Factors Affecting Movement of Freshwater-Saltwater Transition Zones and Future Water Withdrawals on Manhasset Neck, Long Island, New York

A three-dimensional finite-difference model of variable-density ground-water flow and salt transport was used to explore factors affecting movement of freshwater-saltwater transition zones on Manhasset Neck Peninsula, Long Island, New York. 180 stress periods represented the historical record from 1920 to 2007.  Simulations of historical salinity increase at two production wellfields and associated outpost monitoring sites are presented and subjected to sensitivity analysis of scalar parameter multipliers on recharge, hydraulic conductivity, and dispersivity zones. Other aspects of model construction affecting transition zone movement also discussed include: initial (1920) salinity distribution, seasonality of hydrologic stresses, numerical dispersion, and framework conceptualization. Three 46-stress period hypothetical future (2030) scenarios were considered: (1) continuation of present (2007) conditions, (2) rebalancing of public supply pumpage, and (3) extended drought conditions. Each of these scenarios present operational challenges for the water suppliers of Manhasset Neck.