Sengekontacket Pond will reopen Monday to shellfishing, but the source of the bacterial contamination which led to its closure for most of the summer remains a mystery.

Edgartown shellfish constable Paul Bagnall announced on Monday that new tests of the pond’s water, done last week, showed coliform bacteria counts down to within safe levels, allowing most of it to be reopened.

But the water quality problem which kept it closed most of the past summer is yet to be addressed. Until it is resolved, the pond faces closure each summer from the start of April until the start of September.

The first closure of Sengekontacket followed water sampling on June 5, which showed contamination well above the safe level at eight of nine testing points on the pond and right on the upper limit at the ninth.

The pond was retested on June 19 and reopened despite results described as borderline.

But on July 2 another series of nine samples again showed very high coliform counts and the pond was closed again for the remainder of the summer.

As a first step to answering the riddle of the contamination, water samples as well as samples of various possible sources including bird droppings and effluent from septic systems are being tested. Those tests, being carried out at the University of New Hampshire, are still a couple of weeks from completion.

They will be subject to DNA analysis to determine whether the source of the coliform bacteria is bird, animal or human.

To this end, Mr. Bagnall said, he collected and sent off various samples, including one from the septic system of someone he described as a donor on the Boulevard, close to the pond, to be checked.

Now it is a matter of finding the money.

“I’ve now got to take the menu of samples that we need to have run and the cost to the board of the Friends of Sengekontacket and other players around the pond and see what tests we can afford,” Mr. Bagnall said, adding:

“The bill is probably going to be somewhere around $8,000. It’s about $800 a sample.”

He said simple common sense indicates a combination of sources could be responsible for the bacterial contamination.

Cormorants, which fish and nest in large numbers on the pond, were initially widely blamed, and some suggested their numbers might be reduced by culling or other means.

But the scientist running the tests, Dr. Stephen Jones, suggested, purely on the basis of similar work he had conducted elsewhere, that his DNA evidence might see the cormorants absolved of most blame.

“Our previous projects have not shown cormorants to be such a big problem in most cases,” he said. “But Canada geese can be overwhelming.”

The test results will show for sure.

“Then once you find out what percentage the birds are maybe you can get the people to do something about the nesting birds on Sarsen’s Island and elsewhere,” Mr. Bagnall said.

But the prospect of reducing the goose population — organized addling of eggs during the nesting season was discussed at a recent meeting of the Friends of Sengekontacket — also is likely to bring controversy.

Meanwhile, Mr. Bagnall remains hopeful that other measures, such as promoting better flows of water between the pond and the ocean, will help.

“I’m hopeful that just being aggressive about keeping the Little Bridge cleaned out might be enough to keep some of the pond open,” he said, adding:

“The dredging project for the Edgartown side, for which we are just beginning the permit process will help too.”

The problem remains far from solved, but on the up side, Mr. Bagnall said, there should be large numbers of shellfish in the pond to be dug following the summer closure.

“That’s the good news,” he said. “When you leave them alone they make babies.”