Development and Calibration of a 100-Year, Transient, Saltwater Intrusion Model for Long Island, New York

    Presented at the NGWA Groundwater Summit April 2008, Memphis Tennessee

Matthew Gamache CDM Smith, Inc., 50 Hampshire Street, Cambridge, MA 02139

Robert Fitzgerald CDM Smith, Inc., 50 Hampshire Street, Cambridge, MA 02139

Kristina Kirkyla Masterson CDM Smith, Inc., 50 Hampshire Street, Cambridge, MA 02139


Historically, intensive pumping in Brooklyn and Queens caused significant drawdown of groundwater head levels and damaging intrusion of salt water into the groundwater system. From 1900 to 1950 extensive groundwater development occurred in Brooklyn and Queens, resulting in large groundwater drawdowns. Saltwater encroachment at water supply wells in Brooklyn is documented as early as the 1920s. In 1947, all public water supply pumping in Kings county stopped because of widespread saltwater intrusion. Pumping in Queens then increased from approximately 52 MGD in 1950 to approximately 70 MGD in the early 1970s. Nassau and Suffolk county withdrawals increased significantly over this period as well. Groundwater elevations in Queens and Nassau counties decreased, and significant saltwater impacts at public supply wells in Queens occurred in the 1960s. Queens pumping has since been reduced to less than 5 MGD, which has resulted in water level recovery of up to 35 feet during the past 25 years.

A groundwater flow model of the aquifer system beneath Kings, Queens, Nassau and Suffolk counties on Long Island, New York was developed to study groundwater flow patterns and the availability of groundwater for future public water supply. In order to evaluate the feasibility of potential future water supply projects, a 100-year (1905-2005) historical transient simulation was developed to demonstrate that the model can reasonably represent the regional salt water intrusion and groundwater head drawdowns that occurred during the 20th century.

DYNSWIM, a 3-dimensional, finite-element, dual-phase freshwater and saltwater flow code was used in this study. DYNSWIM is part of the DYNSYSTEM suite of groundwater modeling software and was developed for use in coastal aquifer studies. The model adequately simulates the movement of the saltwater wedge, as well as water level fluctuations associated with changes in pumping and groundwater recharge, over time.