Estimating Nitrogen Loading from Groundwater and Potential Effects of Sea-Level Rise in Rhode Island's Salt Ponds Region
The U.S. Geological Survey (USGS), in cooperation with the Southeast New England Program of the U.S. Environmental Protection Agency, is estimating the magnitude, spatial distribution, and travel times of nitrogen-loading to ponds and streams in the Salt Ponds region of southern Rhode Island. This information can be used to guide conservation-management efforts aimed at reducing nitrogen inputs to the Salt Ponds region. Models developed to estimate nitrogen loads can also be used to evaluate potential flooding and groundwater level changes in the region by simulating rising sea levels.
Map showing the Salt Ponds region in southern Rhode Island.
Excess nitrogen is a significant concern in the Salt Ponds region of southern Rhode Island. The salt ponds, or coastal lagoons, in this area are primarily sustained from groundwater discharge, making them susceptible to dissolved nutrients in groundwater that can degrade the water quality of the ponds. Onsite wastewater treatment systems (OWTS), including septic systems, septic tanks, cesspools, and seepage pits, are a major contributor of excess nitrogen. Approximately 14,700 OWTS were in use in the Salt Ponds region between 2010 and 2015. (Narragansett Bay Estuary Program, 2022)
Potential sea-level rise and changing recharge patterns are also a concern for the Salt Ponds region. (Coastal Resources Management Council, 2015) Higher groundwater and sea levels could contribute to flooding of OWTS, basements, and infrastructure, making the land less suitable for future development. This flooding could also disrupt sensitive coastal wetlands and other aquatic habitats. On a regional level, drinking water supplied by public and private wells are potentially vulnerable to saltwater intrusion in the aquifers as sea levels rise. (Panthi and others, 2022)
The goals of this study are to:
- Measure nitrogen levels from groundwater, quantify groundwater flows, and determine how long it takes for nitrogen to reach the ponds and streams of the Salt Ponds region
- Examine how these features may change in response to both continued development and to conservation management efforts
- Study how sea level rise affects flooding and groundwater levels
To accomplish the study goals, USGS is developing a groundwater flow model of the Salt Ponds region. The models created, calibrated, and applied for this study include a local-scale numerical model of groundwater flow and a nitrogen transport model that uses travel times from the groundwater flow model. The groundwater flow model is constructed using MODFLOW 6 to simulate average conditions in the Salt Ponds region of Rhode Island for every month of the year. The model is informed by previous groundwater flow models of the Salt Ponds region by Masterson and others (2007) and of the Long Island Sound watershed by Barclay and others (2024). The groundwater flow model will be calibrated to water levels and streamflow and used to evaluate groundwater flowpaths in the region. The nitrogen transport model will then be developed using information from the groundwater flow model to estimate total rates of nitrogen loading from OWTS, fertilizer, and atmospheric deposition and nitrogen travel times to the ponds and streams in the area. Then, the models will be used to simulate various scenarios that may occur in response to projections of continued development and proposed conservation-management strategies in the Salt Ponds region. Additionally, the groundwater flow model will be used to evaluate the response of the fresh groundwater system to a range of projected sea-level rise scenarios.
References
Coastal Resources Management Council, 2015, The Rhode Island Sea Level Affecting Marshes Model (SLAMM) project: Summary report, accessed April 2025, at https://www.crmc.ri.gov/maps/maps_slamm/20150331_RISLAMM_Summary.pdf.
Narragansett Bay Estuary Program, 2022, Wastewater Infrastructure – OWTS Locations – NBEP2017 (shapefile), accessed November 2024, at https://narragansett-bay-estuary-program-nbep.hub.arcgis.com/datasets/c0aac11088a4407faabeb67ab94d1efc_0/explore?location=41.751069%2C-71.386477%2C9.14.
Panthi, J., Pradhanang, S.M., Nolte, A, Boving, T.B., 2022, Saltwater intrusion into coastal aquifers in the contiguous United States - A systematic review of investigation approaches and monitoring networks: Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2022.155641.
The U.S. Geological Survey (USGS), in cooperation with the Southeast New England Program of the U.S. Environmental Protection Agency, is estimating the magnitude, spatial distribution, and travel times of nitrogen-loading to ponds and streams in the Salt Ponds region of southern Rhode Island. This information can be used to guide conservation-management efforts aimed at reducing nitrogen inputs to the Salt Ponds region. Models developed to estimate nitrogen loads can also be used to evaluate potential flooding and groundwater level changes in the region by simulating rising sea levels.
Map showing the Salt Ponds region in southern Rhode Island.
Excess nitrogen is a significant concern in the Salt Ponds region of southern Rhode Island. The salt ponds, or coastal lagoons, in this area are primarily sustained from groundwater discharge, making them susceptible to dissolved nutrients in groundwater that can degrade the water quality of the ponds. Onsite wastewater treatment systems (OWTS), including septic systems, septic tanks, cesspools, and seepage pits, are a major contributor of excess nitrogen. Approximately 14,700 OWTS were in use in the Salt Ponds region between 2010 and 2015. (Narragansett Bay Estuary Program, 2022)
Potential sea-level rise and changing recharge patterns are also a concern for the Salt Ponds region. (Coastal Resources Management Council, 2015) Higher groundwater and sea levels could contribute to flooding of OWTS, basements, and infrastructure, making the land less suitable for future development. This flooding could also disrupt sensitive coastal wetlands and other aquatic habitats. On a regional level, drinking water supplied by public and private wells are potentially vulnerable to saltwater intrusion in the aquifers as sea levels rise. (Panthi and others, 2022)
The goals of this study are to:
- Measure nitrogen levels from groundwater, quantify groundwater flows, and determine how long it takes for nitrogen to reach the ponds and streams of the Salt Ponds region
- Examine how these features may change in response to both continued development and to conservation management efforts
- Study how sea level rise affects flooding and groundwater levels
To accomplish the study goals, USGS is developing a groundwater flow model of the Salt Ponds region. The models created, calibrated, and applied for this study include a local-scale numerical model of groundwater flow and a nitrogen transport model that uses travel times from the groundwater flow model. The groundwater flow model is constructed using MODFLOW 6 to simulate average conditions in the Salt Ponds region of Rhode Island for every month of the year. The model is informed by previous groundwater flow models of the Salt Ponds region by Masterson and others (2007) and of the Long Island Sound watershed by Barclay and others (2024). The groundwater flow model will be calibrated to water levels and streamflow and used to evaluate groundwater flowpaths in the region. The nitrogen transport model will then be developed using information from the groundwater flow model to estimate total rates of nitrogen loading from OWTS, fertilizer, and atmospheric deposition and nitrogen travel times to the ponds and streams in the area. Then, the models will be used to simulate various scenarios that may occur in response to projections of continued development and proposed conservation-management strategies in the Salt Ponds region. Additionally, the groundwater flow model will be used to evaluate the response of the fresh groundwater system to a range of projected sea-level rise scenarios.
References
Coastal Resources Management Council, 2015, The Rhode Island Sea Level Affecting Marshes Model (SLAMM) project: Summary report, accessed April 2025, at https://www.crmc.ri.gov/maps/maps_slamm/20150331_RISLAMM_Summary.pdf.
Narragansett Bay Estuary Program, 2022, Wastewater Infrastructure – OWTS Locations – NBEP2017 (shapefile), accessed November 2024, at https://narragansett-bay-estuary-program-nbep.hub.arcgis.com/datasets/c0aac11088a4407faabeb67ab94d1efc_0/explore?location=41.751069%2C-71.386477%2C9.14.
Panthi, J., Pradhanang, S.M., Nolte, A, Boving, T.B., 2022, Saltwater intrusion into coastal aquifers in the contiguous United States - A systematic review of investigation approaches and monitoring networks: Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2022.155641.