The short version
Falmouth is weighing a plan to connect more homes to sewers, treat the wastewater at its Main Wastewater Treatment Facility and send it through an ocean outfall pipe into Nantucket Sound, instead of letting it soak into the ground. That would take water out of the aquifer that now returns to it, so the town asked the U.S. Geological Survey (USGS) what would happen to groundwater levels and streams. USGS updated its computer model of western Cape Cod's groundwater and tested two sewering scenarios. In both, the changes were small compared with the natural ups and downs from year to year: the water table dropped about 0.1 to 0.2 foot across most affected areas, with the largest drop, about 1.1 to 1.3 feet, right at the treatment plant's sand beds. Stream flow fell by less than 1% in total for partial sewering and 3% for town-wide sewering. The study modeled water quantity only.
What the researchers did
The authors, Kendall Goldstein and Timothy McCobb, took an existing USGS groundwater model of western Cape Cod and brought it up to date, then changed the wastewater inputs to match each proposal.
- They started with a three-dimensional USGS model of the Sagamore flow lens, the large underground freshwater lens beneath Falmouth, that represents long-term average conditions (a "steady-state" model).
- They updated the model to average 2019–23 conditions: public-supply pumping, treated wastewater returned at the two town treatment plants, septic-system flow, newly sewered neighborhoods, and the new sand beds at the Main plant. They did not recalibrate the model.
- Scenario 1 sewers all properties in the water-system service area south of Route 28 and ends the return of treated wastewater to the ground at the Main plant. The authors say this approximates the town's possible sewer expansion over the next 20–30 years.
- Scenario 2 sewers every property in Falmouth, to show the largest possible effect, and also ends return flow at the Main plant.
- They compared each scenario with the 2019–23 condition, looking at water-table height and at the groundwater-fed flow of six streams in Falmouth and Mashpee: the Coonamessett, Backus, Bourne, Childs and Quashnet Rivers and Red Brook.
- They also compared the 2019–23 model with the earlier 2010 version to show how much recent changes in pumping and discharge had already shifted the water table.
What they found
- Water table barely moves. In scenario 1 the water table fell by about 0.1 foot south of Route 28, where septic systems were removed. In scenario 2 it fell by about 0.1 to 0.2 foot over a wider area of Falmouth, reaching north into Bourne (Goldstein & McCobb, 2025).
- The biggest drop is at the treatment plant. Near the Main plant the water table fell about 1.1 feet in scenario 1 and about 1.3 feet in scenario 2. The authors explain that years of treated water spreading onto the sand beds probably raised the water table there.
- Small next to nature. At a USGS monitoring well in Falmouth, the water table rose and fell by an average of 1.92 feet a year from 2019 to 2023 (range 0.49 to 5.57 feet). That natural swing is larger than the 1.3-foot maximum change in either scenario.
- A modest share of the water. Total wastewater returned to the ground would drop by 180,343 cubic feet a day (35.7%) in scenario 1 and 503,385 cubic feet a day (99.6%) in scenario 2. Even the larger figure is about 5% of all the recharge over the model area.
- Coonamessett River feels it most. As the stream closest to the Main plant, its simulated flow fell 0.1 cubic feet per second (1.1%) in scenario 1 and 0.6 cubic feet per second (5.4%) in scenario 2.
- Other streams change even less. For all six streams together, flow fell 0.3 cubic feet per second (0.7%) in scenario 1 and 1.4 cubic feet per second (3.0%) in scenario 2. The Childs River dropped 0.4% and 2.6%, and the Quashnet River 0.3% and 1.1%.
- What the model can't say. It shows long-term averages only. The authors note a time-varying model would be needed to address seasons, droughts, heavy summer pumping, water quality or effects on the marine environment.
What it means for Waquoit Bay today
This study covers Falmouth's groundwater and streams. It also covers the same sandy aquifer that feeds the bay, and two of the streams it checked, the Childs and Quashnet Rivers, flow toward Waquoit Bay. Their simulated flows changed by under 3% even with town-wide sewering. For neighbors worried that sewers and an ocean outfall would drain ponds and rivers, the answer here is that the hydrologic effect looks small, with the clearest change at the Main plant. The Childs River has just been through a very different kind of change, a dam and bog removal; see our post on results one year after Childs River restoration.
For whether sewering would clean up the bays, and how fast, see our Research Library posts on USGS monitoring after the Little Pond neighborhood was sewered and on USGS modeling of Popponesset Bay in Mashpee. Both suggest that nitrogen loads respond over years, and that where treated water is discharged matters. The Falmouth Enterprise's account of the town committee hearing this USGS report is summarized in our news post from August 2025.
Read the report
Goldstein, K. M. F., & McCobb, T. D. (2025). Simulated hydrologic responses to proposed wastewater-return-flow scenarios in Falmouth, Massachusetts (Version 1.1, 2026; Scientific Investigations Report 2025–5066). U.S. Geological Survey. https://doi.org/10.3133/sir20255066
License: The report is a U.S. Geological Survey publication, prepared in cooperation with the Town of Falmouth. USGS says it is largely in the public domain, so we host the PDF here as a US federal work and also link to the USGS copy through the DOI. The summary is based on the full report, not just the abstract. The card image is a title card made from the citation, because the report's first page is a cover.
Drafted with AI assistance from the linked source.
