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

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.

 

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 .

 

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).