MODFLOW-NWT and MODPATH6 Models Used to Simulate Groundwater Flow in the Regional Aquifer System of Long Island, New York, for Pumping and Recharge Conditions in 2005-2015 (ver. 2.0, December 2021)
Description
In 2016, the United States Geological Survey (USGS) began development of a regional-scale numerical
model of the Long Island aquifer system, as part of the National Water Quality Assessment (NAWQA)
Program. The three-dimensional groundwater-flow model was developed to evaluate 1) responses of the
hydrologic system to changes in natural and anthropogenic hydraulic stresses 2) the subsurface
distribution of groundwater age, and 3) the regional-scale distribution of groundwater travel times and
the source of water to fresh surface waters and coastal receiving waters. The model also provides the
groundwater flow components used to define model boundaries for possible inset models used for
local-scale analyses. Unconsolidated sediments underlying the Island comprise a sole source aquifer
that supplies water to about 2.9 million people in Nassau and Suffolk Counties; the aquifer also
contributes groundwater discharge to freshwater and marine ecosystems. Anthropogenic activities
have affected both the quantity and quality of groundwater, owing to the Island's large population and
the generally unconfined conditions prevalent across the aquifer system. Groundwater withdrawals,
particularly in the western part of the Island, have resulted in large declines in water-table altitude and
in the landward movement of the freshwater/saltwater interface encroaching on local water supplies.
Subsurface contamination emanating from numerous point sources, often associated with industrial
sites in developed areas in western Long Island, adversely affect downgradient water supplies. In
central and eastern Long Island, nutrients emanating from non-point sources associated with
residential development and agricultural activities have degraded water quality in shallow parts of
the aquifer system. The model uses the numerical code MODFLOW-NWT to represent steady-state
conditions for predevelopment and 2005-2015 average groundwater pumping and aquifer recharge.
The particle-tracking algorithm MODPATH was used to simulate advective transport in the aquifer,
to delineate the areas at the water table that contribute recharge to coastal and freshwater bodies,
and to estimate total travel times of water from the water table to discharge locations. This USGS
data release contains all of the input and output files for the simulations described in the associated
model documentation report (https://doi.org/10.3133/sir20205091). UPDATE: In July 2021, the
MODFLOW-NWT output for the steady-state 2005-2015 model were used with the particle-tracking
algorithm MODPATH6 to estimate the recharge areas to 1,662 simulated public-supply wells in the
aquifer system underlying Long Island, NY. An array of particles with a uniform spacing of 250 feet
were specified at the water table and tracked forward to model cells containing simulated wells. The
starting locations of the particles terminating in the simulated well represents the recharge area to
that well. The particle starting locations were then georeferenced and used to create a polygon
shapefile of individual recharge areas. This new information has been added to the ancillary directory
of this data releases - December 2021.