Citation:

Cashel, Finnian S. AND Christopher D. Knightes. Using a Multimedia Modeling Approach to Simulate Eutrophication in the Pawcatuck River Estuary. In Proceedings, 11th International Congress on Environmental Modelling and Software, Brussels, BELGIUM, July 05 - 08, 2022. Brigham Young University, Provo, UT, 1-9, (2022).

Impact/Purpose:

The release of nutrients into coastal waterbodies causes decreased oxygen concentrations. Dissolved oxygen is depleted as organic matter released from waste water treatment plants breaks down. Additionally, the release of nutrients into rivers and estuaries results in growht of algae, which further decreases dissolved oxygen concentrations. Low dissolved oxygen concentreations can create dead zones where fish and wildlife cannot survive. The formation of algae, in addition to being unpleasing visually and causing odors shades the sediments of the river, which impacts the growth of sea grass. In this study, we used observed data as well as mechanistic modeling to improve our understanding of these processes within a small, shallow coastal environment, the lower Pawcatuck River and Little Narragansett Bay, which forms the border of Rhode Island and Connecticut. We developed a model system incorporating a series of different modules available within the Water Quality Analysis Simulation Program, a publicly available water quality modeling framework developed by the US EPA. Our study investigates the processes resulting in decreased dissolved oxygen concentrations and demonstrate the feasibility of using a one-dimensional model for this small, shallow estuary, identifying its strengths and weakness and challenges in modeling a complex, small, shallow estuary, which may be applied to other estuaries of similar structure.

Description:

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, and 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 combination of the mechanistic modeling and observed data analysis demonstrated the presence of a strong salinity wedge, which stratified the river system. The model captured decreases in dissolved oxygen (DO) and increases in phytoplankton (chlorophyll a) moving upstream. Due to the constraints of the one-dimensional model, the observed, vertical DO gradient could not be captured, yet the model still provided insight in understanding the system. Our findings emphasize the importance of the salt wedge and the 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 three-dimensions using the Environmental Fluid Dynamics Code (EFDC), as well as simulating the impact of climate change, land-use change, and ocean acidification.

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