Defining Groundwater Flow and Transport to Help Reduce Nitrogen Discharge to Cape Cod’s Coastal Waters

Actions for addressing the ecological impacts on coastal waters of groundwater nitrogen should be based on sound scientific understanding of the groundwater-flow system. Field hydrologic studies and computer modeling during the past four decades have increased the understanding of groundwater contributing areas to coastal waters and groundwater travel times in the subsurface. Questions remain, however, about the processes that affect nitrogen along groundwater flow paths and about discharge locations at the coast. Several studies by the U.S. Geological Survey are addressing these questions. Simulations of transient flow and solute transport demonstrate that groundwater watersheds and associated nitrogen loads to coastal water bodies change over time in response to changes in hydraulic stresses and temporal and spatial patterns of land disposal of wastewater. At the coast, freshwater discharge can occur more than 30 meters from shore and include substantial concentrations of nitrate and ammonium nitrogen, based on recent field sampling at a coastal saltwater embayment. The nitrogen loads may be affected by substantial nitrogen losses along the groundwater flow path, as shown by a detailed three-dimensional examination of a treated-wastewater plume from the Massachusetts Military Reservation. The location of discharge is affected by the position of the freshwater-saltwater interface, which is sensitive to the geometry of the coastline and the hydraulic properties of the bottom sediments of the coastal water bodies. These findings can help inform decisions concerning actions to reduce the impacts of nitrogen loads to Cape Cod’s coastal waters.