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ROBERT SIEGEL, host:

This is ALL THINGS CONSIDERED from NPR News. I'm Robert Siegel.

MICHELE NORRIS, host:

And I'm Michele Norris.

One year ago today, an explosion rocked the Deepwater Horizon oil rig. It was drilling an exploratory well for BP some 40 miles off the Louisiana coast. Fire engulfed the platform and two days later the rig sank to the sea floor. Eleven crew members were killed, 16 were injured and the resulting gusher would last nearly three months and spill roughly four million barrels of oil into the Gulf of Mexico.

SIEGEL: At the time, many along the Gulf Coast feared that BP oil would inundate the coastal wetlands that produce so much of the country's seafood. A year later, you can find oil in about half of the 1,000 miles of marsh and beach that were originally hit by the spill. In a moment, we'll hear why some efforts to study the spill's environmental impact have been delayed or kept secret.

But first, NPR's Elizabeth Shogren reports that the amount of oil still in the Gulf Coast marshes is surprising, even to the experts.

ELIZABETH SHOGREN: On this coastal marsh south of New Orleans, oil still saturates a 30-foot-wide stretch. Where hip-high grass should be, the oil has formed a hard, dark mat.

Mr. SCOTT ZENGEL (Scientist; Contractor, National Oceanic and Atmospheric Administration): It hasn't weathered or degraded much since it came ashore in early June.

SHOGREN: Scientist Scott Zengel has spent a lot of time in this remote area known as Bay Jimmy. He's a contractor for the National Oceanic and Atmospheric Administration, overseeing the marsh survey crews. He says lots of BP oil came ashore here and stayed.

Mr. ZENGEL: It knocked down the vegetation and it formed this tarry coat on top and trapped oil underneath.

SHOGREN: Crews documented this oil for weeks, but it was September before one scientist dug into the matted grass.

Mr. ZENGEL: Let me show you what it looks like underneath here.

SHOGREN: Zengel digs down about six inches. His shovel reveals a thick layer of glistening oil.

Mr. ZENGEL: It's kind of like a thin peanut butter-type consistency.

SHOGREN: The wind, rain, sun and even waves can't get to it to break it down.

Mr. ZENGEL: We just saw how bad it was. I don't think anyone had quite seen this before.

SHOGREN: Biologists are so worried about birds getting oiled here, they set up what they call scare cannons to keep them away.

(Soundbite of cannons)

SHOGREN: Scientists generally agree that after an oil spill, marshes fare best when they're left to recover on their own. Cleaning them usually does more harm than good. These marshes provide a nursery for shrimp, crabs and fish, and protect New Orleans from storm surges. Even before the oil spill, they were rapidly disappearing.

Zengel decided to use this strip of marsh to test a variety of cleanup methods to see if some technique could help save this marsh and other heavily oiled areas.

Mr. ZENGEL: It's kind of like a mad science experiment.

SHOGREN: He shows me plots where workers tried everything from vacuums to power washing to chemicals. Nothing worked. Then Zengel came up with an idea of raking the marsh with pitchforks to get the tarry plants to stand up, and then cutting them with a hedge clipper. That uncovers the gooey stuff scientists call mousse.

Mr. ZENGEL: We tried to smear a lot of that mousse up on to the vegetation that we were raking up so that when we cut it, we could remove all that material.

SHOGREN: The plots where Zengel used this technique do look better than the others. The tarry mat is gone. The surface oil is weathering and when he digs, the oozy layer is barely visible.

Mr. ZENGEL: And, also, compared to where we just were, you see a good bit more vegetation sprouting. Now, is it anywhere close to recovered or looking like the natural marsh behind this area? No.

SHOGREN: Some scientists argue this area should be excavated and replaced with fabricated marsh. Zengel's the first to admit his work is risky.

Mr. ZENGEL: Some of these treatments may still end up being more damaging in the long run. We just don't know. And we had to make a decision, you know, winter and early spring are the time to be in the marsh if you're going to do something.

SHOGREN: First, manual crews started using the technique.

(Soundbite of excavator)

SHOGREN: Now heavy machines are mimicking the method. We watch as a massive long-arm excavator pulls a makeshift rake through the marsh.

(Soundbite of excavator)

SHOGREN: About a third of the 400 miles of Louisiana coastal wetlands hit with BP oil still show some visible signs. But less than 10 miles have enough oil to warrant such an aggressive cleanup. And in many once-oily marshes, green plants are now sprouting.

Still, scientists say BP oil is in this ecosystem for the long term. Tulane University oceanographer Brad Rosenheim says it's very important to tease out whether BP oil is behind bad things that happen to wildlife and plants in the future.

Professor BRAD ROSENHEIM (Oceanographer, Tulane University): For instance, if a section of our wetlands disappears in the next few years, the natural question would be, well, was that because of the oil spill or was that because of business as usual on the Louisiana coastline? And in order to answer that question, we obviously need a pretty good fingerprint of that oil. We need to monitor it through time.

SHOGREN: He's been regularly sampling oil from the marshes in Bay Jimmy and from the beach on Grand Isle. He analyzes the samples to pinpoint chemical markers that will identify BP oil. Rosenheim says one thing already is clear: The oil will be in the beaches and marshes for decades to come.

Elizabeth Shogren, NPR News.

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