Using multiple lines of evidence to evaluate spatio‐temporal trends in nitrogen loading to a eutrophic estuary

Nitrogen (N) pollution in groundwater in coastal watersheds can have negative impacts on the health of estuaries. West Falmouth Harbor, a shallow lagoon on Cape Cod, MA, USA, has experienced a marked increase in N load since the 1990s due to input from a groundwater aquifer contaminated by a municipal wastewater treatment facility. As part of a broader, long-term research effort to understand the changes to the ecosystem associated with this N input, this study takes an integrative approach at quantifying groundwater N load to the estuary from 1985 through 2024 by combining traditional groundwater well sampling with a wintertime open water mixing model. The latter gives a robust spatially averaged nitrate concentration in discharged groundwater, supplementing the spatial and temporal patterns from the well data. We calculate a conservative estimate for the mean N load to West Falmouth Harbor during peak loading years (2008–2013) of 3.1 mmol m−2 d−1. This then decreased by approximately 25% by 2024 as a result of wastewater treatment improvements and should continue to decline over the next decade. The long-term trends observed in the groundwater well network complement the open water sampling, providing confidence in the accuracy of the latter approach and allowing us to interpolate an estimate of N loading to West Falmouth Harbor back to 1985. This integrative approach to evaluating groundwater inputs to this ecosystem dramatically improves our ability to quantify total N load and can be a powerful tool for studies in groundwater-fed lagoonal estuaries with a short residence time.