SIMULATING GROUNDWATER FLOW AND NITROGEN TRANSPORT IN COASTAL WATERSHEDS ON THE NORTH SHORE OF THE LONG ISLAND SOUND

Aquatic systems in and around Long Island Sound (LIS), an estuary that forms the southern coast of Connecticut, provide a variety of ecological and economic benefits. In many areas, aquatic ecosystems in LIS are degraded due to excess nitrogen (N) and little is known about groundwater transport of N in this area. Understanding the magnitude and spatial distribution of groundwater travel times and N transport are essential for optimizing conservation efforts aimed at reducing N inputs. To better characterize the groundwater system on the northern shore of LIS (modeled area of 9,300 km²), we developed a steady-state groundwater-flow model (MODFLOW-NWT) coupled with MODPATH particle tracking. Results indicate that (1) groundwater travel times are relatively short (model-wide median < 2 years), suggesting decade-long N legacies are not widespread, but that seasonal transport dynamics may be important; and (2) direct discharge of groundwater to coastal waters is a minor part of the overall groundwater budget with most groundwater discharging first to inland streams and rivers. Nitrogen in groundwater discharging to inland waters may be further attenuated by in-stream processes, reducing the nitrogen load to sensitive coastal waters. In the current (second) phase of the project, we are refining the groundwater model and using it to quantify N transport dynamics in 111 subwatersheds across the model domain and to assess the effects of potential N management actions such as installation of sewers or reduction in fertilizer use. The subwatershed-scale N transport and scenario analyses will inform conservation actions by state and federal resource managers and policy makers and form the basis for more detailed assessments in priority areas.