The question now: where will the oil go?

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Eighty-six days after the Deepwater Horizon oil rig exploded, BP finally capped the blown-out well, virtually stopping the leak. But with all five states on the northern Gulf Coast reporting oil on their shores, NOAA has issued a long-term model projecting where the spill - the largest in U.S. history - might threaten other shorelines as winds and current carry it farther afield.

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No surprise, the study released July 2 projects an 81 to 100 percent probability of oil impacting the coasts of eastern Louisiana, Mississippi, Alabama and the Florida Panhandle. The probability of an impact in western Louisiana is 61 to 80 percent; east Texas, 21 to 40 percent; most of the rest of the Texas coast, less than 1 percent; Florida's Big Bend region, 21 to 40 percent; Florida's west coast, 1 to 20 percent; southwest Florida, less than 1 percent; southeast Florida and the Keys, 61 to 80 percent; and northeast Florida and Georgia, 1 to 20 percent.

"Potential impacts become increasingly unlikely north of North Carolina as the Gulf Stream moves away from the continental U.S. at Cape Hatteras," the study says.

On July 16, BP capped the blown-out well with an 18-foot-tall, 75-ton apparatus designed to stop the oil from gushing. At press time, all three relief valves on the cap had been closed and all but a "trickle" of oil had been stopped. The cap is a temporary fix, however. BP is relying on one of two relief wells it is drilling to divert the oil permanently so it can plug the blown-out well with mud and cement. The relief wells weren't scheduled for completion until sometime in August.

The Deepwater Horizon rig exploded April 20, killing 11 and unleashing tens of millions of gallons of oil in the Gulf over a period of almost three months. In its projections of how the oil in the Gulf might spread during the summer, NOAA defines a "shoreline impact" as enough oil in the water within 20 miles of shore to cause tar balls or a dull sheen. It does not necessarily mean the oil will run ashore; that will depend on currents and winds.

"The oil could stay in the water and break down through the natural processes that are at work out there," says NOAA spokeswoman Rachel Wilhelm.

The high likelihood of oil reaching South Florida and the Keys is due to the flow of the Loop Current out of the Gulf into the Florida Current and the Gulf Stream. NOAA says by the time the oil reaches South Florida, most of it will have dispersed, evaporated and weathered either into tar balls or heavily weathered streamers - long, narrow ribbons of tar-like oil.

"The oil that reaches Florida, if and when it does, would be highly weathered," Wilhelm says. "It will have been out there a long time."

A NOAA alert for South Florida says, in a reassuring tone: "It is unlikely that large, cohesive oil slicks will reach Florida waters. Instead, on the water surface expect to see floating tar balls and/or scattered patches of weathered oil, perhaps mixed with Sargassum (algae) or other floating material. It's likely that shoreline impacts will be scattered and episodic. On shorelines, there might be small splatters of oil and/or bands of soft tar balls."

NOAA's projections are based on 15 years of historical wind and current data for the 120 days after the rig exploded. NOAA created 500 wind and current models, plugged different historical data into each one, and calculated the probability of the oil reaching a particular shoreline based on what percentage of the models showed it moving that direction.

Wilhelm says the projections are not based on present winds or tides, so the behavior of the Loop Current this summer was not factored in. The Loop Current streams into the Gulf of Mexico from the Yucatan Channel, makes a loop, turns back south and meets up with the northeast-flowing Florida Current and Gulf Stream off the Keys.

The Loop Current has captured some of the oil from the spill. However, instead of carrying it south to the Gulf Stream, the Loop has spun it off into the Franklin Eddy, which lies west of the Loop and is separated from it, so the oil keeps going around in circles. This has delayed its transport to South Florida, the Keys and points north on the Eastern Seaboard. Once in the Loop Current and on a southbound trajectory, the oil will take eight to 12 days - or longer - to reach South Florida, according to NOAA.

The projections do not give any timeline for the oil's spread. That depends on actual conditions, Wilhelm says. However, NOAA does give 24-, 48- and 72-hour daily updates forecasting the oil's movement based on actual oil sightings, and present and forecast winds and currents. That information is available at its website, www.restorethegulf.gov (click below "Maps & Data" near the bottom of the page).

Wilhelm says the purpose of the projections is to give emergency managers a better idea of what to expect so they can plan their responses. She says planners shouldn't have found anything surprising in the projections, since NOAA has been advising them of the probable risks for each region all along.

As of mid-July, oil in one form or another - black crude, orange sludge, tar balls or oily residue - had been reported along a 550-mile swath of the Gulf Coast from Galveston, Texas, to west of Destin in the Florida Panhandle. NOAA's projections are based on a spill rate of 1.7 million gallons per day from April 20 until June 3, when the riser pipe was cut, then 2.5 million gallons a day from June 3 until 90 days after the oil first began to gush. That is when the relief wells were initially supposed to be finished. NOAA also has factored into its models an estimated 294,000 gallons of oil a day captured by skimming or burning, and 840,000 gallons a day captured by the first cap, installed June 5.

The federal government estimates between 1.5 million and 2.5 million gallons of oil were spewing from the well each day before it was capped. At the lower rate, the spill's size in early July was 86 million gallons - nearly eight times the size of the Exxon Valdez spill.

This article originally appeared in the September 2010 issue.