The short version
Many cranberry farms in southeastern Massachusetts sit on old wetlands, and as farms retire, restoring those wetlands is one way to remove nitrogen before it reaches the coast. This study asked which farms could do the most. The team adapted a U.S. Geological Survey groundwater model to map the land that drains through 984 active or retired farms, estimated the nitrogen arriving at each, and applied removal rates taken from published studies of restored wetlands. If every farm were restored, the median cut in nitrogen reaching the bay was under 3% in nine of 24 embayments, 3–10% in seven and 10–30% in eight. The results depend mainly on how much land drains through the farms, which ranges from 1% to 75% of a bay's watershed. The authors call the model a screening tool and say field monitoring is still needed. We read the full paper.
What the researchers did
The authors built a map-based model for the sandy aquifers of Cape Cod and the Plymouth-Carver-Kingston-Duxbury area, where nitrogen moves mostly underground.
- Used outputs of USGS groundwater models to draw the area that drains through each of 984 active or retired cranberry farms (about 4,080 hectares in total), and checked the modeled flows against 89 stream and farm measurements.
- Mapped nitrogen in surface water from 558 monitoring sites compiled from the Massachusetts Estuaries Project, the Buzzards Bay Coalition, the Cape Cod Rivers Observatory, MassDEP and USGS.
- Estimated the nitrogen load reaching each farm in two ways: water flow times nitrogen concentration, and the Massachusetts Estuaries Project's loading rates times drainage area.
- Applied removal rates from 78 published values for created and restored wetlands in temperate regions.
- Compared two scenarios in 24 embayments (bays and estuaries) that have cranberry farms in their watersheds: restoring all farms, or restoring only the one farm that receives the most nitrogen.
- Tested the approach on the Three Bays estuary in Barnstable, using a stream gauge at the outlet of a farm scheduled for restoration.
What they found
- Large share of land drains through farms. About 30% of the land in the modeled area drains through cranberry farms. In the 24 bays studied, 419 farms drained about 25% of the land, and the nitrogen passing through them was 17% to 25% of the watershed's load, depending on the method (Wiegman et al., 2025).
- Restoring every farm: a wide range of payoffs. Median nitrogen reductions were under 3% in nine embayments, 3–10% in seven and 10–30% in eight. Cranberry farms intercepted from under 1% to about 75% of the nitrogen reaching a bay, and in ten bays more than a quarter.
- Drainage area matters most. Drainage areas varied about 10,000-fold across the farms, while nitrogen concentrations varied far less. The authors conclude that the area draining through a farm is the main reason some farms could remove much more nitrogen than others.
- One well-placed farm can do a lot. In seven embayments a single farm received more than 25% of the watershed's nitrogen, and restoring that one farm gave a median cut of at least 10%. The authors say targeting sites could stretch conservation money further.
- Fertilizer is a small piece. Fertilizer running off active farms was a median 0.9% of a bay's nitrogen load. The Wareham River estuary was the exception at 16%. Retiring farms without restoring wetlands would trim the load only slightly.
- Low nitrate may limit removal. Nitrate (the form of nitrogen wetland microbes can convert to gas) was under half of the nitrogen in all but three embayments, and just 12% in the Wareham River watershed. The authors suggest this, along with short water residence times, could hold removal below the published averages.
- Three Bays check. A USGS gauge at the outlet of a 5.4-hectare farm, part of a 31.6-hectare cluster due to be restored, measured about 5,138 kg of nitrogen a year. The model estimated about 3,498 kg (flow-times-concentration) and about 5,075 kg (loading-rate method). Restoring the cluster could meet roughly 4% to 23% of the bay's roughly 20,500 kg a year reduction target, depending on how well the wetland works.
- A screening tool, not a forecast. The model is meant to pick candidate sites for closer study. It may overestimate drainage areas by about 11%, and the authors call for field monitoring of former cranberry wetlands. They also say that cutting nitrogen at its sources through septic upgrades and sewers is still required.
What it means for Waquoit Bay today
The study covers southeastern Massachusetts and Cape Cod, and Waquoit Bay is one of the 24 embayments modeled. The paper reports that only 17% of the cranberry farms in the Waquoit Bay watershed were still active, so most are already retired. That makes the question of what happens to those bogs, restored wetland or not, a live one for the Bay. Local results so far are modest: restored bogs on Falmouth's Coonamessett River removed well under 1% of the river's nitrogen (see groundwater seeps in a restored bog and early data from the Coonamessett). This paper helps explain why: how much a restoration removes depends on how much water and nitrogen flows through the site.
The state's legal limits require roughly halving the nitrogen reaching Waquoit Bay, mostly from septic systems (see the 2020 limits explained). Targeted bog restoration might supply part of that, in bays where retired farms sit on large drainage areas. The authors are clear it complements wastewater treatment and does not replace it. A companion field study from the same research group found a restored farmed wetland held back about 28% of incoming nitrate (see Can restoring an old farmed wetland stand in for sewers?).
Read the paper
Wiegman, A. R. H., Kennedy, C. D., Neill, C., Jakuba, R. W., Welsh, M. K., Millar, D., & Buda, A. (2025). Modeling attenuation of nitrogen loads delivered to coastal bays from ecological restoration of cultivated wetlands. *Journal of Geophysical Research: Biogeosciences*, 130(9), e2025JG008742. https://doi.org/10.1029/2025JG008742
License: This article is open access under a Creative Commons Attribution-NonCommercial (CC BY-NC) license, and work by U.S. Government employees is in the public domain in the USA. We host the PDF here under CC BY-NC and also link to the publisher's copy (AGU/Wiley). This summary is based on the full text. The card image is a title card made from the citation.
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