully documented set of publicly accessible delineations of the recharge areas and resulting traveltimes for groundwater discharge to Long Island streams and estuarine embayments is essential for developing informed strategies to address pollutant loading to these systems, to provide the basis for further scientific study, and to engage the public.
 
Background
The U.S. Geological Survey (USGS) has been instrumental in the development of virtually all regional and County-wide groundwater-flow models on Long Island and in many other parts of the U.S. This has been facilitated through the local development and sharing of numerous hydrogeologic-framework and hydrologic-conditions datasets and model archives. This has also been enabled through the national development of the mathematical and computer methods of model simulations. Recently, the USGS developed an updated groundwater-flow model for Long Island as part of its groundwater availability study of the North Atlantic Coastal Plain (NACP)  aquifer system (http://ny.water.usgs.gov/projects/NACP/). Although the study is regional in  scope, it compiled detailed information on the processes that govern how water recharges, flows  through, and discharges from the aquifer system of Long Island and other parts of the NACP  (Masterson and others, 2013). In 2015, work funded by the USGS National Water Quality  Assessment (NAWQA) initiative will begin to collect Long Island water quality data in direct  support of flow modeling. NAWQA will use this data to create a Long Island regional model  focused on better understanding of water quality variations. This work will provide a rigorous  foundation for the delineation of groundwater recharge areas proposed here. This proposed
project would be conducted in cooperation with NAWQA/NACP researchers who have  previously delineated over 500 separate recharge areas of Cape Cod areas, Mass. (Walter and  others, 2004; http://pubs.usgs.gov/sim/2004/2857/). Using similar methods to those applied on  Cape Cod, the Long Island results would potentially yield several thousand separate  groundwater recharge area delineations, and may be used to generate seamless and(or) scalable  map products that could hang on many walls.
 
Objective
The primary objectives of the proposed investigation are to (1) delineate present-condition  ground-water recharge areas and traveltimes to Long Island streams and estuarine embayments,  and (2) produce geospatial layers and metadata that describe these delineations for public  dissemination via the Internet.
 
Approach
The approach is divided into three basic categories: flow model development, generation of  recharge areas and traveltimes, and report production.
Flow model development
The starting points of the approach are NACP/NAWQA groundwater-flow model archives and  contact with NACP/NAWQA researchers. The NACP project is described in Masterson and  others (2013), with a modeling report and archive anticipated in mid-2015. This modeling uses  the USGS MODFLOW suite of open source codes (Harbaugh, 2005), including modules for  freshwater-saltwater interface representation (Langevin and others, 2007) and particle-tracking  analysis (Pollock, 1994). Using NAWQA funds, the NACP model will be reworked as a new  Long Island regional model with emphasis on improving accuracy in traveltime simulation.  NAWQA-funded work includes model grid and boundary condition refinements, and water  quality data collection for use in model calibration. In the first step of the flow model  development phase of this proposed project, model datasets will be obtained from  NACP/NAWQA researchers. The data will be checked against published results of the NACP
project, and ported into the USGS open-source graphical user interface ModelMuse  (http://water.usgs.gov/nrp/gwsoftware/ModelMuse/ModelMuse.html).
 
Step two is to conduct model modifications to produce a working steady-state model representing Long Island-wide present-conditions, for use in delineating groundwater recharge areas and traveltimes. The following subtasks are included: (a) refining model grid discretization; (b) incorporating local-scale features not represented in the NACP/NAWQA model; (c) compiling local-scale model calibration data; (d) evaluating sensitivity of parameters that may affect recharge area delineation such as hydraulic conductivity and porosity; and (e) evaluating boundary conditions that may affect recharge area delineation such as recharge rate, surface water elevations and discharge rates, and production wells pumping rates. Local-scale data are summarized and may be interactively viewed at the USGS State of the Aquifer (SOTA) website .
 
Recharge areas and traveltimes
Step three is to conduct particle-tracking simulations. To delineate ground-water recharge areas (groundwatersheds), receiving surface-water bodies must be identified to provide particle ending locations. The identification process will be done in consultation with the project cooperator(s), and involves identifying the name and extent of each surface-water feature. Particles are tracked from points of entry into the groundwater system to surface-water discharge locations. The distribution of points of entry for particles that ultimately exit at identified surface waters delineates the respective ground-water recharge areas for these receiving waters. In addition to ground-water recharge area delineation, subareas with similar groundwater traveltimes, along with groundwater discharge rates, will be reported. These results will include geographic information system (GIS) polygons of ground-water recharge areas, their subareas with similar groundwater traveltimes and groundwater-discharge rates. Recent developments in USGS modeling techniques (summarized at http://water.usgs.gov/software/lists/groundwater) and NACP/NAWQA results will be reviewed at this time.
Step four is to present and discuss final results with cooperator(s) and implement a strategy for reporting and dissemination. A technically acceptable first draft of a USGS Scientific Investigations Report will be available at this time for cooperator review. Provisional GIS polygons of ground-water recharge areas, their subareas with similar groundwater traveltimes and groundwater-discharge rates also will be shared with the project cooperator(s).
 
Report production
Steps five and six are to produce the final report documenting the changes to the NACP/NAWQA model and an archive for the island-wide model that was employed in the ground-water recharge area analysis. Geospatial (GIS) layers and metadata that describe the delineations of ground-water recharge areas and traveltimes will also be produced and used to map a seamless recharge mosaic. The GIS files will include polygons of ground-water recharge areas, their subareas with similar groundwater traveltimes and groundwater-discharge rates.
These GIS files will be disseminated publicly via the Internet from the website for the USGS in  New York. (See the GIS metadata format for Cape Cod given at  http://water.usgs.gov/GIS/metadata/usgswrd/XML/ds451_gwcontrib_areas.xml for example.)
 
Products
Results of the investigation will be disseminated through a final online report in the USGS  Scientific Investigations Report series, a link to an online file archive where model data may be  publicly accessed, and an online repository of geospatial layers and metadata. All numerical  models used will be those developed by USGS and available publicly at http://water.usgs.gov/software/lists/groundwater. The study products will document application  of a groundwater-flow model to comprehensively delineate the ground-water recharge areas
associated with estuaries and streams of Long Island. The products will include:
• the flow model grid, including comparison of NACP/NAWQA grids and Long Island  study area grid,
• maps and sections presenting the three-dimensional distribution of the hydrogeologic  units,
• the present steady-state distribution of groundwater recharge,
• simulated potentiometric-surface and water-table maps of principal aquifers,
• statistics related to flow model calibration, sensitivity analysis, and model uncertainties,  and
• recharge area delineations and associated traveltimes.
 
References Cited
Harbaugh, A.W., 2005, MODFLOW-2005, the U.S. Geological Survey modular ground-water model -- the Ground-Water Flow Process: U.S. Geological Survey Techniques and Methods 6-A16.
 
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.
 
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.
 
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.
 
Walter, D.A., Masterson, J.P., and Hess, K.M., 2004, Ground-Water Recharge Areas and Traveltimes to Pumped Wells, Ponds, Streams, and Coastal Water Bodies, Cape Cod, Massachusetts, Scientific Investigations Map I-2857, 1 sheet.

Project
Location by County

Kings County, NY, Queens County, NY, Westchester County, NY, Nassau County, NY, Bronx County, NY, Suffolk
County, NY, New York (Manhattan) County (FIPS 36061), NY, Richmond County, NY