An iterative approach to developing and refining a regional groundwater model to provide actionable understanding of groundwater flow and nitrogen transport in coastal subbasins

Abstract:

Aquatic systems in and around Long Island Sound (LIS), an estuary that forms the southern coast of Connecticut, USA, 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 to optimizing conservation efforts aimed at reducing N inputs. To address the gap in actionable information about the groundwater system across the region, we cooperated with state resource managers to develop a steady-state groundwater flow model of the northern shore of LIS (modeled area of 9,300 km²). The model relies on recently developed scripting tools, water-use estimations, and an iterative refinement approach that reduced model development time. The initial model served as a framework for understanding the groundwater component of flow to the coastal watersheds of the LIS and for simulating N transport in one subbasin. Results from the initial model 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) groundwater discharge directly to coastal waters is substantial in some areas. In the current (second) phase of the project, we are iteratively refining the groundwater and N transport models based on the intial results and using them to quantify N transport dynamics in 109 subbasins 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 subbasin-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.