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 2005-15

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 Islands 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. 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).