The short version
CPWB asked water resources consultant Scott Horsley to sketch what it would take to restore the Moonakis River, the tidal lower stretch of the Quashnet that flows into Waquoit Bay. Working with Tufts University graduate students, he found that the river receives about 8,500 kg of nitrogen a year and needs to lose about 3,000 kg, roughly 36%, to recover. The plan shows that four steps together could get there: requiring nitrogen-reducing septic systems for new building, upgrading 250 existing septic systems, upgrading two small treatment plants, and treating road runoff. It is a starting point for discussion.
What the report covers
The report reviews the state’s nitrogen studies for the river, the Falmouth and Mashpee wastewater plans, the Cape Cod Commission’s 208 Plan and recent test results from the Massachusetts Alternative Septic System Test Center. It then builds a preliminary plan to meet the state’s nitrogen target.
- The watershed runs from Mashpee and Sandwich at the headwaters to Falmouth near the Bay.
- It compares conventional sewering with non-traditional options: enhanced septic systems, permeable reactive barriers, stormwater treatment and fertilizer reduction.
- It estimates how much nitrogen each option would remove and lays out a 20-year schedule, starting with a first five-year phase.
What it found
- A 36% cut is needed, and the target is moving. Using the state’s 2013 figures, the river receives about 8,497 kg of nitrogen a year against a target of 5,463 kg, so it needs a cut of about 3,035 kg. Growth since then, including an expansion of the Southport wastewater plant to 172,000 gallons a day, means the real cut needed is now somewhat larger.
- Doing nothing makes it worse. If the watershed builds out with standard Title 5 septic systems, which remove almost no nitrogen, the load could rise 67%, to about 14,189 kg a year. No nitrogen mitigation is currently required for new development.
- New septic technology works much better than the old. Older alternative septic systems cut nitrogen by only about 27%. Newer woodchip-based systems tested on Cape Cod and Long Island are reaching 5–8 mg/L in their discharge, removing roughly 75–90% of the nitrogen.
- Underground barriers are promising. Permeable reactive barriers, buried trenches of woodchips that strip nitrogen from groundwater before it reaches the water, removed about 90% in pilots in Orleans and Eastham. One installed beside Waquoit Bay has been working for more than 15 years with little sign of wear.
- Road runoff can be treated. Several roads pipe stormwater straight into the river. Swales, rain gardens and other green infrastructure can remove up to 65% of the nitrogen and also catch bacteria, metals and oil.
- Four steps would meet the target. The plan estimates yearly reductions of 980 kg from upgrading 250 existing septic systems, 1,837 kg from upgrading the Southport and Mashpee High School treatment plants from 10 to 3 mg/L, and 678 kg from stormwater retrofits. Together that is about 3,495 kg, more than today’s required cut. A health regulation requiring enhanced septic systems for new construction would prevent about 3,870 kg a year of future growth.
- Acting early saves money. Adding nitrogen treatment while a new septic system is being built costs about $20,000 extra, compared with $30,000–35,000 to retrofit one later.
What it means for Waquoit Bay today
The Moonakis/Quashnet is one of the most nitrogen-damaged parts of the Waquoit Bay system (see the Mashpee monitoring results), and its watershed crosses the Falmouth–Mashpee line. The plan’s main message is that the river does not have to wait for sewers: proven, mostly passive technologies could meet the state target if the two towns act together, starting with the homes closest to the river, where groundwater arrives fastest.
The figures are preliminary estimates built on the state’s 2013 numbers (our summary of the state report) and need updating. The report is meant to grow into a formal targeted watershed plan and, eventually, a MassDEP watershed permit. Read about CPWB’s Moonakis River project.
Read the report
Horsley, S. (2023). Moonakis River Watershed Restoration Plan: A Preliminary Assessment. Prepared for the Citizens for the Protection of Waquoit Bay, March 3, 2023, with Tufts University graduate students Jenna Van Holten, Elizabeth Gilbert, Peter Federico, Melissa Landman, Kristen Homeyer and Nicholas Stewart.
License: Prepared for CPWB, which publishes it here. This copy is compressed for the web.
Drafted with AI assistance from the linked source.
