Cape Cod Times
October 8, 2017
Restoring a Mill Pond
Zenas Crocker, executive director of the Barnstable Clean Water Coalition, says it’s time to shout “hair on fire” about the degrading quality of the Cape’s water resources — from its groundwater to fresh water ponds and salty estuaries.
Formerly Three Bays Preservation Inc., the Barnstable Clean Water Coalition now works to restore and preserve clean water throughout the entire town of Barnstable. Crocker and his team warn that the Cape’s water is impaired, mostly by excess levels of nitrogen. On average, 85 percent of nitrogen in our waters comes from septic systems, 10 percent from fertilizers that most of us dump on our lawns every year, and 5 percent from stormwater runoff.
The Massachusetts Estuaries Project has determined that the Three Bays estuary and embayment system, comprised of West Bay, North Bay and Cotuit Bay on the south side of the Cape, receives about 46,000 kilograms of nitrogen per year from septic systems, fertilizer and stormwater runoff. The total watershed limit for Three Bays is about 26,000 kilograms per year. As a result, about 20,000 kilograms of nitrogen must be removed from the watershed.
How is that accomplished?
According to the Cape Cod Commission, about half the excess nitrogen, or 10,000 kilograms, could be removed by aquaculture alone. Oysters and quahogs filter nitrogen from the water. About 43 acres of aquaculture beds would be needed to remove this amount of nitrogen, and finding suitable sites may be the most challenging obstacle. After all, oceanfront homeowners have gone to court to stop aquaculture projects near their beaches.
Of course, mitigating stormwater runoff and managing fertilizers would also reduce the nitrogen load.
But in one of the most ambitious projects to reduce nitrogen in the Three Bays watershed, the Coalition also supports restoring Mill Pond off Route 149 in Marstons Mills. Mill Pond dates back to the 17th century when it was created by a small dam, built to power a gristmill. The Cape is home to scores of mill ponds.
Currently, Mill Pond receives most of its nitrogen from the Marstons Mills River, which flows into the pond from the north. But because the pond is very shallow, less than two feet deep on average, most of the nitrogen flows right through the pond and continues heading south to Three Bays.
The plan is to dredge the pond to restore its ability to act as a nitrogen trap or “sink.” This important ecological role has been significantly diminished due to significant eutrophication and sedimentation over the past 300 years.
“Longer (nitrogen) residence time provides for more settling of solids and increased biological uptake and dentifrification processes,” according to a report last year from the Cape Cod Commission.
Currently, the pond retains only about 25 percent of the nitrogen that flows through it. By dredging sediments that have built up over the centuries, the pond could retain as much as 60 percent of the nitrogen.
According to a study conducted by Lycott Associates Inc. in 2008, about 90 percent of the pond contains organic sediment in excess of seven feet deep. In 2012, under contract with the Cape Cod Water Collaborative, Horsley Witten Group Inc. prepared an Environmental Notification Form (ENF) for the Massachusetts Environmental Policy Act office (MEPA). The ENF was submitted for the proposed Mill Pond Improvements Project with the overall goal of nutrient attenuation, water quality, habitat improvements in Mill Pond and water quality improvements in the downstream systems.
But dredging requires a series of state and local permits. And according to the Natural Heritage & Endangered Species Program (NHESP), Mill Pond contains the bridle shiner, a small fish protected under the Massachusetts Endangered Species Act. According to the Cape Cod Commission, the NHESP provided recommendations to alter the project footprint to confine dredging to a more limited portion of the pond to protect the shoreline habitat as well as provide an area of refuge in the southeastern portion of the pond during dredging. Based upon this information, a revised project has been designed that includes a more limited dredging area.
Crocker believes the Mill Pond project could be “a living laboratory,” where lessons from project could be applied to all the other mill ponds across the Cape.