Gulf spill: How 3.6 million barrels of crude can disappear

Three quarters of some 4.9 million barrels of crude disgorged into the Gulf of Mexico from BP's ruptured oil well has been neutralised by nature or human efforts, according to a US government report released Wednesday.

Only 26 percent of the oil remains close to its original form, floating on the ocean water or suspended under the surface, said the report by the National Oceanic and Atmospheric Administration (NOAA).

The other 74 percent has either evaporated or been burned, skimmed, recovered from the underwater wellhead or dispersed, a panel of government and independent scientists found.

"Three quarters is not surprising, indeed it is very consistent with our own preliminary estimates," said George Peigne, an expert on ocean oil spills at CEDRE, a French government pollution research centre.

"Indeed, it is possible that more than 75 percent has been eliminated," he told AFP.

Here is how the NOAA estimates breaks down:

REMOVAL AT SOURCE: After several false starts, some 17 percent of the oil was recovered directly from the busted wellhead a mile below the ocean surface using a riser pipe insertion tube and so-called "top hat" systems.

By the end of this week, the well - ruptured after a violent explosion on April 20 - could be sealed for good.

BURNING AND SKIMMING: Up to five percent of the crude was burned as it floated on the water, and another three percent was skimmed off the surface by a small armada of boats and ships.

NATURAL DISPERSION: NOAA estimates that 16 percent of the crude was dispersed naturally into the ocean water. As the oil gushed out of the well at high pressure, some of it sprayed apart into droplets less than 100 microns across, the diameter of a human hair.

Drops this small are neutrally buoyant and do not rise to the surface, and thus begin to biodegrade. "Oil-eating bacteria are particularly abundant in the Gulf due to the oil extraction industry and warm water temperatures," explained Peigne.

CHEMICAL DISPERSION: An additional eight percent of the oil was broken down with chemicals, leaving droplets just below the surface and deep in the water column.

As with the natural process, the oil is dispersed into tiny droplets. But until they are biodegraded, they remain potentially toxic to marine organisms.

EVAPORATION AND DISSOLUTION: The scientific panel calculated that 25 percent of the crude "quickly and naturally evaporated or dissolved into the water column".

In the latter process, individual hydrocarbon molecules in the oil separate and dissolve into the water much as sugar dissolves into coffee or tea.

For evaporation, molecules in the oil rise into the air "like an exhaust gas", said Peigne. "But they don't disappear entirely."