Santuit Pond Study Finds Phosphorous Comes From Within
By: Brian Kehrl
A draft report has identified the cause of the widespread algal blooms in Santuit Pond, and much of the cause is now from within the pond itself.
More than three-quarters of the nutrient pollution fueling algae growth is generated from sediment within the pond, according to a draft consultant’s report on the pond provided to the town last week.
Other causes of excessive phosphorous in the pond include man-made sources like road runoff, septic systems, and the nearby cranberry bogs, which each account for a few percentage points of the total load, according to the report by the environmental consulting firm AECOM. But the vast majority of the nutrient is released from sediment on the bottom of the pond, which has built up over time with highly nutrient rich material.
The level of “internal loading” is extraordinarily high for a pond, and initial reactions to the report from two members of the town Environmental Oversight Committee portrayed the consequences of results for restoration in strikingly different lights.
The report, the first scientific study of the pond’s sharp decline over the last decade, is hoped to provide local officials with the information needed to plan an approach to improving water quality in the shallow, 174-acre pond in northeast Mashpee. The pond is ranked five out of five on the Clean Water Act list of impaired waterbodies, which requires the state to develop a phosphorous target that the town must meet.
Concerns about Santuit, long voiced by residents around the pond, have come to the fore in recent years when potentially toxic algal blooms have caused state and town officials to warn against swimming in it.
The report recommends two main solutions to the problem of high phosphorous levels in the pond: mechanically mixing the pond water using a bubbler or a circulator, or treating the pond with a chemical known as alum, which binds the phosphorous making it unusable by algae.
A third approach, dredging, was found to be too costly with a $12 to $16 million estimated price tag.
Representatives from AECOM have been invited to the environmental oversight committee’s meeting next month, on April 8 at 7 PM at Mashpee Town Hall, to discuss the report, answer questions, and get feedback from the town before completing the final draft later this spring.
Shellfish Constable Richard H. York Jr., who serves on the environmental oversight group and has been helping to oversee a water quality monitoring instrument in the pond, said the conclusion that most of the phosphorous is generated internally is “really good news” for curing the pond’s most egregious ills.
He said mixing the pond water with a solar-powered circulator should provide a relatively low-cost solution to improving the water quality. Permitting, purchasing, installing, and running a circulator for 15 years is estimated to cost between $215,000 and $315,000, a cost that Mr. York said the town could find some assistance with through a grant.
The phosphorous is released from the sediment when the oxygen level drops in the water just above it, which tends to happen when there is little wind and under certain conditions at night. But because the surface water typically has adequate oxygen, if the water is mixed together, the oxygen in the water in the deeper areas never drops and the phosphorous in the sediment is not released, according to Mr. York and an explanation in the report.
The generation of phosphorous in the pond creates a self-feedback loop in which algae helps create the low-oxygen events, which releases the phosphorous from the sediment, which in turn helps more algae grow, and so on. Circulating the water is meant to break that cycle.
“It is good news because we can fix it fast, and it is a really environmentally friendly fix,” Mr. York said, adding that solar-power circulators could be installed as floats on the pond. “If we can do this fast enough, and get it up and running, we might be able to prevent another big bloom this summer.”
“It is never going to be completely clear from this fix, but it is going to be better for all the things that we want it for, like fish and recreation,” he said. “We have got our answers. Now we just have to implement it.”
However, Edward A. Baker, another member of the environmental oversight committee and president of the nonprofit Mashpee Environmental Coalition, said, “My view is that it is a very difficult thing to remove the internal load. And you either have to dredge to take it out or you have to isolate it using something like an alum treatment or I suppose the bubblers would work. But if you think about it, are you going to run the bubblers forever?”
He said bringing the phosphorous levels down from the average of 81 to the target of 15 micrograms per liter (or parts per million) identified in the study will require the town to take on the other sources, like road runoff and the contributions of the two sets of cranberry bogs next to the pond.
“They want a target of 15 micrograms per liter in the pond, and even without the regeneration, you are getting more than that,” he said.
Solutions like mixing the water or treating it with alum will not prevent the additional phosphorous from coming in, which will continue to cause problems, he said.
He pointed to the alum treatment at Ashumet Pond, which was done first in 2001 and which the Air Force Center for Engineering and the Environment is planning to do again this summer because too much phosphorous is continuing to leach into the pond.
AECOM estimated an alum treatment for Santuit Pond will cost between $180,000 and $200,000.
He said he is wary of mechanical fixes because of concerns about vandalism. He said Barnstable installed a bubbler at the nearby Lovells Pond, but last summer the air filter had been ripped out and the machine appeared to not be working.
Mr. Baker said the report did not address how much phosphorous leaves the pond down the Santuit River, which flows over a dam and fishway at the southern end and down to Shoestring Bay, a key piece of information to determine whether altering the pond level could help keep the nutrient level down.
“I would think that the quick fix should come after we get the input low enough. If you don’t solve the input, you haven’t solved anything,” he said.
The report establishes Santuit Pond as an outlier on the spectrum of water quality in Cape Cod ponds. Of the 175 ponds sampled for the Cape Cod Commission to compile the Cape Cod Pond and Lake Atlas in 2003, the median phosphorous level was 27 micrograms per liter, compared to Santuit’s 81.
Thomas C. Cambareri, water resources program manager for the commission, said phosphorous levels above 10 micrograms per liter indicate impact by humans. “A concentration above 10 means it has been impacted from phosphorous loads that you wouldn’t otherwise expect a healthy pond to have, he said. “It is an indication that there is something a little out of whack in the pond.”
“So, yes, 80 some odd is way high,” he said, adding that it may be as high as any other pond on Cape Cod.
According to the AECOM report, Santuit Pond historically had clear water, but water quality in the pond declined somewhat through the 1980s and then saw a marked drop in the 1990s and early 2000s.
‘This decrease in clarity corresponded to the shift from an aquatic macrophyte dominated pond to an algal dominated one,” according to the report. “Santuit Pond is one of the most productive Cape Cod ponds and has historically provided an excellent warmwater fishery.”
“Given the current high phosphorous loads in Santuit Pond, it may be difficult to attain an in-lake concentration of 15 [micrograms per liter], but it is possible. AECOM recommends an aggressive phosphorous management strategy that works towards a target of an in-lake concentration of 15 [micrograms per liter]. Improvements in water quality will occur at higher in-lake concentrations,” according to the report, which also states that reducing phosphorous from road runoff and septic systems is “vital” to restoring the pond.
The report recommends curtailing the use of inorganic, phosphorous-heavy lawn fertilizers near the pond; improving stormwater treatment in areas around Briants Neck, Beechwood Point Drive, and Cranberry Lane; surveying septic systems around the pond to identify failing systems; and maintaining or restoring vegetated buffers around the pond.
Mr. Baker said the drainage improvements made recently along Timberlane Drive, at the intersection with Lantern Lane, where stormwater has been directed into catch basins instead of running directly into the pond, has likely made a significant improvement in phosphorous loading already.
But he said the town should be looking at several issues affecting the pond and the area around it, from wastewater treatment to stormwater, water quality, and the state mandate to improve the dam at the south end of the pond in a more comprehensive manner.
“We have a problem with wastewater that we are attempting to resolve, that people keep talking about it can be up to half a billion dollars, and we have this dam problem that we are trying to get a quarter of a million grant for, and then we have this blue-green algae problem in the pond. And they are related to each other. They may not be brother and sister, but they are at least cousins,” Mr. Baker said.
“We are concerned about the dam, so we are working the dam. We are concerned about the blue-green algae, so we are working that. And we are concerned about wastewater, so we are working that. But I don’t see people looking at relationships between them. And I keep saying to myself, if the ability to manipulate the height of the pond is related to the flushing rate of the pond, which is related to the phosphorous of the pond, then there is a connection to the dam. And if there is stuff coming in from septic systems, then there is a relation to wastewater. To me, those things should be built in rigorously to each other in the planning activities. And that is not happening,” he said.
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