Although oil has stopped gushing from the Macondo well in the Gulf of Mexico and spilled oil has been disappearing rapidly from the water surface, it will take a long time to determine the long-term environmental impacts of the Deepwater Horizon disaster. But, even in the early days of the spill, as estimates of oil volumes escalated, people began making comparisons between the new unfolding disaster in the Gulf and the infamous Exxon Valdez disaster that fouled about 1,300 miles of Alaska shoreline a couple of decades ago.
So, how do these two oil spills compare?
Obviously, in both incidents, very large volumes of crude oil were spilled in situations where ocean currents can carry oil slicks over large distances. And any spilled oil can smother animals and plants, as well as spread chemicals that are harmful to living organisms.
Significant differencesBut there are probably more differences than similarities between the Exxon Valdez and Deepwater Horizon incidents, especially when it comes to the volumes and types of crude oil involved, and to the circumstances in which each spill occurred.
The tanker Exxon Valdez spewed about 250,000 barrels of North Slope crude oil, to form a thick oil slick on the waters of the western Prince William Sound. The spilled material was crude that had been processed on the North Slope to remove water and natural gas.
According to information published by the National Oceanic and Atmospheric Administration, North Slope crude is generally a medium-grade oil, 15 to 20 percent of which may evaporate within 24 hours of a spill, depending on the wind and sea conditions, with relatively little of the oil likely to disperse as droplets through the ocean water column.
And a U.S. Coast Guard and U.S. Department of Transportation report on the Exxon Valdez disaster says that the North Slope oil tends to contain inorganic sulfur compounds and relatively high concentrations of aromatic hydrocarbons.
Given the relatively cold conditions in Prince William Sound in March 1989, when the Exxon Valdez struck Bligh Reef and started discharging its cargo, the oil reportedly formed a thick slick close to the stricken vessel. A Coast Guard officer who boarded the vessel within a few hours of the grounding reported “about six to ten inches of oil on the surface and oil billows on the port side for about half the length of the ship, which were boiling about three to four feet above the surface of the water.”
But less than three days after the accident, when relatively little of the spilled oil had been recovered or dispersed, a severe storm blew up, emulsifying much oil into a thick mousse, pushing the oil over huge distances, and stranding thick, emulsified oil along shorelines.
The resulting mess severely curtailed the possibility of recovering oil on water and created a shoreline cleanup nightmare.
Light crudeBy contrast, the material that gushed from the Macondo well consisted of an unprocessed mixture of water, natural gas and light Louisiana crude oil. Light Louisiana crude typically contains a relatively high proportion of light, volatile components and it contains no hydrogen sulfide. Scientists have estimated that as much as 40 percent of this oil may evaporate within 24 to 48 hours of a spillage, according to a fact sheet published by the Deepwater Horizon response unified command.
The Deepwater Horizon accident occurred much farther offshore than the grounding of the Exxon Valdez, and in warm water that would tend to encourage the evaporation of some oil components. Also, bugs that devour tiny droplets of dispersed oil in the water column tend to flourish in the warm Gulf waters, where natural oil seeps occur on the seafloor.
But the Macondo well ejected a very much larger volume of oil than escaped from the Exxon Valdez — on Aug. 2 an interagency team of scientists published an estimate that 4.9 million barrels of oil had flowed from the well.
As the oil squirted upwards from the out-of-control wellhead, some of it dispersed into the seawater as oil droplets. Much of the oil, however, floated to the sea surface and spread across a vast area of the Gulf, eventually reaching land and hitting an extensive amount of shoreline.
Deepwater Horizon spill response teams removed oil from the water using skimmers, through the in-situ burning of corralled oil pools, and by the application of chemical dispersants (dispersants cause oil to spread though the water column as minute droplets for consumption by oil-eating bugs).
26 percent remainsA team of scientists from the U.S. Department of the Interior and NOAA has estimated that much of the 4.9 million barrels of oil that leaked from the Macondo well has now been removed or dispersed. And in an Aug. 4 report on this analysis NOAA said that natural evaporation, coupled with the dissolving of some oil components in seawater, accounts for the loss of about 25 percent of the original oil. Natural dispersion into the water column would account for another 16 percent. Eight percent would have been dispersed by chemicals, 5 percent would have been burned and 3 percent would have been skimmed. Direct recovery of oil from the wellhead using capping devices would have accounted for 17 percent of the oil flowing from the well.
“The residual amount, just over one quarter (26 percent), is either on or just below the surface as residue and weathered tar balls, has washed ashore or been collected from the shore, or is buried in sand and sediments,” NOAA said in announcing the new estimates. “Dispersed and residual oil remain in the system until they degrade through a number of natural processes. Early indications are that the oil is degrading quickly.”
The fate of this remaining oil and its effect on the Gulf ecosystem will obviously be the focus of intense scrutiny in the coming months and years.