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The Gowanus Canal is polluted with heavy metals — but for one day only it’s going acoustic.

The second-ever Banjo Toss contest is capping-off this year’s Brooklyn Folk Festival, giving attendees of the weekend-long hootenanny of American roots music a chance to strum the rustic instrument before taking turns throwing one in the waterway.

The Banjo Toss, along with the festival, is the brainchild of jug band musician and Red Hook resident Eli Smith, who said the idea for the competition came out of his love-hate relationship with the instrument he’s played for 15 years.

“Sometimes I just want to throw my banjo across the room,” said Smith. “And I thought, that might be a pretty fun idea for a contest.”

To avoid destroying numerous banjos, Smith decided to have contestants pitch one instrument repeatedly into a body of water — which meant the game couldn’t go down in 2012 when the festival was held at a landlocked venue. But this year’s Gowanus-based Bell House will allow folks to launch the five-stringed javelin right into the Lavender Lake.

A rope tied to the sacrificial banjo will act as a watermark to determine who throws it the farthest, as well as a life-line to draw the instrument back on to land.

The clawhammer master added that he will provide contestants with gloves to protect them from the toxins, oil, human waste, and gonorrhea floating in the Venice of Brooklyn. The winner will receive a free banjo, which they can either learn to play or — since they’ve had enough practice already — sling into the canal.

Smith said competitors are free to try any track-and-field technique they want in the banjo toss.

“It’s really freestyle,” said Smith, who personally favors a shot put approach. “If you have an idea how to throw it, go for it.”

Of course, the fun won’t be limited to chucking long-necked music-makers into a federally-designated Superfund site.

The folk festival, which kicks off April 19, will showcase dozens of bluegrass, American folk, jazz, blues, gospel, soul, and foreign ethnic artists based in Brooklyn — plus a couple of nationally-known acts like the Cactus Blossoms and the Great Smoky Mountain Bluegrass Band.

Smith said he modeled the exhibition on the classic Newport Folk Festival, with all genres of traditional music, acoustic and electric alike, represented.

“It’s got a great feeling to it, it’s a welcoming place,” said Smith. “There’s so much great talent in Brooklyn, I was surprised no other venue or promoter has tapped into it already.”

The Fifth Annual Brooklyn Folk Festival at the Bell House [149 Seventh St. between Second and Third avenues in Gowanus, (718) 643–6510, www.thebellhouseny.com]. Begins April 19, 6:30 pm. Banjo Toss April 21, 1 pm.

Reach reporter Will Bredderman at wbredderman@cnglocal.com or by calling (718) 260-4507. Follow him at twitter.com/WillBredderman.