By LAUREN SURBEY Mashpee Enterprise
Mashpee-Wakeby Pond has received its diagnostic pond study results, and a management plan is in the works to improve the water quality.
“We are finally here to discuss the highly anticipated Mashpee-Wakeby Diagnostic Study and the Mashpee Water Quality Report for the estuaries from 2019 to 2023,” said Ashley Fisher, the town’s water quality coordinator, to the Mashpee Select Board on Monday evening, December 16.
Ed Eichner, an adjunct professor and senior water scientist with the School of Marine Science and Technology at the University of Massachusetts Dartmouth, managed the study with a team and presented the data to select board and to residents.
“We proposed the study that we have, a diagnostic study and a management plan. The diagnostic study portion is looking at the pond and figuring out how the ecosystem works,” Mr. Eichner said. “The management plan is figuring out the ways that you can restore the water quality in the water column.”
Mr. Eichner said that Mashpee and Wakeby are two interconnected basins in Mashpee and Sandwich. He said that while nitrogen is the primary focus in estuaries, phosphorus is the key to managing ponds.
“Wakeby has higher phosphorus concentrations pretty much throughout the water column on a regular basis compared to Mashpee,” Mr. Eichner said. He explained what happens to each basin to cause the pond’s poor water quality.
“In the spring, the whole water column mixes. There is enough wind energy that’s going across that the whole water column, top to bottom, mixes,” Mr. Eichner said. He said that this happens every year from November until April.
“Once it starts to warm up, the upper layer of the pond warms faster than the wind can mix the whole water column, and you get this temperature layering,” Mr. Eicher said. “The upper layer is called the epilimnion; it continues to get mixed.”
“The transition layer between the epilimnion and a colder, bottom layer is called the hypolimnion. That hypolimnion is cut off from the atmosphere of mixing and if the sediments remove dissolved oxygen from that layer, you get anoxia,” Mr. Eicher said to the select board. “You get a complete lack of oxygen. That is something that happens in Mashpee-Wakeby, in both basins.”
Mr. Eichner said as part of the diagnostic study, he and his team looked at sediments, phytoplankton surveys, streamflows in Mashpee River, the watershed, prudent plants and mussels and stormwater.
Both Mashpee and Wakeby have been impaired since 2001, Mr. Eichner said.
“It was impaired at the deep level, so you wouldn’t necessarily see it,” Mr. Eichner said, after looking through past pond and lake steward reports and snapshots from 2001 until now. “Upper portions of the pond seemed to be okay. That’s sort of the history that you have seen at this point. You got to having these blooms in Wakeby only recently. But the bottom, deeper portions of this pond have been impaired for quite a long time.”
Mr. Eichner said looking at the sediments at the bottom of the ponds are key to monitoring them.
“I think this is why you’re getting the blooms that you’re getting in Wakeby Pond. These are being exposed to anoxic conditions, and you’re getting a lot of phosphorus that is coming off of the bottom. You get the bloom,” Mr. Eichner said. “And then that upper layer… goes deeper in the pond and goes away. It’s the function of phosphorus coming off these high phosphorus sediments, and that’s what is causing what is happening.”
Mr. Eichner said that Mashpee Pond has more phosphorus than Wakeby Pond, due to its structure. Phosphorus starts in Wakeby, and then makes its way to Mashpee.
“Water is in Mashpee for about two years. Flow from Wakeby is 68 percent of the water that gets into the Mashpee basin. Groundwater is only 8 percent of that input,” Mr. Eichner said. “It’s not going out through the Mashpee River; some portion of it is going out back into the groundwater and flowing away from the pond.”
As for the Wakeby Pond, water stays there for about a year. According to Mr. Eichner, 80 percent of the water that enters the pond is from groundwater. Only 3 percent of the water entering is wastewater.
There are external and internal sources to review, Mr. Eichner said. The biggest external source is the town’s recent wastewater project.
“Basically, both ponds are impaired,” Mr. Eichner said. “Phosphorus management really is what you need to focus on.”
Mr. Eichner told the select board that there are three plausible options to managing the pond’s poor water quality: alum treatments, hypolimnetic aeration and dredging.
Alum treatments are used in ponds to reduce phosphorus loads in order to improve water quality. Mr. Eichner said this option has been done on the Cape. For Mashpee Pond, it would cost $960,000. For Wakeby, it would be $725,000.
Hypolimnetic aeration is a technique by which a pump on the shoreline adds oxygen to the bottom layer of the pond, where most of the sediment and phosphorus is located. It has not been done on the Cape before.
“The idea is you’re trying to maintain that habitat and that cold layer without disturbing it,” Mr. Eichner said. “And you are pushing all of that phosphorus that’s in that bottom layer up into the warmer layers.”
Hypolimnetic aeration in Wakeby would cost $1.5 million, and $2 million in Mashpee Pond.
Dredging, Mr. Eichner says, is the most expensive option, and is not the most logically or structurally sound. It would cost $21 million to do.
“We’ve had other projects where we’ve looked at dredging. It was a roadblock because of sediment reuse,” Ms. Fisher told the select board on Monday evening. “So when you remove the soil from the bottom of these ponds, you have to safely be able to put it somewhere else, and not affect groundwater or drinking water sources. And that is very difficult.”
Ms. Fisher added that the permitting process for dredging would become a big hurdle to the project. There also have to be plans to reuse the sediments in the pond, and other contaminants in the pond that might cause the dredging to fail.
“Dredging isn’t really an option,” Ms. Fisher said. “It would be great in a perfect world, but we don’t live in a perfect world.”
Mr. Eichner recommended an alum treatment only in Wakeby Pond to the select board, since Wakeby is the source of the phosphorus. Wait two to four years, he recommended, and after monitoring Wakeby, decide if Mashpee would need an alum treatment as well.
“We have all of our data. We know what we need to do. We know how we’re going to tackle the major forces of phosphorus: through sewering and pond treatment,” Ms. Fisher said. “We have to do our external treatment at the same time we do our internal treatment. We’re going to take a look at the septic systems, we’re going to take a look at the stormwater, we’re going to take a look at the cranberry bogs. It’s a collective source. It’s what we can control. We know we can control those sources.”
Ms. Fisher said that the internal forces they will also tackle include the town’s on-site septic systems and yard practices.
“Phosphorus is in everything. So we need to look at external and internal. We have a good management plan moving forward to do that,” Ms. Fisher said. “This is just a Band-Aid for the real problem out there. And that’s the external sources, our wastewater.”
