The short version
Seacoast Shores in East Falmouth is a narrow peninsula with about 1,000 homes on septic systems and cesspools, and their nitrogen travels through the groundwater into the Eel River, the western arm of Eel Pond in the Waquoit Bay system. US Geological Survey (USGS) scientists sampled that groundwater in the last few feet before it seeps up through the riverbed. In places, microbes in low-oxygen sediment where fresh groundwater meets seawater were turning nitrate into harmless nitrogen gas. That natural cleanup was strongest a little offshore, under sediment that is always underwater, and weak right along the shoreline. The study did not measure how much nitrate still reaches the estuary.
What the researchers did
This 2015–16 field study followed up an earlier USGS study at the same site (Colman and others, 2018), which had found signs of nitrate removal in the groundwater beneath the peninsula.
- They installed shallow sampling points 0.3 to 1.2 m (1 to 4 feet) below the riverbed, from 1 to 13.5 m (3 to 44 feet) offshore, plus deeper wells 1.8 to 4.9 m down and 4.3 to 14.3 m offshore.
- They measured which way the groundwater was flowing, how salty it was, and its nitrate and oxygen levels.
- To catch nitrate being destroyed, they measured dissolved nitrogen gas against argon. Nitrogen gas above the expected level is the fingerprint of microbes converting nitrate.
What they found
- Groundwater pushes up into the river. In October 2016, groundwater beneath the shore and shallows flowed upward toward the estuary at every stage of the tide, most strongly at low tide.
- Plenty of nitrate arrives. Nitrate in the shallow groundwater ranged from none detected to 600 micromoles per liter (8.4 mg/L as nitrogen). It was highest in June 2016, 4 to 9 m offshore. Salty water near the riverbed surface, and all samples 10 to 13.5 m offshore, generally had none.
- Low oxygen is common. About 60% of the groundwater sampled under the shore and shallows was nearly out of oxygen, the condition these microbes need.
- The cleanup happens mostly offshore. The largest losses, an estimated 230 to 430 micromoles per liter of nitrate converted to gas, were in nearly fresh groundwater 10 to 13.5 m offshore. Within 10 m of shore, evidence of removal was sparse despite high nitrate. In one June 2016 transect, the data fit complete removal of nitrate from fresh groundwater.
- Mixing zones matter. Removal showed up where fresh groundwater mixes with seawater soaking down into the sediment. Seawater and estuary mud may supply the organic carbon and other fuels the microbes need.
- No full accounting yet. Some fresh, nitrate-rich groundwater reached the shallowest samples, which may mean nitrate is seeping directly into the river in places. The authors call for more sampling to measure how much nitrate ultimately gets through.
What it means for Waquoit Bay today
This is good news with a big asterisk. Some septic nitrogen from dense shoreline neighborhoods is destroyed in the riverbed before it reaches the water, a process the report notes is not counted in the conservative assumptions behind the state’s nitrogen limits (see the 2020 Waquoit Bay TMDL). But the removal was patchy, weakest right at the shoreline, and never added up to a total. It does not change the basic picture: Eel Pond and the Eel River remain well above their nitrogen targets, and septic systems are the main source.
The authors say the results may help plan future nitrogen work in crowded coastal neighborhoods that may be candidates for sewers. They also hint at why engineered approaches can work: the same low-oxygen, carbon-fed conditions are what a buried woodchip barrier tries to create on purpose (see the woodchip barrier test at the Waquoit Bay Reserve).
Read the report
Huntington, T. G., Kroeger, K. D., McCobb, T. D., Böhlke, J. K., Colman, J. A., Brooks, T. W., & Szymczycha, B. (2024). Evidence of nitrate attenuation in intertidal and subtidal groundwater in a subterranean estuary at a Cape Cod embayment, East Falmouth, Massachusetts, 2015–16 (U.S. Geological Survey Scientific Investigations Report 2024–5100). Prepared in cooperation with the U.S. Environmental Protection Agency. https://doi.org/10.3133/sir20245100
License: This is a US federal government report and is in the public domain (USGS notes that it may contain copyrighted material where marked in the text). The PDF below is reproduced unmodified from USGS. The supporting data are in a separate USGS data release (doi:10.5066/P13LGNTT). The card image is the report’s cover, with a photograph by Timothy D. McCobb, USGS.
Drafted with AI assistance from the linked source.
