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Scientific Investigations Report 2008–5166

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

Simulation of Variable-Density Ground-Water Flow and Saltwater Intrusion beneath Manhasset Neck, Nassau County, New York, 1905–2005

By Jack Monti, Jr., Paul E. Misut, and Ronald Busciolano

ABSTRACT

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The coastal-aquifer system of Manhasset Neck, Nassau County, New York, has been stressed by pumping, which has led to saltwater intrusion and the abandonment of one public-supply well in 1944. Measurements of chloride concentrations and water levels in 2004 from the deep, confined aquifers indicate active saltwater intrusion in response to public-supply pumping.

A numerical model capable of simulating three-dimensional variable-density ground-water flow and solute transport in heterogeneous, anisotropic aquifers was developed using the U.S. Geological Survey finite-element, variable-density, solute-transport simulator SUTRA, to investigate the extent of saltwater intrusion beneath Manhasset Neck. The model is composed of eight layers representing the hydrogeologic system beneath Manhasset Neck. Four modifications to the area’s previously described hydrogeologic framework were made in the model (1) the bedrock-surface altitude at well N12191 was corrected from a previously reported value, (2) part of the extent of the Raritan confining unit was shifted, (3) part of the extent of the North Shore confining unit was shifted, and (4) a clay layer in the upper glacial aquifer was added in the central and southern parts of the Manhasset Neck peninsula.

Ground-water flow and the location of the freshwater-saltwater interface were simulated for three conditions (time periods) (1) a steady-state (predevelopment) simulation of no pumping prior to about 1905, (2) a 40-year transient simulation based on 1939 pumpage representing the 1905–1944 period of gradual saltwater intrusion, and (3) a 60-year transient simulation based on 1995 pumpage representing the 1945-2005 period of stabilized withdrawals.

The 1939 pumpage rate (12.1 million gallons per day (Mgal/d)) applied to the 1905-1944 transient simulation caused modeled average water-level declines of 2 and 4 feet (ft) in the shallow and deep aquifer systems from predevelopment conditions, respectively, a net decrease of 5.2 Mgal/d in freshwater discharge to offshore areas and a net increase of 6.9 Mgal/d of freshwater entering the model from the eastern, western, and southern lateral boundaries. The 1995 pumpage rate (43.3 Mgal/d) applied to the 1945–2005 transient simulation caused modeled average water-level declines of 5 and 8 ft in the shallow and deep aquifer systems from predevelopment conditions, respectively, a net decrease of 13.2 Mgal/d in freshwater discharge to offshore areas and a net increase of 30.1 Mgal/d of freshwater entering the model from the eastern, western, and southern lateral boundaries. The simulated decrease in freshwater discharge to the offshore areas caused saltwater intrusion in two parts of the deep aquifer system under Manhasset Neck. Saline ground water simulated in a third part of the deep aquifer system under Manhasset Neck was due to the absence of the North Shore confining unit near Sands Point.

Simulated chloride concentrations greater than 250 milligrams per liter (mg/L) were used to represent the freshwater-saltwater interface, and the movement of this concentration was evaluated for transient simulations. The decrease in the 1905–1944 simulated freshwater discharge to the offshore areas caused the freshwater-saltwater interface in the deep aquifer system to advance landward more than 1,700 ft from its steady-state position in the vicinity of Baxter Estates Village, Long Island, New York. The decrease in the 1945-2005 simulated freshwater discharge to the offshore areas caused a different area of the freshwater-saltwater interface in the deep aquifer system to advance more than 600 ft from its steady-state position approximately 1 mile south of the Baxter Estates Village. However, the 1945–2005 transient simulation underestimates the concentration and extent of saltwater intrusion determined from water-quality samples collected from wells N12508 and N12793, where measured chloride concentrations increased from 625 and 18 mg/L in 1997 to 821 and 128 mg/L, respectively in 2004. The underestimation in the simulated concentration could be a combination of limitations inherent in the ground-water-flow model, such as the coarseness of the model mesh represented in this area.

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

http://ny.water.usgs.gov

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Suggested citation:

Monti, Jack, Jr., Misut, P.E., and Busciolano, Ronald, 2009, Simulation of variable-density ground-water flow and saltwater intrusion beneath Manhasset Neck, Nassau County, New York, 1905–2005: U.S. Geological Survey Scientific Investigations Report 2008–5166, 69 p.



Contents

Abstract

Introduction

Purpose and Scope

Previous Studies

Description of Study Area

Population and Land Use

Ground-Water Withdrawals

Hydrogeologic Framework

Hydraulic Properties

Coastal-Aquifer System

Hydrologic Boundaries

Freshwater-Saltwater Interface

Ground-Water Levels and Flows

Simulation of Variable-Density Ground-Water Flow beneath Manhasset Neck

Description of the Ground-Water-Flow Model

Model Grid

Boundary Conditions

Stresses (Withdrawals)

Predevelopment

1905–1944 Stress

1945–2005 Stress

Model Calibration

Saltwater Intrusion beneath Manhasset Neck, 1905–2005

Ground-Water Levels

Predevelopment Condition

1905-1944 Condition

1945-2005 Condition

Freshwater-Saltwater Interface

Predevelopment Condition

1905–1944 Condition

1945–2005 Condition

Ground-Water Budget

Sensitivity Analysis of the Model

Limitations of the Model

Hydrogeologic Controls on Position and Movement of the Freshwater-Saltwater Interface

Extent of Raritan Confining Unit

Extent of North Shore Confining Unit

Clay Layer within the Upper Glacial Aquifer

Summary and Conclusions

Acknowledgments

References Cited

Appendix 1. Sutra Model Layers



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