More than 20 years after the Exxon Valdez foundered off the coast of Alaska, puddles of oil can still be found in Prince William Sound. Nearly 25 years after a storage tank ruptured, spilling oil into the mangrove swamps and coral reefs of Bahia Las Minas in Panama, oil slicks can still be found on the water. And more than 40 years after the barge Florida grounded off Cape Cod, dumping fuel oil, the muck beneath the marsh grasses still smells like a gas station.

"The conventional wisdom then was that the oil would only last for a few days," says marine chemist Chris Reddy of the Woods Hole Oceanographic Institution, who is part of the "third or fourth" generation of scientists to study the Florida spill. "But in this small area you have chemical warfare still going on."

That spill was only roughly 190,000 gallons—or less in total than the ongoing BP spill in the Gulf of Mexico has gushed every day since April 20. The lightest parts of the current catastrophe may soon wash ashore from Louisiana to Alabama, and the thicker stuff is just a few kilometers behind at this point. So how long will the damage from this oil spill last?

"If the [oil] mousse gets into the marshes, it can last a real long time," says environmental chemist Jeffrey Short of environmental group Oceana, who has studied the aftereffects of the Exxon Valdez spill. "Once there's no oxygen, it doesn't break down fast at all; it's a long-term toxic reservoir."

There is no cure. "The only way to remove it is mechanically, and that will destroy further the whole habitat," says marine biologist Héctor Guzmán of the Smithsonian Tropical Research Institute (STRI) in Panama, who is part of a team that conducted a long-term study of the impacts of the Panama oil spill in 1986. "The priority is to set up barriers and stop the oil."

Oily fate
Adding up all the spills from natural seeps, drilling, leaky vessels or pipelines and refueling means roughly 78 million gallons of oil enter U.S. waterways each year, according to a 2003 report from the National Academy of Sciences—dwarfing the roughly 4 million gallons (at least) of BP's Gulf oil spill, so far, based on the spill rate of roughly 200,000 gallons per day. So the marine environment is already dealing with lots of oil—how bad can it be?

The toxic compounds in oil vary, but largely fall in the group known to chemists as polycyclic aromatic hydrocarbons (PAHs), such as napthalenes, benzene, toluene and xylenes. All are known human carcinogens with other health effects for humans, animals and plants. "These hydrocarbons are particularly relevant if inhaled or ingested," says environmental toxicologist Ronald Kendall of Texas Tech University. "In the bodies of organisms such as mammals or birds, these aromatic hydrocarbons can be transformed into even more toxic products, which can affect DNA." In other words, the effects of the oil spill will linger in the genetics of Gulf coast animals long after the spill is gone, resulting in mutations that could lead to problems ranging from reduced fertility to cancer.

The oil from the Mississippi Canyon 252 well, which started leaking on April 20 when BP's Deepwater Horizon rig exploded, has been described as light, sweet crude, meaning that it may contain more such compounds that dissolve in water or evaporate. "There are components that make up that oil that will have some affinity to dissolve into the water, but how fast and to what extent I just don't know at 5,000 feet in this incredibly turbulent flow," Reddy says. "It's not how much of the oil got spilled, it's the concentration and the duration—how much of the components that have bioactive tendencies are in the water and are going across the gills of fish and for how long?"

Already, scientists in the Gulf have found plumes of oil floating roughly 1,000 meters beneath the surface—rather than rushing to the surface from the more than 1,500-meter-deep well as anticipated. And that means some of these compounds are literally washing off the oil and into the water. "It's going to be taking hours to get up," Reddy says.

Once the oil reaches the surface, it begins to evaporate, losing as much as 20 to 40 percent of the original hydrocarbons. "Evaporation is good; it selectively removes a lot of compounds we'd rather not have in the water," like PAHs, Reddy notes. It also emulsifies, forming the now ubiquitous mousse—a frothy mix of hydrocarbons and water—or clumps into so-called tar balls, like those found on the shore of Dauphin Island in Alabama on May 12.

The properties of the oil also change depending on whether it is at the release point more than 1,500 meters down, directly above the leak at the surface or a few kilometers east or west as it drifts. But "light [crude] components can be more pervasive in finding ways to infiltrate a salt marsh and impacting for a long period of time," Reddy says. And that's where the real problems begin.

Lifestyle choice
There are nearly 16,000 species of plants and animals in the Gulf of Mexico, according to marine biologist Thomas Shirley of Texas A&M University, "not counting microbes."

"There are a diversity of types of habitats in the Gulf, many very important in support of a variety of wildlife and fisheries," added marine biologist Jane Lubchenco, director of the U.S. National Oceanic and Atmospheric Administration (NOAA), at a May 12 press conference on the oil spill. "Many are at risk of being affected, but we don't have any direct way to know which ones or in what amount."

Looking specifically at the area impacted by the spill, more than half of the species—from 1,270 kinds of fish to 1,461 types of mollusks—call that region home. "Those that live in the area and are air breathing are most at risk," Shirley says. "Anything that's in the upper water column is going to be exposed to volatiles coming out of the oil."

Worse, sunlight can interact with PAHs to turn them deadly. In essence, PAHs act as catalysts to shovel energy from the sunlight into oxygen molecules, shifting them into a more reactive form and thereby oxidizing living cells. If oxygen naturally existed in that state "the whole Earth would burn up," notes Short.

That's bad news for the millions of translucent sea creatures out there—zooplankton—and could ultimately end up having cascading effects up the food chain. "If you start removing pieces of this big food web out there, what's going to happen?" Shirley asks. "We don't really know but probably not good things."

And the dispersed oil more readily crosses membranes as well as being more easily taken up by filter feeders, such as the deep-water coral in the vicinity or oysters nearer to shore. "Oysters will bioconcentrate this so fast," Kendall says.

Plus, spring is breeding season for species ranging from migratory birds to sea turtles, all congregating along the Gulf shore. "This is the time of year for larvae," Shirley notes, meaning that entire generations of short-lived species such as shrimp or crabs may disappear. "It's going to take immigration to replace some of those lost-year classes for things to get back to the level they were."

And there is the potential for the impacts of this oil spill to reach west to the Flower Garden Banks reefs off Texas or east to the coral reefs of the Florida coast and beyond. "My big nightmare is that this oil is going to get carried around to the Florida Keys and up the Eastern Seaboard," Shirley says. "That will happen. It's a matter of how much and when."

But it's when the oil gets into the marshes that the effects really start to accumulate. "That's your nurseries," Kendall notes, for species ranging from fish to birds. Adds Short: "It sets the stage for impacts from embryo toxicity. It gets into the developing eggs and induces aberrations in development. Even the smallest aberration in the field is lethal.... These marshes are important nursery areas for pretty much everything."

How's the weather?
Whether the oil can be kept out of the wetlands all comes down to one thing: the weather. A relatively calm week has allowed containment efforts to proceed smoothly, but even one day of rougher seas—one- to two-meter swells—would swamp the booms keeping oil off the coast and would inundate the marshes in petroleum. "The longer [the oil slick] stays offshore and continues to weather and turn into mousse, the less likely it is to impact sensitive habitats on shore," Short says. And the oil that doesn't turn to mousse may clump into tar balls that "either land on a shoreline or become part of the ocean's immense tar ball population."

Unfortunately, NOAA predicts landfall of the leading edge of the slick from Isle Dernieres to the barrier islands off Gulfport in Louisiana by this weekend. And June 1 marks the official start of hurricane season, which would stir the Gulf dramatically. "A hurricane or even just a tropical depression could be catastrophic," Kendall says. "It will push oil into places that it's difficult to clean up."

After all, sea otters still routinely dig oil out of Prince William Sound in Alaska in their hunt for clams in the intertidal zone—and there are populations of sea birds, fish and other species that have never recovered from the Exxon Valdez oil spill there, Shirley and Short note.

Of course, the warmer conditions of the Gulf of Mexico will help bacteria and other natural forces more quickly degrade the oil in this case, particularly with the help of chemical dispersants—hundreds of thousands of gallons of which have been deployed, including an experimental deployment of more than 28,000 gallons at the site of the oil spill itself, 1,500 meters down, though they carry their own risks and toxicity. "When an oil spill occurs there are no good outcomes," Lubchenco said. "It's a trade-off decision to reduce the impact of the oil on the shoreline and to sensitive wildlife."

But "once the oil, because of high tides or high winds, gets into the coastal wetland, it gets trapped in the sediment," notes STRI's Guzmán. "Then for decades you continue to see oil coming back out, this chronic pollution." The coral reefs in Panama have never recovered, and oil is still found in the mangrove swamps.

The most important task is stopping the oil from spilling—a prospect that remains out of reach nearly a month after it began gushing from BP's deep water well in the Gulf of Mexico. "We've got to stop this spill. We have to shut off the valves," Reddy says. "This is like someone telling you I'm going to punch you in the face but I won't tell you when or how often. That's a miserable existence."