Oil released by the Deepwater Horizon spill could reach the western North Atlantic Ocean by November. That’s according to researchers from the US and Germany who have modelled the transport of dye from the site.
“It is likely that small amounts of either oil or an oil-water-dispersant mixture will, at some time in the six months or so following the spill, make its way into the Gulf Stream and the North Atlantic,” Mat Maltrud of Los Alamos National Laboratory, US, told environmentalresearchweb. “In order to detect this, it would be necessary to make multiple sensitive measurements below the surface, for example in the Florida Straits, since concentrations will be low and mitigation efforts have been concentrated at the surface of the ocean.”
Maltrud was keen to stress, however, that the model does not predict oil washing up on the eastern shores of North America or reaching Europe in detectable amounts. Instead, the calculations indicate that the oil will become extremely diluted in a region centred around the Gulf Stream in the North Atlantic.
“It is likely that long-term ecological and coastal impacts will be small within the Atlantic Ocean due to the very high dilution factors, but these impacts remain unquantifiable at this time,” write Maltrud and colleagues from the US National Center for Atmopsheric Research and IFM-GEOMAR, Germany, in Environmental Research Letters (ERL).
The Deepwater Horizon rig suffered an explosion and began to leak oil on 20 April 2010. The oil flow was stemmed on 15 July; in total around five million barrels of oil (roughly 795 million litres) were released, making it the largest ever accidental oil spill.
Modelling the long-term fate of the oil is difficult because it depends in part on the highly variable ocean circulation in the region. It is also not clear how quickly the oil is degraded in the ocean or how much has been removed, although one estimate puts the clean-up total at 800,000 barrels.
To get a handle on the problem, Maltrud and colleagues used a high-resolution global-ocean model, running it for a number of different conditions of ocean current. Because of the absence of robust data on oil degradation rates, they simulated the release of a passive dye rather than oil.
“We released this dye at four different depth intervals to model the subsurface distribution, while others were at the time using massless particles at the ocean surface,” said Maltrud. “This is important because we know not all of the oil reached the surface – in part due to the use of dispersants at the wellhead – and this subsurface oil will likely have a longer lifetime and a larger impact on ocean chemistry.”
Because the leak was still ongoing at the time of the study, in May 2010, the team ran the model to simulate four months following a two-month oil spill under 20 different ocean-current conditions, and for eight months following a four-month oil spill under five different ocean scenarios.
In all of the two-month spill simulations, relatively dilute dye in the top 800 m of the ocean reached the Florida Straits, on the border of the Atlantic, between 30 and 150 days after the initial spill.
“The time that it takes to get there depends on the state of the Loop Current in the Gulf of Mexico, with the longer timescales reflecting model scenarios in which the Loop Current detached a Loop Eddy,” said Maltrud. “This was in fact what happened in June and July in the Gulf.”
The team found that, depending on the state of the ocean, as much as 75% or as little as 25% of the dye may leave the Gulf of Mexico and enter the North Atlantic after six months.
“The model does not include any physical, chemical, or biological processes that result in degradation of real oil, so actual amounts of oil will be lower than these numbers,” said Maltrud. “Any removal of oil in the Northern Gulf would mean there was less available to be transported out.”
According to Maltrud, if oil degradation processes work very fast compared with transport of the oil by currents, almost all of the oil should disappear before it reaches the North Atlantic. “If the entirety of over 4 million barrels of oil can be removed from the Gulf on a timescale of roughly 3 months, then it is quite possible that no traces of oil will ever enter the Atlantic,” he said. “However, if the timescales for removal of this amount of oil are much longer, then it is very likely that the ocean currents will transport the oil (or oil-water-dispersant mixture) to the Atlantic. Ultimately, measurements will be needed to establish actual amounts.”
Although researchers at the University of Georgia and University of South Florida are measuring subsurface oil in the northern Gulf, Maltrud is not aware of any cruises planned to sample oil or dispersant in the Florida Straits.
“Because of the use of chemical dispersants in the Gulf, it appears that by early August 2010, most of the oil remaining in the Gulf was subsurface,” said Maltrud. “The pathways the model predicts for the dye should also reflect approximate pathways for a subsurface oil-water-dispersant mixture.”
The researchers chose this approach because they felt it was an opportunity to make a unique contribution to understanding the long-term fate of the oil. “When we started this study in late May 2010, other groups were performing short-term forecasts, but no other modelling group had considered the possible longer-term fate of oil by using information from many different realizations of a high-resolution global ocean circulation model,” said Maltrud.