Using a Multimedia Modeling Approach to Simulate Eutrophication in the Pawcatuck River Estuary

The Pawcatuck River Estuary (PRE), composed of the Pawcatuck River and Little Narragansett Bay (CT/RI, USA), is a coastal plain estuary subject to environmental stress. Anthropogenic nutrient loading, watershed land use changes, and urbanization have contributed to eutrophication, which results in hypoxia and seagrass loss. To understand these processes, a one-dimensional, numerical, process-based model was developed to interpret and predict spatial variations in water quality, using WASP8 (Water Analysis Simulation Program, v 8.32). Our PRE WASP Model included five different sub-modules: Dynamic Wave, or DYNHYD5, was used for hydrodynamics, the heat module for water temperature, the advanced eutrophication module for water quality and biological components, and the sediment diagenesis module for sediment oxygen demand and benthic nutrient exchanges. A Hydrological Simulation Program - FORTRAN (HSPF) model, developed by RESPEC (2022), modeled the Wood-Pawcatuck watershed to provide upstream boundary inputs and watershed loads. The multimedia modeling approach created a WASP model which represented hydrodynamics and water quality. The combination of the modeling effort and analysis of observed data demonstrated the presence of a salinity wedge reaching further upstream than originally anticipated. The model captured decreases in dissolved oxygen (DO) and increases in phytoplankton (chlorophyll a) moving upstream. The simulation of macroalgae confirmed nutrient depletion and shading in Little Narragansett Bay. Due to the constraints of the one-dimensional model, the observed, vertical DO gradient was not captured. Our findings also emphasize the importance of the salt wedge and impact of sediment oxygen demand on hypoxia in benthic waters. These insights will be used to support management efforts and total maximum daily load (TMDL) development for ecosystem restoration. Further research will include exploring trade-offs of expanding into two- and three-dimensions using Environmental Fluid Dynamics Code (EFDC), as well as simulating the impact of climate change scenarios and ocean acidification.