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Scientific Investigations Report 2013–5201

Prepared in cooperation with the
Town of North Hempstead and the
New York State Department of Environmental Conservation

Simulation of Groundwater Flow Pathlines and Freshwater/Saltwater Transition Zone Movement, Manhasset Neck, Nassau County, New York

By Paul Misut and Omkar Aphale

Thumbnail of and link to report PDF (7.78 MB)Abstract

A density-dependent groundwater flow and solute transport model of Manhasset Neck, Long Island, New York, was used to analyze (1) the effects of seasonal stress on the position of the freshwater/saltwater transition zone and (2) groundwater flowpaths. The following were used in the simulation: 182 transient stress periods, representing the historical record from 1920 to 2011, and 44 transient stress periods, representing future hypothetical conditions from 2011 to 2030. Simulated water-level and salinity (chloride concentration) values are compared with values from a previously developed two-stress-period (1905–1944 and 1945–2005) model. The 182-stress-period model produced salinity (chloride concentration) values that more accurately matched the observed salinity (chloride concentration) values in response to hydrologic stress than did the two-stress-period model, and salinity ranged from zero to about 3 parts per thousand (equivalent to zero to 1,660 milligrams per liter chloride). The 182-stress-period model produced improved calibration statistics of water-level measurements made throughout the study area than did the two-stress-period model, reducing the Lloyd aquifer root mean square error from 7.0 to 5.2 feet. Decreasing horizontal and vertical hydraulic conductivities (fixed anisotropy ratio) of the Lloyd and North Shore aquifers by 20 percent resulted in nearly doubling the simulated salinity(chloride concentration) increase at Port Washington observation well N12508. Groundwater flowpath analysis was completed for 24 production wells to delineate water source areas. The freshwater/saltwater transition zone moved toward and(or) away from wells during future hypothetical scenarios.

First posted March 5, 2014

Revised July 11, 2014

For additional information, contact:
Director, New York Water Science Center
U.S. Geological Survey
425 Jordan Road
Troy, NY 12180
(518) 285-5600
http://ny.water.usgs.gov

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Suggested citation:

Misut, Paul, and Aphale, Omkar, 2014, Simulation of groundwater flow pathlines and freshwater/saltwater transition zone movement, Manhasset Neck, Nassau County, New York (ver. 1.1, June 2014): U.S. Geological Survey Scientific Investigations Report 2013–5201, 44 p., http://dx.doi.org/10.3133/sir20135201.

ISSN 2328-0328 (online)



Contents

Abstract

Introduction

Groundwater-Flow System

Simulation of Groundwater Pathlines and Freshwater/Saltwater Transition Zone Movement

Summary and Conclusions

Selected References

Appendix 1. List of wells, Manhasset Neck Peninsula, New York

Appendix 2. Slice-sequence Animations of Hydrogeologic Units, Manhasset Neck Peninsula, New York


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