Modeling attenuation of nitrogen loads delivered to coastal bays from ecological restoration of cultivated wetlands

Abstract

Nitrogen (N) pollution is a major threat to coastal ecosystems, worsened by the loss or degradation of natural wetlands, which historically acted as N sinks. In the glacial outwash plain of Southeastern Massachusetts, N pollution primarily from human waste and turf fertilizer has caused coastal eutrophication. Social and economic factors have driven ecological restoration efforts on wetlands previously modified for cranberry farming. These restoration projects offer a chance to enhance ecosystem N attenuation, but the extent and spatial distribution of watershed N loads through these farms remain poorly understood. To address this gap, we adapted a U.S. Geological Survey (USGS) groundwater model to identify wetland contributing areas and model potential N load reduction from the retirement and restoration of 984 cranberry farms. Using modeled contributing areas and data and assumptions about attenuation rates, we estimated N load reductions for farm retirement and restoration scenarios in 24 embayments. For restoration of all farms, median N load reductions were less than 3% in nine embayments, 3%–10% in seven embayments, and 10%–30% in eight embayments. Attenuation was limited by the contributing area intercepted by cranberry farms, ranging from 1% to 75% of watershed areas. Our model serves as a screening tool to identify farms with high potential to reduce watershed N loads, but more field monitoring is needed to refine N attenuation estimates in former cranberry wetlands. This work highlights the critical linkage between wetlands, development patterns, and ecosystem health, emphasizing the need for sustainable resource management approaches.

Key Points

• Restoring functioning wetlands on retired cranberry farms can substantially decrease watershed N inputs to coastal embayments
• Potential N attenuation due to wetland restoration was strongly influenced by the drainage area which varied 10,000-fold across ∼1,000 farms
• Wetland restoration can be spatially targeted to areas with high N load reduction potential to make efficient use of conservation resources

Plain Language Summary

Coastal water quality in Southeastern Massachusetts is declining due to diffuse nitrogen pollution primarily from human waste, turf fertilizer, and atmospheric deposition. The region is densely covered with groundwater-fed wetlands that occupy glacially formed kettle-hole depressions and coastal valleys. Many of these wetlands have been modified for agriculture, particularly cranberry farming, which has decreased their extent and limited their capacity to attenuate (trap and remove) inflowing nitrogen. In most regions, watershed boundaries can be mapped using surface elevation, but in the coastal plain of Southeastern Massachusetts, nitrogen travels primarily with groundwater rather than overland flow. To address this, we used U.S. Geological Survey groundwater models to develop a tool to rapidly estimate contributing areas and identify which cranberry farms receive the greatest nitrogen loads from upstream sources. Our analysis shows that restoring ecologically functioning wetlands on select farms may substantially reduce nitrogen inputs to coastal bays and improve water quality. This approach demonstrates how targeted wetland restoration, informed by groundwater models, can enhance nutrient attenuation in agriculturally influenced landscapes. The findings offer practical guidance for improving the efficiency of restoration programs and may be applicable to other regions of the world that suffer from groundwater nitrogen pollution and degraded wetlands.

Acknowledgments

We sincerely thank the late Brian Howes and his team of associates at UMass Dartmouth for their service to the Municipalities of Southeast Massachusetts by collecting data, developing models, and documenting efforts to support watershed nitrogen management as part of the Massachusetts Estuaries Project (MEP). Without data from the MEP this study would not be possible. We would also like to thank John Masterson and Tim McCobb of U.S. Geological Survey for providing MODFLOW model files for PKCD (Masterson) and Cape Cod aquifers (McCobb). Ed Eichner of TMDL Solutions and longtime colleague of Brian Howes provided geospatial data and supporting information on the MEP reports.

Data Availability

Water quality, stream flow, and MODFLOW data files are publicly available and cited within the main article (Jakuba et al., 2021; MEP, 2025; WPP, 2024; Woodwell Climate Research Center, 2024; USGS-NWIS, 2024). These data as well as scripts for preprocessing, geospatial modeling, and postprocessing are hosted on the authors GitHub: https://github.com/arhwiegman/Cran_Q_Crepository and preserved on Zenodo https://doi.org/10.5281/zenodo.15757982 (Wiegman, 2025).