Data shows dredging improved Sengekontacket water quality

Photo by Steve Myrick

Water quality data collected over the past five years shows that Sengekontacket Pond is improving since two major dredging projects changed the circulation flow inside the popular salt pond, according to local shellfish wardens.

The Massachusetts Division of Marine Fisheries (DMF) ordered Sengekontacket Pond closed to shellfishing during the summer months from 2007 to 2011 because of high fecal coliform detected during routine water quality testing. DMF allowed most areas of the pond open to shellfishing last summer, except for short periods following rainfall, when contaminants wash into the pond from shorelines, parking areas, boat ramps, and from Beach Road. Sengekontacket Pond is under the same rainfall closure restrictions this summer.

Those restrictions require the pond closed to shellfishing for five days after a rainfall of more than two-tenths of an inch in July. In August, the pond must be closed for five days after a rainfall of an inch or more.

Rain over the weekend forced Oak Bluffs and Edgartown shellfish wardens to extend a closure of Sengekontacket Pond to shellfishing, first put in place on Friday, July 12, another five days until Friday, July 19.

Long process

In 2008, following a three-year permitting process, Edgartown dredged about 10,000 cubic yards of sand from a large area just east of big bridge, near the Beach Road side of the pond. Edgartown estimated the cost of the project, considerably reduced by using the town owned dredge, at $300,000.

In 2010 and 2011, Oak Bluffs dredged about 57,000 cubic yards of sand, creating a channel from big bridge to little bridge. The cost of that project was $1.2 million. Oak Bluffs taxpayers borrowed $500,000 to finance part of the project. The balance was funded by the sale of dredged sand to a private homeowners association.

Cleaner water

Edgartown shellfish constable Paul Bagnall is convinced by water quality data and his own observations that the dredging has improved water quality.

“I think we’re getting more flow between the bridges,” Mr. Bagnall said. “The area that is the biggest problem, down by Trapps Pond, is getting better. We’re not getting more tide, but once the water is in the pond, it’s moving around more efficiently.”

Bacteria counts

The DMF is responsible for insuring that shellfish taken from the pond are safe to eat. Most strains of coliform are not harmful to humans, but they are considered an indicator that other dangerous pathogens, or disease causing bacteria, are present. Common symptoms caused by the harmful pathogens are diarrhea and vomiting. Rarer, but more serious diseases caused by pathogens are dysentery and polio.

In recent years, DMF has taken water samples from 16 different locations, or stations, in Sengekontacket Pond. While it is difficult to compare water quality measurements from year to year, because the amount of testing varied widely, the data does show a general trend toward cleaner water.

In 2007, for example, only 45 samples were taken, but 44 percent of them were above the limit considered safe. In 2009, after the first phase of dredging on the Edgartown side of the pond, DMF took 108 water samples; 34 percent exceeded the safety standard.

In 2010 and 2011, when the Oak Bluffs project created a deeper channel between Big Bridge and Little Bridge, sampling was far more extensive. In 2010, 17 percent of the samples exceeded the standards, in 2011 16 percent exceeded the standards. In 2012, only 1 percent of the samples exceeded the acceptable level of bacteria.

That trend has continued in 2013, according to Mr. Bagnall.

“This winter, we didn’t have a single station that flunked,” he said.

Nitrogen problem

While bacteria counts are reason for optimism, it appears that dredging has done little to improve nitrogen levels, considered the most serious threat to the pond’s health, according to Oak Bluffs shellfish constable David Grunden.

Excessive nitrogen from shoreline septic systems, fertilizer runoff, and natural sources could eventually threaten shellfish and other aquatic life, according to a Massachusetts Department of Environmental Protection (DEP) study of the pond.

Excessive nitrogen reduces oxygen in the water, which kills off eel grass and other plants critical to aquatic life.

State environmental officials are in the process of setting targets for the town to reduce nitrogen going into the pond.

Few expected dredging to address nitrogen loading.

“I don’t think the dredging touched much at all with nitrogen loading,” Mr. Grunden said. “There was very small expectation for that. Nitrogen load numbers are coming back about where they were.”

Mr. Grunden did not respond to repeated requests for data that would illustrate the trend of nitrogen loading in Sengekontacket Pond, then said he could not find the test result records.

Oysters to the rescue

This month, Edgartown began an oyster cultivation project in Major’s Cove, a part of Sengekontacket Pond where water samples are frequently higher than the accepted standards. Oysters are efficient at removing nitrogen from the water.

Edgartown voters approved $48,500 in April to establish an oyster fishery in Sengekontacket Pond.

The money will fund the equipment to grow the oysters, and buy 500,000 oyster seed. In two or three years, about 500,000 bushels of oysters should be ready for harvest.

Sengekontacket has never supported wild oysters, but a test project last summer showed good growth and survival rates, according to Mr. Bagnall.