Hydrogeologic Framework and Extent of Saltwater Intrusion in Suffolk County, Long Island, New York
Results of Recent Exploration Drilling
Presented By:
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Frederick Stumm PG, Ph.D
United States Geological Survey
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Abstract: In 2016, the U.S. Geological Survey (USGS) began a multiyear cooperative study with the New York State Department of Environmental Conservation to evaluate the sustainability of Long Island’s sole-source aquifer system through hydrogeologic mapping, compilation of groundwater chloride concentrations, and groundwater flow modeling. In the initial phase of the island wide study, the hydrogeologic framework and extent of saltwater intrusion in aquifers in Kings, Queens, and Nassau Counties on western Long Island, NY were investigated.
Beginning in 2021, the eastern part of Long Island which includes Suffolk County, was studied using drill-core samples, chloride sampling, borehole geophysical logs, and surface geophysical methods. The aquifer system underlying eastern Long Island has been under stress from pumping of public, private, and irrigation (golf course and agriculture) wells. Saltwater intrusion has occurred from the surrounding embayments (Long Island Sound, the Great South Bay, the Peconic Bay, and the Atlantic Ocean) under both natural conditions and in isolated areas due to pumping.
Between 2021 and 2024, 29 boreholes were drilled and core samples collected to obtain hydrogeologic, geochemical, and geophysical data for delineating the complex subsurface hydrogeology and extent of saltwater intrusion in Suffolk County. Of these, 20 boreholes were drilled by USGS, and 9 were existing wells provided by cooperating agencies. Evaluation of the new cores, reexamination of legacy core descriptions, and analysis of new and legacy borehole geophysical logs were used to refine the previously published hydrogeologic framework of Pleistocene and Cretaceous unconsolidated sediments in the area, including delineation of a previously undefined hydrogeologic unit between the upper glacial aquifer or Gardiners confining unit and the Raritan confining unit, herein named the “Magothy confining unit.”
Deposits of the Magothy confining unit were originally correlated with the Monmouth greensand of New Jersey; however, new drilling and recent pollen analysis indicate that the unit is part of the Matawan-Magothy undifferentiated formation.
Nuclear magnetic resonance (NMR) logging was used to provide estimates of the hydraulic properties of the major aquifer and confining units. NMR logs record responses to the presence of hydrogen protons in the formation fluid to determine water fraction and pore-size distribution. NMR log analysis provided estimates of the clay-bound, capillary-bound, and mobile water fractions and hydraulic conductivity of aquifers and confining units penetrated by 9 wells in Suffolk County.
Pumpage for private, public-supply and irrigation wells in Suffolk County began in the early part of the 20th Century with small, localized private and irrigation wells, mostly from the upper glacial aquifer. Suffolk County’s population increased from about 200,000 in 1940 to 1.1 million in 1970, and 1.53 million in 2023. Correspondingly, groundwater withdrawals increased from 40 million gallons per day (mgd) in 1950 to 155 mgd in 1970, and 217 mgd in 2023, primarily from the upper glacial and Magothy aquifers. Evidence of saltwater intrusion in the 20th century impacting the upper glacial–Magothy–upper Raritan complex and the Lloyd aquifer was documented in the North and South Forks as early as the early 20th century. In some areas of eastern Suffolk County, saltwater intrusion has led to reduced pumpage or the shutdown of public-supply wells. Time-domain electromagnetic soundings were collected in the eastern parts of Suffolk County to delineate the depth of the saltwater interface. When combined with well chloride sampling, and borehole geophysical logs the TDEM the saltwater interface was delineated within Suffolk County.
Saltwater intrusion in the upper glacial-Magothy-upper Raritan complex and Lloyd aquifers were documented in the North and South Forks of Suffolk County in the early part of the 20th century. The saltwater interface in these areas was onshore and somewhat close to its present position during predevelopment. Most of the main body of Suffolk County has not been impacted by saltwater intrusion, except along the northernmost and southernmost coastal areas, and the North and South Forks. Movement of the saltwater interface has increased during the 20th century and continues to increase to the present (2025) in the Upper glacial-Magothy aquifer system in the North and South Forks in Suffolk County.
Please note: PDH Course Approval Pending
*Bio: Dr. Stumm is a Research Hydrologist with the U.S. Geological Survey New York Water Science Center in Coram, Long Island, NY. He holds a B.S. in Geology from Hofstra University, an M.S. in Geology from California State University Fresno, a Ph.D. in Earth and Environmental Science from the City University of New York Graduate Center and is a licensed Professional Geologist in the State of New York. He has published over 30 papers and reports on fractured-rock groundwater flow, the hydrogeologic framework of Long Island, and saltwater intrusion in Manhattan and Long Island, New York. His current research is applying Nuclear Magnetic Resonance logs, electromagnetic induction borehole logs, and time domain electromagnetic soundings to remap the hydrogeologic framework and extent of saltwater intrusion in the coastal plain aquifers throughout Long Island as part of the Long Island Sustainability project.
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