WASHINGTON—Unknowns abound, but with oil no longer flowing freely into the Gulf of Mexico, a grim calculus is beginning to define the dimensions of the worst spill ever.

Assuming no more oil is released — and for many skeptics, that alone is the bravest of assumptions — 85 days of slow-motion disaster added up to some 700 million litres of crude by some estimates. Put another way, about 17 times more oil than gushed from the Exxon Valdez when it broke open off Alaska in 1989.

Those in search of silver linings say the warmth, depth and distance of the Deepwater spill — a full 80 kilometres offshore — means that barrel for barrel, the Gulf may not pay the same price as Alaska simply because oil breaks down more quickly in warmer water.

Friends of the Earth, in its running tally of dead wildlife, count 1,387 sea birds, 444 sea turtles and 53 mammals dead in the Gulf thus far. The Alaska spill, by contrast, is believed to have killed 36,000 sea birds.

But spill-watchers were loath to express optimism, given the as yet unknown toll to wetlands, estuaries and beaches already degraded by decades of human development along the Gulf coast. Weather (read hurricane) remains a vexing wild card to those trying to anticipate what comes next.

“It is awkward to use the phrase ‘best-case scenario’ but if not another ounce of oil is released that’s obviously good. We can only hope so,” said Hugh Kaufman, senior policy analyst with the Environmental Protection Agency’s Office of Solid Waste and Emergency Response.

“What that will leave us with, however, is a 20-year reckoning of the damage to the environment, to the fisheries, to tourism and beyond. People will still be sending in the bills long after I’m dead and buried.”

Kaufman is a leading critic of BP’s decision to use the chemical dispersant Corexit, which was described during hearings in Washington on Thursday as the potential “Agent Orange of the Gulf.” Marine biologists at this point simply cannot calculate the impact of the nearly 7 million litres of Corexit now in the water.

“What we do know is that the dispersants put all that oil in play — there is no skimming or burning it off anymore. Instead you have a vast expanse of oil droplets, together with Corexit chemicals, that’s going to do what it will do,” said Kaufman.

“The result may be short-term gain for very long-term pain. More complex pain, in fact, because we’ll be sorting through questions of both the oil and the dispersant impacts. Anyone who looks on the label of a bottle of Corexit might decide on that alone they don’t want to ever again swim in this water or eat this fish.”

Marcie Keever, a Washington-based campaigner with Friends of the Earth, said environmentalists understand now that only a tiny fraction of the oil will ever be cleaned up.

“Looking for a best-case scenario when you are talking everything from tuna spawning grounds to an already fragile fishery and ecosystem is almost beyond us at this point,” Keever told the Star.

“One possibility is that the spill will bring about a public demand to end the era of the oil industry’s ability to basically write its own regulations, another is that this tragedy will ultimately persuade Americans that we need to move away from fossil fuels altogether.”

But Keever admits even those scenarios are blunted by the ever-weakening U.S. climate bill now working its way through Congress. Few in Washington expect the bill to become law before the end of the year. And if and when it does, fewer still expect it to contain a robust cap-and-trade element that might spark a dramatic shift toward alternative energy sources.

The EPA’s Kaufman, meanwhile, cautions watchers of the famous underwater oil spout not to be deceived by still waters. BP remains hesitant to declare the well under control for good reason.

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“This isn’t over. Nobody really has any idea how much stress the cap can take. A day, a week, a month, two years? Eventually it is at risk of blowing again, until they can get the relief well aligned, pump in the mud and properly shut this thing down,” he said.

“For now, that is the best-case scenario, that we can buy ourselves enough time to end this well for good before a catastrophe becomes even worse.”