Groundwater models represent our knowledge of how groundwater-flow systems work, and they provide tools that water-resources managers can use to effectively plan for sustainable aquifer development. However, existing models lack the necessary geologic information to fully assess the sustainability concerns of the Long Island aquifer system. To improve the existing model the USGS will map new geologic information by drilling groundwater wells throughout the island. The extent of saltwater intrusion will be identified by monitoring these new wells. Mapping and monitoring results will be used to improve existing models.