Citation:

Cashel, F. AND Chris Knightes. Mechanistic Modeling Using the Water Quality Analysis Simulation Program (WASP8): Assessing Eutrophication in the Pawcatuck River Estuary. American Geophysical Union, NA, Virtual, December 13 - 17, 2021.

Impact/Purpose:

This work investigates the release of nutrients released from man-made sources, like waste water treatment plants, and their impacts on a coastal river and bay (Pawcatuck River on the border of Connecticut and Rhode Island and the Little Narragansett Bay). These waters have decreased dissolved oxygen concentrations and the presence of algae due to these nutrient loads. We've developed a mathematical model to predict the nitrogen, phosphorous, dissolved oxygen, and algae growth in this system. The goal of this work is to be able to take this model as an example in New England so that other similar system along the coast line can be better understood to improve water quality within the region.

Description:

Coastal plain estuaries are environments of substantial ecological importance, characterized for their productivity and the services they provide to both juvenile and adult marine fauna. Despite their importance, human development in watersheds has created a litany of downstream issues connected to the influx of excessive nutrients, eutrophication, and habitat degradation. A numerical and process-based model was used to accurately simulate the temporal and spatial variations of water quality parameters in a complex New England estuary (Pawcatuck River and Little Narragansett Bay, CT/RI). A 1-D WASP model was developed using the dynamic wave flow function to simulate hydrodynamics, advanced eutrophication for nutrient dynamics and algae, and sediment diagenesis for sediment oxygen demand (SOD) and benthic nutrient fluxes. The dynamic wave functionality uses 1-D momentum equations to solve for the continuation of a long wave through a shallow system, while accounting for variable upstream inflows and downstream tidal heads. This design was used to evaluate how effectively a 1-D model can characterize eutrophication dynamics to support management strategies in other New England estuaries. Initial results indicate that the model was able to simulate the dynamics of the system, particularly the factors governing eutrophication such as hydrodynamics, water temperature, and nutrient concentrations, as well as phytoplankton and macroalgal growth. This research highlighted the importance of the tidal influence on the salinity front, and the importance of SOD on dissolved oxygen concentrations [DO] in the estuary. Using a 1-D model, we were unable to capture the vertical or lateral [DO] gradients. Further research will expand the modeling effort into two- and three-dimensions using the Environmental Fluid Dynamics Code (EFDC) to simulate hydrodynamics to investigate the trade-offs on improved model simulation accuracy with increased model complexity and simulation run time. Additionally, we will investigate the implications of climate change scenarios, oyster aquaculture restoration, and other ecological processes that could impact the water quality of the Pawcatuck River estuary.

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