NOAA's Response and Restoration Blog

An inside look at the science of cleaning up and fixing the mess of marine pollution


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Solving the Case of the Mystery Sheen

Ocean with sheen. Image: U.S. Coast Guard.

Can you see the sheen in the distance? That lighter blue just below the horizon caught the attention of the U.S. Coast Guard helicopter crew that led to the discovery of a natural oil seep off the coast of San Diego, California. The sheen’s narrowing on the left with broader “feathering” on the right suggested a submerged source. Image credit: U.S. Coast Guard.

By Jordan Stout, NOAA Scientific Support Coordinator

In early March 2017, a U.S. Coast Guard helicopter was returning to its home base when the aircrew spotted a silvery sheen in the water about 35 miles west of San Diego, California.

I imagine the conversation among the crew went something like this:

Hey, that looks like it might be oil…

Is that from yesterday’s spill?  No sir, too far away…

Did any vessels sink out here recently?  Nothing’s been reported…

Do you see any debris in the water?  No sir, but I think I see bubbles coming to the surface within the sheen…

We’re pretty far offshore.  How deep is the water here?  Chart says about 300 fathoms (roughly 1,800 feet)…

Any other petroleum sources out here?  Not that I’m aware of.  Let’s call it in…

Reporting that finding of a mysterious sheen of oil on the ocean’s surface triggers a forensic process that typically requires the highly skilled staff of multiple federal agencies. In this instance, it included U.S. Coast Guard, several NOAA offices, and the U.S. Geological Survey.

Most Coast Guard aircrew members have overflown an oil spill at some point in their careers and many have seen our overflight job aid or taken the NOAA on-line training to help them identify and describe oil on the water.

In this case, the aircrew took photos and reported their observations to their command center. The crew’s initial report stated the sheen spanned approximately 400 yards by 10 yards, was patchy, thin, and unrecoverable.

Initial theories about the mystery sheen included:

  • A passing ship that spilled oil (by accident or intentionally)
  • A recent vessel casualty
  • An old shipwreck
  • An offshore disposal site, or
  • A natural oil seep

Theories in place, the next step was going through the list and systematically eliminating what could be causing the sheen.

Sheen shape, size, and area traffic

The Coast Guard is well known for conducting search and rescue operations when vessels are in distress and lives are at stake. It’s also responsible for ensuring safe and lawful maritime commerce. A check showed no large vessels had been in the area recently nor had any vessels (small or large) been reported missing.

The aircrew’s report of an oil “sheen” indicated to folks on shore that they had seen a very thin layer of oil (<50 microns) on the water’s surface.  That’s pretty darned thin, if you consider that a normal sheet of paper is about 100 microns thick.  Such thin layers don’t normally persist very long in the environment, so it wasn’t expected to stick around very long.

If the sheen had been spread out and patchy, it might be consistent with a spill from a passing ship or an earlier spill that had moved some distance over time.  Instead, the photos showed the sheen as a long linear feature, very narrow at one end and spread out and dissipating at the other (downstream) end.

During subsequent overflights for the next few days, the Coast Guard continued to see a similar sheen in the same location.  Because the original sheen would have dissipated in a matter of hours, these repeated sheen observations seemed to confirm an on-going, fixed source. But, what was it?

Both sunken vessels and natural seeps can release gases, so observing bubbles was interesting but not conclusive.  It could have been a sunken vessel, but in most cases, a sunken vessel will only release bubbles over a short period of time until the pockets of trapped air are all gone.  However, natural seeps can release gas bubbles continuously or sporadically for years.

Satellites, sunken ships and chemical analysis

Satellites – We checked with NOAA’s environmental satellite office, the National Environmental Satellite, Data, and Information Service, to see if any of their environmental satellite data/imagery had picked anything odd at that location recently or in any of their archived imagery.  They looked but they didn’t see anything obvious, but that’s not conclusive either.  Even the best satellite sensors for detecting oil on water have limitations.  There may very well have been a sheen out there, but it could not be distinguished with satellite data either because the winds were not optimal, the sheen was too small, or there was too much background “noise” in the data.

Maritime history – NOAA plays an important national role in identifying and protecting our nation’s maritime history.  As part of that stewardship role, NOAA and the Coast Guard partnered to evaluate which of the 1,000’s of shipwrecks in United States water might pose a substantial pollution threat.  This effort, called Remediation of Underwater Legacy Environmental Threats, or RULET, resulted in a series of reports in 2013.  No potentially polluting shipwrecks were identified off San Diego through the RULET program.

Charting – Another data source, the Resources and Under Sea Threats, called RUST, which includes shipwrecks and other potential pollution sources, only identified an ammunition dumpsite offshore of San Diego.  That site appears on the NOAA nautical charts, but is over 13 miles away.

Chemistry – A Coast Guard ship was sent out to obtain some sheen samples.  Chemical analysis from their Marine Safety Lab revealed the sheen contained petroleum oil with characteristics most resembling those of moderately weathered crude oil.  A vessel leaking fuel would not show a crude oil signature, but a natural seep would.

Two men on boat's deck taking water samples. Image USCG.

The helicopter crew guided a boat-based sampling team to the area. Samples were sent to the Coast Guard’s Marine Safety Lab for analysis. Image credit: U.S. Coast Guard.

And the Mystery Sheen is: a natural seep?

 The seas off Southern California are known to have hundreds of sub-sea natural oil seeps. Most of them are found off Santa Barbara, and quite a few off Los Angeles, according to Thomas Lorenson, research geologist with the U.S. Geological Survey at the Pacific Coastal and Marine Science Center.

“Recent seafloor mapping south of Santa Catalina Island shows subsea features like mounds that are often associated with oil or natural gas seepage, so it is not too surprising to discover another seep,” said Lorenson. “Luckily a person can pay a visit to a famous oil seep found on land at the La Brea Tar Pits in Los Angeles and imagine what they may look like underwater.”

So though we’ve not (yet) gotten visual confirmation of seep with a submersible or remotely operated vehicle, a natural seep on the sea floor remains the best explanation for this mystery.

 

Jordan Stout is the Scientific Support Coordinator in California, providing scientific input to the U.S. Coast Guard and Environmental Protection Agency for spills of oil and hazardous material.

 


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Incident Responses for July 2017

Alaska coastline with mountains. Image: NOAA.

The U.S. Coast Guard requested a vessel drift analysis and trajectory for the 400 gallons of diesel fuel associated with the FV Grayling that capsized off the coast of Kodiak, Alaska July 21, 2017. The Alaska ShoreZone photo shows the gravel shoreline most immediately adjacent to the sinking location of the Grayling. Image credit: NOAA.

Aug. 3, 2017 – Every month our Emergency Response Division provides scientific expertise and services to the U.S. Coast Guard.

Our services include everything from running oil spill trajectories to possible effects on wildlife and fisheries, and estimates on how long the oil may stay in the environment.

In July, our scientific support coordinators responded to requests for a vessel drift analysis and trajectory, an analysis of currents and winds to help identify the potential source of an oil sheen, and list of sensitive species and resources that could be effected from warehouse fire near a river.

Our Incident News website has information on oil spills and other incidents where we provided scientific support.

Here are some of this month’s responses:

Drifting Fisheries Buoy Trajectory

FV Grayling, Kodiak, AK

Tanker Truck Spill Florida Keys MM 70

Mississippi Canyon 736 Platform Discharge

North River Street Fire – Portland, OR

Wreck 1487

UTV Eric Haney

FV Donna

FV Ketok

FV Bunchie


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Understanding How Oil Reacts on Water: A Simple Experiment

Rainbow sheen.

Rainbow sheen, such as the one shown here from a different incident in the Gulf of Mexico, has been spotted near the leaking natural gas well off the Louisiana coast. (NOAA)

Have you ever seen a rainwater puddle on a street and wondered why it seemed to have a rainbow floating on top? That rainbow effect is caused when oil on the street floats to the top of the puddle.

Understanding how oil and water react together is an essential part of the science of cleaning up oil spills. One of the goals of NOAA’s Office of Response and Restoration (OR&R) is to share our scientific expertise and experience. Fostering scientific understanding of oil spills helps everyone prevent and prepare for marine pollution.

Here is a simple experiment for elementary-aged children that can be done with common household items to understand how oil reacts in water.

OR&R has more experiments and activities for elementary school students and life-long learners on our education page.


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Mysterious Oil Spill Traced to Vessel Sunk in 1942 Torpedo Attack

Aerial photo of an oil sheen on the ocean.

U.S. Coast Guard overflight photo taken on July 17, 2014, showing a mysterious oil slick off the North Carolina coast. (U.S. Coast Guard)

A few weeks ago a North Carolina fisherman had a sinking feeling as he saw “black globs” rising to the ocean surface about 48 miles offshore of Cape Lookout. From his boat, he also could see the tell-tale signs of rainbow sheen and a dark black sheen catching light on the water surface—oil. But looking around at the picturesque barrier islands to the west and Atlantic’s open waters to the east, he couldn’t figure out where it was coming from. What was the source of this mysterious oil?

Describing what he saw, the fisherman filed a pollution report with the U.S. Coast Guard. On July 17, 2014, a U.S. Coast Guard C-130 aircraft flew over the site and confirmed the presence of a sheen of oil in the same vicinity. Based on the location and persistence of the sheens, the responders suspected the oil possibly could be leaking from the sunken wreck of the steamship W.E. Hutton, 140 feet below the water surface. Shortly after, archeologists confirmed that to be the case.

Balck and white photo of a ship in 1942.

A 1942 photo of the W.E. Hutton. (U.S. Coast Guard)

At the Bottom of the Graveyard of the Atlantic

This area off of North Carolina’s Outer Banks is known as the Graveyard of the Atlantic. The combination of harsh storms, piracy, and warfare have left these waters littered with shipwrecks, and because of the conditions that led to their demise, many of them are broken in pieces. In the midst of World War II, on March 18, 1942, the W.E. Hutton was one of three U.S. vessels in the area torpedoed by German U-boats. Tragically, 13 of the 23 crewmembers aboard the ship were killed. The Hutton’s survivors were rescued by the Port Halifax, a British ship.

When the steam-powered tanker was hit by German torpedoes, the Hutton was en route from Smiths Bluff, Texas, to Marcus Hook, Pennsylvania, with a cargo of 65,000 barrels of #2 heating oil. An initial torpedo hit the starboard bow, and the second hit to the port side came 10 minutes later. The ship sank an hour after the first hit, eventually settling onto the seafloor. Today, it is reportedly upside down, with the port side buried in sand but with the starboard edge and some of its railing visible.

The wreck of the W.E. Hutton also is located in the NOAA Remediation to Undersea Legacy Environmental Threats (RULET) Database. Evaluated in the 2013 NOAA report “Risk Assessment for Potentially Polluting Wrecks in U.S. Waters,” this wreck was considered a low potential for a major oil spill because dive surveys “show all tanks open to the sea and no longer capable of retaining oil.”  However, as the fisherman could observe from the waters above, some oil clearly remains trapped in the wreckage.

This shipwreck was described by wreck diver and historian Gary Gentile as having “enough large cracks to permit easy entry into the vast interior.” Another wreck diver and historian, Roderick Farb, noted that the largest point of entry into the hull is “about 150 feet from the stern,” through a “huge crack in the hull full of rubble, iron girders, twisted hull plates and other wreckage.”  This wreck is the closest one to the spot where the fisherman first saw the leaking oil, and given the Hutton’s inverted position and such cracks, we now realize the possibility that the inverted hull has been trapping some of the 65,000 barrels of its oil cargo as well as its own fuel.

An image of the wreck of the W.E. Hutton laying on the ocean floor.

A multibeam scan of the wreck of the W.E. Hutton taken in 2010. (NOAA)

Solving the Problems with Sunken Shipwrecks

On July 21, 2014, a commercial dive company contracted by the U. S. Coast Guard sent down multiple dive teams to the Hutton’s wreck to assess the scope and quantity of the leaking oil. The contractor developed and implemented a containment and mitigation plan, which stopped the flow of oil from a finger-sized hole in the rusted hull. It is not known how much oil escaped into the ocean or how long it had been leaking before the passing fisherman noticed it in the first place.

NOAA’s Office of Response and Restoration, led by Scientific Support Coordinator Frank Csulak, provided the U.S. Coast Guard access to historical data about shipwrecks off of North Carolina, survey information, including underwater and archival research, and the animals, plants, and habitats at risk from the leaking oil. Our office frequently provides scientific support in this way when a maritime problem occurs due to sunken wrecks. They may pose a significant threat to the environment, human health, and navigational safety (as an obstruction to navigation). Or, as in this case, shipwrecks can threaten to discharge oil or hazardous substances into the marine environment.

Last May, our office released an overall report describing this work and our recommendations, along with 87 individual wreck assessments. The individual risk assessments highlight not only concerns about potential ecological and socio-economic impacts, but they also characterize most of the vessels as being historically significant. In addition, many of them are grave sites, both civilian and military. The national report, including the 87 risk assessments, is available at “Potentially Polluting Wrecks in U.S. Waters.” Several of those higher-risk wrecks also lie in the Graveyard of the Atlantic, but as we discovered, it is difficult to predict where and when a rusted wreck might release its oily secrets to the world.

The Office of Response and Restoration’s Doug Helton and Frank Csulak contributed to this post.