Study holds hope, caution for Edgartown Great Pond, with future growth an issue
The final report of the Massachusetts Estuaries Project study of Edgartown Great Pond released last week concludes that the cherished natural resource is, in general, "moderately impaired," a description that means "it's close to being good," according to a scientist who reported the results of the study.
The study suggests that nitrogen loading from future development must be mitigated in order to restore the 890-acre salt pond to prime health, and it suggests that some combination of reduced nitrogen loading and frequent opening of the pond to the ocean's cleansing tides are the most effective way to preserve the pond's water quality in the near future.
"The system is not severely degraded," said Brian Howes, a scientist who helped write the report, and explained it in a presentation Monday in Edgartown. "Moderately impaired means it's close to being good."
By contrast, Mr. Howe noted that the state has studied a number of salt ponds on Cape Cod where nitrogen loading has virtually wiped out plant and animal life.
Mr. Howe and other scientists who have studied 11 years of data collected from Edgartown Great Pond and its surrounding watershed have created a mathematical model that can be used to predict how development, sewering, septic systems, wastewater plant discharge, and other variables will affect the pond.
"It's good news," said Edgartown shellfish constable Paul Bagnall. "If you want to have sustainable development within a watershed, you need to have very good numbers. [If] you know what the number is, you know what the expected result is. Let the voters decide. Not all the land is subdivided, not all the uses are set, in terms of what you can do with a buildable lot. As we make those decisions, we have better information."
Source, of course
The study focused squarely on the amount of nitrogen introduced into the pond. Nitrogen promotes plant growth, including invasive weeds, and also algae. That, in turn, reduces the oxygen levels in the water. Without adequate oxygen levels, the pond's ecosystem is disrupted. Beneficial plants, such as eel grass, and animal species, such as clams, oysters, and fish, cannot survive.
The study confirmed earlier estimates about nitrogen levels and refined the measurements to a much higher level of accuracy, according the scientists and others with a stake in water quality issues.
It showed that the largest source of nitrogen entering Edgartown Great Pond, 37 percent of the total, is wastewater from the septic systems of homes located within the watershed. The second largest source, 27 percent, is acid rain, generated mostly from air pollution carried by prevailing winds from Northeastern and Midwestern industrial areas. The next highest source, 16 percent of the total, is effluent from the town's wastewater treatment plant. The study concludes that nitrogen from the treatment plant will be significantly reduced in future years, because of an upgrade to the plant completed in 1996. The upgrade decreased the amount of nitrogen flowing out of the plant by more than 71 percent, according to figures from the study. The plume of reduced nitrogen in wastewater from the plant is just now entering the pond, after traveling through the groundwater system.
The wastewater plant, and its effect on the Great Pond, has been the subject of a series of costly lawsuits by citizens groups, dragging on for more than a decade, and costing the town about $400,000 in legal fees.
"I was pleased to hear that the old treatment plant is becoming an insignificant amount," said Joe Alosso, the plant manager. "We've spent $13 million upgrading it, and it has been a success."
Scientists, town officials, and others concerned about the health of Edgartown Great Pond agree that the study provides invaluable tools for making decisions, and they agree those decisions will be difficult, and probably costly.
The study suggests that reducing the amount of nitrogen from private septic systems coming into the pond, along with opening the barrier beach to allow tidal flushing every 45 days, would produce an immediate improvement in the health of the pond.
One way to reduce the wastewater load is to expand the town's sewer system. Mr. Alosso said work is completed, or in some stage of planning, to sewer an additional 394 homes.
"The developments that have sewers are Edgartown Meadows, Old Purchase Road, and Donald-Bin/Duncan-Close, 119 homes," said Mr. Alosso. "The subdivision we are working on now is Island Grove, 150 homes." He added that infrastructure recently installed by The Field Club would allow an additional 125 homes on Meshacket Road, Llewellyn Drive, Road to the Plains, and the Kitts Field Development to tie in to the sewer system. While tie-in is still optional, if all those homes were hooked up, it would reduce nitrogen loading in the watershed by about 30 percent, as suggested by the Massachusetts Estuaries Study, according to Mr. Alosso. He said town officials would soon discuss mandatory sewer hook-ups within certain districts.
"We're going to meet with the board of health to talk about the possibility of having an overlay district," said Mr. Alosso. "You need something to base that overlay district on, and this report may be just that tool."
Bill Wilcox, water resource planner for the Martha's Vineyard Commission, is concerned about the pressure future development might put on the pond.
"What happens when we have another 300 to 400 houses in the watershed that aren't tied into the sewer?" said Mr. Wilcox. "We need to address the nitrogen load coming in, that's where it gets costly, and it needs to be done in a well thought out manner."
Planners can immediately begin using the mathematical model from the project to evaluate various development scenarios, said Mr. Wilcox. "We can start to look at the watershed of these ponds and the nitrogen issues in a science-based, and comprehensive way."
According to figures compiled by the Martha's Vineyard Commission, annual nitrogen loading could nearly double from today's levels, if all development allowed by current zoning regulations happens.
Mr. Bagnall said tidal flushing has proved beneficial. He supervised four openings this past summer, and noted an improvement in the growth of eel grass, which serves as a breeding ground for organisms at the bottom of the food chain. But he cautions that creating a breach every 45 days may not be possible.
"Our breach schedule is to open it as aggressively as we can, that's basically about every 60 days," said Mr. Bagnall. The timing of the breach is determined by how fast the pond fills up from rain and freshwater springs, once the breach is closed. "If you don't have two feet above sea level, you can't open the pond."
In his presentation explaining the report, Mr. Howes said the state is expected to use the report's mathematical models and other data to determine the total maximum daily load of nitrogen that the pond can absorb and remain healthy. That will be a critical figure, used by the state Department of Environmental Protection (DEP) as a benchmark for future development permits.