Enlarge By Patrick Semansky, AP Oiled marsh grass is seen underneath absorbent booms in Barataria Bay on the coast of Louisiana June 20. Months after the start of the worst offshore oil spill, scientists say wetland damage is severe in some spots, especially in reedy swamps at the Mississippi River's mouth.

Out of sight, out of mind?

With surface oil slicks fading from view in the Gulf of Mexico, courtesy of the capped Macondo well, we'd be out of our minds to think that the oil still isn't there, warn forensic toxicologists.

"We're finding less and less oil as we move forward," disaster response chief Thad Allen said last week, noting skimmer boats having trouble finding slicks. The retired Coast Guard admiral also pointed out that some 40% of the leaked oil, more than 90 million gallons of crude by U.S. Geologic Survey scientist estimates, is missing. "There's the issue of whether or not we may find oil under the water," Allen added.

Under the water is where the oil is, say environmental chemists such as Jeffrey Short of the conservation group, Oceana, and not just in deep sea clouds of oil reported by National Oceanic and Atmospheric Administration (NOAA) scientists. "Oil tends to congeal and where you saw a broad slick, you now have a lot of droplets and tarballs," he says. Whether floating as tarballs, buried under Mississippi River mud or carried off in currents to the Atlantic, much of the spilled oil remains in the water, Short says.

"A chemical spill in the ocean is what this (Gulf of Mexico) leak is, really," says chemist Kim Anderson of Oregon State University in Corvallis. "The crude oil contains diesel, it contains gasoline, it contains kerosene, it contains methane and it contains chemicals that are unfortunately, carcinogenic. Literally there are hundreds of chemicals in crude oil."

FULL COVERAGE: The latest on the oil spill

"Very hazardous groups of chemicals are released in any spill," adds chemist Trevor Penning of the University of Pennsylvania. The exact amount varies with the exact nature of the crude, he adds, with heavier oil containing more toxic stuff. The 2002 Prestige oil tanker spill in the Mediterranean, which released heavy bunker oil — literally the gloppy "bottom of the barrel" oil burned in ship's engines — was particularly toxic, while the lighter crude of the Gulf should contain less-heavy concentrations of the worst toxins, the chemists suggest.

Anderson heads a team tracking just how much of these worst toxins— organic chemicals called polycyclic aromatic hydrocarbons — have been dumped in the water by the spill. They'll be measured at four sites off the coast of Louisiana, Mississippi, Alabama and Florida. Earlier samples from Louisiana alone, showed that by June 7, Gulf of Mexico water concentrations of the toxic chemicals had risen 40 times higher than levels on May 1, although the water looked clear of oil. The toxin increase reflects the rise after the April 20 Deepwater Horizon subsea drilling rig explosion that killed 11 people and started the Gulf oil spill, the nation's worst ever. The team's next survey results, from Louisiana, Mississippi, Alabama and Florida sites, should come this week.

Each kind of chemical in the crude oil responds to tides, currents, saltiness of the sea water, and natch, chemistry, in different ways, Anderson says. "Tracking them all down is like chasing bees from a smashed hive, they go every which way possible."

Complicating the search for the chemicals is the amount of dispersant, about 1.84 million gallons, applied to oil from the leak. The dispersant has done its job, acting like dish soap on bacon grease, congealing the oil into tiny droplets that microbes can begin eating. "That means they are in the food chain." Short says. "Whether people will want to swim or eat food from water that looks clear but has high concentrations of (toxins) will be interesting," she says.

Another open question is the issue of photo-enhanced toxicity from the chemicals, says Short, a former NOAA scientist who worked on the 1989 Exxon Valdez spill's aftermath. In the marshes, oil may be cleaned from foliage and end up buried by Mississippi mud, where it ends up near the roots of growing plants. "The toxins get inside the surfaces of cells and release oxygen in response to sunlight," he says, burning up the plants from the inside. Mangrove swamps in Panama were hit hard by this reaction after a 1986 spill, and the effects are still seen today, according to Smithsonian Tropical Research Institution scientists.

In a July, environmental scientist Arne Jernelöv of Sweden's Institute for Futures noted in the journal Nature, that shrimp, squid and some fish populations were "severely hit" by the 1979 Ixtoc oil rig spill in the Gulf of Mexico, recovering in a few years after a fishing moratorium.

As the oil spill response shifts from capping the leak to fixing the damage, we have seen only the start of the story, Anderson concludes. "Years. I'll be here for years."