Study Pinpoints Source of Pond Pollution
By MARK ALAN LOVEWELL
The primary reason for the decline in water quality in Lake Tashmoo and Lagoon and Sengekontacket ponds is nitrogen coming from septic systems.
This is the conclusion of the Nitrogen Isotope Analysis Project, a study of shellfish and eelgrass conducted last summer. Samples were collected from 13 sites in the three coastal ponds between May and September; the samples then were analyzed by the U.S. Environmental Protection Agency lab in Narragansett, R.I., to determine the source of the nitrogen: animal/human waste, fertilizer or acid rain.
In all but one of the 13 sample areas, the source was identified as animal/human waste - in other words, nitrogen leaching from the septic systems of homes along the watershed.
"The study of nitrogen isotope is a tool we can use right off the bat to demonstrate wastewater nitrogen is reaching our coastal ponds," Martha's Vineyard Commission water resource planner Bill Wilcox said. "If we are able to continue this study we will be able at some point to determine the percentage of what is coming from wastewater and by what percentage is coming from fertilizer and the atmosphere."
He added: "This is a first step. It is a good first step."
The results will be useful in approaching town boards of health about changing bylaws to require denitrifying systems or encouraging homeowners to install them on their own initiative.
"We need to start to treat our own waste differently to maintain and/or improve the water quality of our coastal ponds," the report states.
The project was funded by the Massachusetts Coastal Zone Management office, through its Non-Point Source Pollution Grants Program. "This is the first time they have funded this kind of analysis in the state," said Mr. Grunden, who will discuss the results in a public meeting at noon today at the MVC offices in Oak Bluffs.
Nitrogen loading in Island ponds has been a problem for years. At excessive levels, the buildup causes algal blooms, which choke off life in the pond and in particular is harmful to shellfish.
"The plankton, those single-celled plants become so thick in a pond that it cuts down on the sunlight penetrating through the water column," Mr. Grunden said. "The result is that the eelgrass below doesn't get the sunlight it needs.
Along with the eelgrass, bay scallops and oysters are among the first to become stressed as living conditions deteriorate. Fisheries biologists already know the bay scallop decline in Vineyard waters is linked to the collapse of the eelgrass beds years ago. Recovery is slow, particularly in Sengekontacket Pond.
The report states: "The consequences of adding ever greater amounts of nitrogen included reduced water clarity, growth of macro-algae, loss of eelgrass, increase in phytoplankton populations, increase in organic matter smothering bottom dwelling shellfish and a shift away from desirable species like the bay scallop to the less desired species such as snails and other detritus feeders."
The study involved sampling shellfish or eelgrass at three sites in Lake Tashmoo and five each in Lagoon and Sengekontacket ponds. A site in Narragansett Bay was used as the control.
Samples of eelgrass and meats from quahaugs, bay scallops and ribbed mussels were gathered every two weeks, then dried at the Martha's Vineyard Shellfish Group's solar hatchery at Lagoon Pond. The dried parts were ground down with a mortar and pestle and made into small samples measurable down to one hundredth of a microgram. These were sent to the Narragansett lab.
Only one site - the culvert that connects Sengekontacket to Trapp's Pond in Edgartown - produced low nitrogen readings and signs that the existing nitrogen came from fertilizer. The samples were lower than the control site in Narragansett Bay.
Mr. Grunden speculated the low reading was due to the site's proximity to the Caroline Tuthill Preserve, a 150-acre parcel of conservation land on the shore of Sengekontacket that belongs to Sheriff's Meadow Foundation. Conservation land insulates the pond from nitrogen loading, he said.
He added that he wondered whether the fertilizer in the Trapp's Pond samples might be coming from the Edgartown Golf Club.
The Coastal Zone Management grant totaled $5,361, most of which was used to purchase the needed $3,200 scale that was used to measure the weight of the samples down to a one hundred of a milligram. The efforts of three town shellfish departments, The Martha's Vineyard Shellfish Group, the Lagoon Pond Association, Tisbury Waterways Inc., and Friends of Sengekontacket Inc. were of great help.
Mr. Grunden said the collection of samples was done by many volunteers, one in particular Bob Ford of Oak Bluffs.
Mr. Wilcox said the results of the study will be valuable in encouraging efforts at the municipal level to protect and improve water quality in the ponds.
"There is a whole dialogue that needs to happen with the town boards of health, as to what role they should play in managing nitrogen entering our ponds. The boards of health main area of focus is public health. I believe the quality of the public ponds is part of their area of focus," he said.
Individual homeowners also may undertake treatment options which remove anywhere from 50 per cent to 90 per cent of the nitrogen from waste. They range from putting an additive in the septic tank to retrofitting a device that pumps air into the septic system.
"Acid rain is a source of nitrogen but that requires national level regulation. Fertilizers are a source but there is no regulatory hook to control its use, only added education," Mr. Wilcox said. "But wastewater is probably the source that we have a chance to do something about at the local level."