Why coastal oil spills can pollute for decades: study

Marlowe Hood Yahoo News 17 Jan 10;

PARIS (AFP) – Oil from the 1989 Exxon Valdez tanker spill that devastated Alaska's Prince William Sound still lies trapped beneath its beaches, continuing to pollute once pristine shores, scientists reported Sunday.

As climate change opens the Arctic region to oil exploration and shipping, the findings could prove crucial in devising effective methods for cleaning up future spills, the researchers said.

Up to now, experts puzzled over why remnants of the 11 million gallons of crude that fouled some 1,300 kilometers (750 miles) of Alaskan coastline have persisted for so long.

At first it seemed that nature, with some help from technology, would soon wash away one of the worst environmental disasters in history.

The spill decimated the region's wildlife as well as the state's fishing industry.

But within a decade it became apparent that the rate at which the oil was disappearing had dramatically slowed, from 70 percent per year to about four percent.

Today, it is estimated that some 20,000 gallons remain.

Michael Boufadel and Hailong Li of Temple University in Philadelphia, Pennsylvania wanted to find out why this oil was not been broken down through biodegradation and weathering, as had been widely predicted.

Collecting field data and running computer simulations, they found the key lay in the fact that affected beaches consisted of two layers, each with different properties.

The geographically variable impact of rising and falling water tables also played a critical role.

Oil was temporarily stored in the porous upper layer, slowing the rate at which it was subject to weathering, according to the study, published in the journal Nature Geoscience.

An environment lacking the kind of nutrients needed by oil-eating micro-organisms to thrive further protected the fossil fuel.

The second layer, while composed largely of the same materials, was far less porous: on average, water moved through the top layer 1,000 times faster.

When the water level from declining tides fell below the interface between the two layers, oil seeped from the upper to the lower stratum, especially where there was little or no freshwater discharge to compensate.

"Once the oil entered the lower layer, it became entrapped by capillary forces and persisted," the authors said.

Because of the even lower oxygen content in the sub-stratum, the crude was not degraded and has remained suspended.

The study also said that oil tends to linger on gravel beaches more than on sandy ones, pointing to evidence from previous spills: the Arrow in Nova Scotia, Canada (1970), the Metula in the Strait of Magellan, Chile (1974), and the Amoco Cadiz along the French coast of Brittany (1978).

"As global warming is melting the ice cover and exposing the Arctic to oil exploration and shipping through sea routes such as the Northwest Passage, the risk of oil spills on gravel beaches in high-latitude regions will be increased," it said.

Gravel beaches trapping oil from 1989 Exxon spill
Mary Pemberton, Associated Press Yahoo News 17 Jan 10;

ANCHORAGE, Alaska – An engineering professor has figured out why oil remains trapped along miles of gravel beaches more than 20 years after the Exxon Valdez tanker disaster in Prince William Sound.

An estimated 20,000 gallons of crude remain in Prince William Sound, even though oil remaining after the nearly 11-million-gallon spill had been expected to biodegrade and wash away within a few years.

The problem: The gravelly beaches of Prince William Sound are trapping the oil between two layers of rock, with larger rocks on top and finer gravel underneath, according to Michel C. Boufadel, chairman of the Department of Civil and Environmental Engineering at Temple University. His study appeared Sunday in Nature Geoscience's online publication and will be published in the journal later.

Boufadel found that water, which could have broken up and dissipated the oil, moved through the lower level of gravel up to 1,000 times slower than the top level.

Once the oil entered the lower level, conditions were right to keep it there, he said. Tidal forces worked to compact the finer-grained gravel even more, creating a nearly oxygen-free environment with low nutrient levels that slowed the ability of the oil to biodegrade.

"The oil could be maybe one foot below the beach surface and in contact with sea water with a lot of oxygen, but the oxygen doesn't get to it," Boufadel said.

He found that the upper layer of beach is so permeable that the water table falls within it as fast as the tide. However, the permeability of the lower level is so low that the water table does not drop much within it, he said.

Boufadel said the study points out the susceptibility of beaches worldwide to long-term oil contamination, especially at higher latitudes where beaches tend to be gravel or a mixture of sand and gravel.

"As global warming is melting the ice cover and exposing the Arctic to oil exploitation and shipping through sea routes such as the Northwest Passage, the risk of oil spills on gravel beaches in high-latitude regions will be increased," the study says.

Boufadel and his team dug about 70 pits between 3-feet and 5-feet deep on six beaches during summers from 2007 to 2009. His report focuses on data collected on Eleanor Island, about 15 miles away from Bligh Reef where the Exxon Valdez grounded on March 24, 1989.

Peter Hagen, program manager for Exxon studies for the National Oceanic and Atmospheric Administration, said Boufadel's study is a continuation of previous work that began in 2001 when 9,000 pits were dug around the sound, confirming the presence of oil.

While the remaining oil likely remains somewhat locked up in the beaches, the spill's lingering effects are ongoing, Hagen said. Sea otters, sea ducks and some sea birds are producing an enzyme showing exposure to oil.

Boufadel's study was funded by a $1.2 million, three-year grant from the Exxon Valdez Oil Spill Trustee Council. The council was formed after the environmental disaster to oversee restoration of the sound.

Boufadel doesn't know how long it might take for the remaining oil to finally disappear but predicted it will take a long time.

"It will be a slow process because the oil is relatively sheltered from water motion," he said.

Beaches trapping some oil from Exxon Valdez spill
JoAnne Allen, Reuters 17 Jan 10;

WASHINGTON (Reuters) - A lack of oxygen and nutrients below the surface of beaches in Alaska's Prince William Sound is slowing the dissipation of oil remaining from the 1989 Exxon Valdez spill, U.S. researchers said on Sunday.

The team conducted field studies over the past three summers using geologic information and hydraulics to try to determine why patches of oil linger on the beaches 20 years after the worst oil spill in U.S. history.

The supertanker Exxon Valdez spilled more than 11 million gallons (50 million liters) of crude oil, blackening some 1,300 miles of Alaska's coastline. An estimated 20,000 gallons (90,920 liters) remain, the researchers said.

They found that the oil remaining was trapped between two layers of beach and sheltered from the elements, according to the study posted on the journal Nature Geoscience's website (www.nature.com/ngeo/index.html)

"The oil that is in the upper layer either gets flushed out or biodegraded. In the lower layer, we found out there's not enough fresh water exchange to cause any flushing," co-author Michel Boufadel of Temple University said in a telephone interview.

Boufadel also said oxygen levels in the lower layer were not high enough for the oil to disappear through natural biodegradation.

"Microorganisms that are indigenous in beaches are capable of breaking down the oil, of eating the oil, provided that they have oxygen to breathe and nutrients such as nitrate and phosphate," Boufadel said.

He said earlier research uncovered nutrient deficiency in the area. But his team was the first to detect the low oxygen levels and the two-layer beaches.

In the first five years after the accident, the oil was vanishing at a rate of 70 percent a year and calculations showed it would be gone within a few years, the researchers said.

About eight years ago, the disappearance rate slowed to 4 percent a year.

Boufadel's team is exploring ways to deliver oxygen and nutrients to the lower layer of beach in an effort to spur biodegrading of the remaining oil.

The study was funded by the Exxon Valdez Oil Spill Trustee Council, established in 1991 when Exxon settled civil and criminal charges filed by the Alaska and the U.S. governments.

The council has administered the $900 million that Exxon paid to settle the state and federal civil cases from the disaster.

(Editing by Eric Beech)