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When the current 520 bridge goes away, it may not go far.

The Duwamish River, best known for decades of pollution and superfund status, may see new life through the old 520.

For a couple of Seattle architects the 33 leftover pontoons produced some food for thought. Not so much for what they'd add to the Duwamish, but for what they'd subtract.

I like to think of it as kind of a floating planter box. You're taking away concrete and you're filling them in with soil to grow things or the opportunity to allow them to host different wetlands, says architect Nicole Lew.

Each pontoon amounts to 10 million pounds of concrete. You can't put that in a landfill-- but you can put them in the Duwamish River to support Lew and David Dahl's South Park Food Bridge.

It'd be for urban agriculture and really creating a waterfront park because I think people love being on the water, says Lew.

The view from above shows the 33 pontoons lining the banks of the Duwamish. The boardwalk a barrier between a community farm on one side and a floating wetland on the other.

You would see a linear park created by stringing the pontoons together, says architect David Dahl.

Dahl and Lew presented the winning concept in a contest on how best to turn the old 520 into something new and sustainable.

I think people were really intrigued by the idea and maybe it could become a reality, says Lew.

Lew and Dahl have only floated the idea, but it is gaining ground. A concrete concept they hope one day takes root in the Duwamish.

The current plan is to break down the pontoons into smaller pieces and remove them from the site. But a spokesman for the company building the new 520, Kiewit/General/Manson, says it will consider the ideas that came out of the contest.

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