In a rare gift from up-Island to down-Island, the town of Oak Bluffs has received a stormwater treatment system from the Wampanoag Tribe of Gay Head (Aquinnah).

The town selectmen presented a certificate of appreciation to the tribe Tuesday, in recognition of efforts to protect the town’s main harbor from pollutants.

“We appreciate the opportunity to help,” tribal chairman Cheryl Andrews-Maltais said at the selectmen’s meeting.

The project has been in the works since 2003, when town shellfish constable David Grunden first suggested New York avenue as a possible site for a stormwater treatment system. During storm events, the harbor is vulnerable to runoff from New York avenue, which slopes uphill from the harbor and is highly-trafficked throughout the year. At the gathering Tuesday night Mr. Grunden estimated that during a half-inch rain event, runoff from the road could deposit as much as 35 pounds of sludge into the harbor.

“It was one of the sites that was giving us most trouble with suspended solids entering the harbor in each rainstorm,” Mr. Grunden recalled from a 2003 Martha’s Vineyard Commission study on the water quality in town.

Now during a storm, runoff is directed underneath New York avenue and into a stormwater treatment system.

The main element of the system is an oil grit separator, which looks like a cement block from the outside, said Bret Stearns, director of natural resources for the tribe. The separator measures 12 feet long and 8 feet wide, and requires a crane to move it. Runoff from the road enters the tank and passes through three chambers, where oil and solids are removed, before spilling out into the harbor. The entire system is powered by gravity.

“[The unit] slows the water down, so it allows for the suspended solids, sand, grit and anything heavy to sink to the bottom of the tank and before it goes into the last chamber, there is a level that skims any petroleum product off the water,” Mr. Grunden explained earlier this week.

A storm drain system was installed by the state several decades ago, but was not set up to treat the water entering the harbor. Instead, its purpose was to preserve the integrity of the road by diverting the untreated runoff into the harbor. The new treatment system, which is designed to protect the road as well as the harbor, is tied into the existing storm drain system.

This project will help protect the Oak Bluffs harbor, open in the off-season to shellfishing and home to quahaugs and steamer clams, Mr. Grunden said.

“It will improve the quality of the water that the shellfish are living in,” the constable said. He’s also looking at possible stormwater treatment projects in other parts of town, including protecting Inkwell Beach from road discharge.

The town highway department is charged with cleaning the unit of grit, sand and petroleum products periodically, which is done using a vacuum pump. According to EPA estimates, most oil grit separators have a life span of 50 years.

The town plans to begin sampling the harbor during storm events to assess the efficacy of the unit.

The EPA awarded nearly $150,000 in pollution prevention funding for the project several years ago, but because of some engineering and state highway department hurdles, the project advanced slowly at first. The road, which is owned by the state, was repaved in 2010, and state law prohibits cutting into a state road within three years of repaving. “There were some delays, but everybody just stuck together,” Mr. Stearns said.

After posing for a picture with tribal representatives, the selectmen thanked Mr. Stearns and Mr. Grunden for their efforts.

“It’s a real good thing for the town, and I am sure it will do good things for the harbor,” said board chairman Walter Vail.

The storm drain system is one of the biggest projects the tribe’s natural resources division has completed outside of tribal property, said Mr. Stearns, who acted as project manager.

“We try to enhance water quality throughout Martha’s Vineyard when we can,” he said. “It’s exciting for us to do something that we know is creating a positive impact for people on the Island of Martha’s Vineyard.”