A construction crew blasting a ledge in Annisquam to widen Bennett Street North at the foot of Washington Street ruptured a sewer main yesterday, sending an estimated 600 gallons of untreated sewage through a catch basin and into Lobster Cove.

The break in a two-inch, high-pressure line off the sewer main under Washington Street took place about 2:45 p.m., and it was isolated at about 4 p.m., according to a report written by City Engineer Frank Ventimiglia.

Public Works Director Joseph Parisi said the state Department of Environmental Protection, Division of Marine Fisheries, Shellfish Warden David Sargent and the Board of Health were notified.

The shellfish hot line reported an "indefinite emergency closing" of flats in the Little River and around Wheeler's Point.

Sargent could not be reached for comment, so it could not be determined last night if the closings were ordered because of yesterday's daylong rain, which often triggers a closing, or the ruptured pipe and spill.

The break in the line that carried sewage from the homes along the higher elevations of Bennett Street occurred near the intersection with Washington Street, said Parisi.

Cardillo Construction has been widening Bennett Street North, which has been long notorious as the city's worst paved road.

Ventimiglia said public health sanitarian Jennifer Olivier went to the scene and reported "that sewage was overflowing the trench and running down Washington Street and into a catch basin and eventually into Lobster Cove," which is surrounded by some of the city's oldest and finest homes.

"There was a 15- to 20-minute period when the sewage was being pumped out of the trench into the city's pump truck," Ventimiglia reported. "The remainder of the time the sewage was overflowing the trench."

At 10 gallons per minute, about 600 gallons could have gone into Lobster Cove, said Parisi. Lobster Cove is a thin finger at its end that is crossed by a narrow foot bridge before opening into the mouth of the Annisquam River just west of the mouth of Goose Cove.

Many fishing and pleasure boats are moored in Lobster Cove.

It was the second time in two years that raw sewage backed up into the Annisquam estuary.

Two springs ago, the sewage treatment plant malfunctioned, forcing about 600 gallons of raw sewage through a catch basin on the property and under Essex Avenue and into the river.

Parisi said he believed no homes were harmed by the break yesterday in the sewer line.

The system in Annisquam, as well as in Bay View and Lanesville to its north, are served by a balky mechanical technology - the septic tank effluent pump or STEP system. It was installed in the 1990s to carry the flows up and down the hilly terrain in the city's northernmost villages.