Santuit Pond Nutrient Reduction Project
Santuit Pond Nutrient Reduction Project
What is being done to improve water quality in Santuit Pond?
The Mashpee Wampanoag Tribe and the Town of Mashpee have collaborated to take a strategic, watershed-based approach utilizing a combination of structural and non-structural best management practices (BMPs) to improve water quality in Santuit Pond. Studies conducted over several decades have shown that Santuit Pond is suffering from excessive phosphorus from stormwater, agricultural inputs, septic and the release of phosphorus from muddy bottom sediment. Learn more about the phosphorus problem here. Measures are being implemented by the Town of Mashpee to address stormwater runoff, septic system inputs, and historic agricultural sources that account for approximately 20 percent of the phosphorus sources in Santuit Pond.
What has not been addressed to date is the source of the remaining 80 percent of the phosphorus that comes from accumulated sediment at the bottom of the pond. The only viable approach to addressing phosphorus within the pond’s sediment is to implement a nutrient inactivation program. The Mashpee Wampanoag Tribe and the Town of Mashpee teamed up to secure a grant from MassDEP in 2025 to implement the program, and treatment is set to begin in fall 2026.
What is nutrient inactivation?
Nutrient inactivation applies a binding agent known as “alum” that acts as a “lock” to stop the release of phosphorus from the sediment into the pond water. After treatment, the phosphorus in the sediment is “inactive” or unavailable as a food source for algae.
Implementing a nutrient inactivation program in Santuit Pond will improve water quality, habitat available to fish and wildlife, and reduce the public health risk from harmful algal blooms. The project builds on ongoing outreach and engagement activities targeted at residents in the neighborhoods surrounding Santuit Pond, Town officials, and the Mashpee Wampanoag Tribe.
Is nutrient inactivation safe?
Yes, it is safe when correctly applied. Nutrient inactivation applies a binding agent called Alum (aluminum sulfate). Alum is a compound commonly used by public water suppliers to clarify drinking water during the treatment process. When added to water, alum forms a solid, heavy cluster called a “floc”, which traps phosphorus and settles it onto the bottom of a water body.
Nutrient inactivation has been successfully used in dozens of water bodies over the last few decades, including a number on Cape Cod:
Cliff Pond – Brewster / Orleans
Long Pond – Harwich
Hamblin Pond – Barnstable
Ashumet Pond – Mashpee / Falmouth
Great Pond – Eastham
Herring Pond – Eastham Mystic Pond – Barnstable
Lovells Pond – Barnstable
Stillwater Pond – Chatham
Lovers Pond – Chatham
Parker Pond, Barnstable/Osterville
The ponds treated with nutrient inactivation have a marked improvement in water quality.
How do we pay to treat the pond?
In May of 2025, Mashpee Wampanoag Tribe and the Town of Mashpee received a grant from the Massachusetts Department of Environmental Protection for $700,000. The grant award will cover the cost of the alum treatment that will be applied to the pond to reduce the availability of phosphorus sources from sediment over the course of two years.
How will the nutrient inactivation treatment be implemented?
The release of phosphorus from the sediment in Santuit Pond will be addressed through nutrient inactivation to permanently bind phosphorus over an approximately 145-acre area of the pond bottom where low dissolved oxygen occurs most frequently (i.e., below the 4-foot contour). The nutrient inactivation treatment (alum treatment) includes the use of two chemicals—alum and sodium aluminate—that will be applied from a barge. Because alum alone can make water acidic, sodium aluminate will be added as a buffer to keep the water’s chemistry stable and safe for fish and other wildlife throughout the process. Alum comes in liquid form, and treatment is a precision process that requires specialized equipment and professional training. The alum treatment must be injected under the surface of the water at a controlled rate so that the treatment is applied evenly and the pond is not overloaded with too much alum in one place or at one time.
The alum will “trap” excess phosphorus, which is the main “food” that causes excessive algae growth. When these chemicals enter the water, they form a fluffy particle called “floc” that grabs onto phosphorus and other particles. This mixture then sinks to the bottom, forming a thin layer that locks the phosphorus into the mud so it can no longer be used by algae for food.
The treatment will be conducted by TRC, a company specializing in lake management. TRC collected sediment core samples to determine the amount of phosphorus present in the bottom sediment throughout the pond. TRC determined that different parts of the pond will need different amounts of treatment, as more alum will need to be applied in the deeper areas because that is where phosphorus concentrations were found to be highest. Therefore, two alum treatments are anticipated, one in fall 2026 and a second in fall 2027. One treatment will focus on portions of the pond located between the mapped 4- and 8-foot contour, and the other treatment will focus on areas below the mapped 8-foot contour. It is anticipated that each alum treatment will take place over a minimum of four days. Alum treatments do not produce a perceptible odor, and once treatment is complete, recreational activities may resume.
TRC and the Mashpee Wampanoag Tribe will be on-site to monitor the water before, during, and after the application to ensure everything stays within a healthy range.
What permits are needed?
The nutrient inactivation treatment will be conducted within the boundaries of the Town of Mashpee. Regulatory approvals will require preparation and filing of a Notice of Intent pursuant to the Massachusetts Wetlands Protection Act and its implementing regulations, as well as Chapter 172 of the Mashpee Code (Wetlands Protection Bylaw) and its implementing regulations thereunder. Natural Heritage, State Fisheries and Wildlife as well as MassDEP, will be engaged in oversight of the project.
What will be done to ensure that the treatment is applied in accordance with the plan?
The Mashpee Wampanoag Tribe and their selected management consultants will perform all field sampling before, during and after the application of alum to ensure that the application minimizes harm to the environment. A Quality Assurance Project Plan (“QAPP”) and Sampling and Analysis Plans (“SAPs”) have been created to guide monitoring activities.
How is the public being engaged?
Come and join the Mashpee Department of Natural Resources at a series of public outreach events and meetings aiming to discuss and help residents better understand ongoing water quality improvement efforts at Santuit Pond.
- The Mashpee Wampanoag Tribe and TRC hosted a table at the Honor the Earth Fair on Saturday, April 18th to share about the Santuit Pond Nutrient Reduction Project and ways to reduce the amount of excess phosphorus transported to our waters.
- A public hearing was held with the Town of Mashpee Conservation Commission on June 26, 2026 to review the Notice of Intent for the nutrient inactivation treatment
- Learn more about upcoming events here.