Gulf spill: Where did the oil go – and what did it do?

In November, the technical group released a 217-page document – a final estimate of the situation as of July 14 – that spelled out in detail the approaches it used to reach its estimates.

A full, detailed partitioning of the oil's fate may never appear, researchers say. Currents and bacteria have long since dissipated once-coherent plumes of oil or gases such as methane.

To complicate matters, even biological processes can leave oily residue. Many mud samples taken from the Gulf sea floor held oil-munching bacteria that had died and carried their oily intake to the bottom with them when they sank, reported Samantha Joye, a marine scientist at the University of Georgia, during February's annual meeting of the American Association for the Advancement of Science.

Effects of the oil trapped in marshes, mud, and sand

It's the balance-sheet's "other" category -- oil in marshes, beaches, and sea-floor sediment -- that many scientists are tracking and analyzing on a regular basis.

They want to assess the oil's current effects on organisms, and furthermore determine what hazards remain for plants and animals – whether the oil continues to degrade or remains as a lump of tar for decades.

Oil and its residues contain several hundred chemical compounds in varying concentrations and with different environmental effects, says Dr. Reddy, who works at the Woods Hole Oceanographic Institution in Woods Hole, Mass. Knowing those mixtures can help determine whether removing any remaining reside could be more damaging to plants, fish, animals in the area than the blow-out leftovers themselves.

One location that has proven tough to assess and treat is the surf zone, where mats of oil can gather in natural trenches between sand bars. It's a tough place to try to hold a boat stable and gather detailed measurements, let alone clean up, according to report issued on February by a scientific advisory panel.

For his part, Reddy and a colleague have been combing beaches in Florida, Alabama, and Mississippi for tar balls, which they bring back to the lab for analysis.

"I know what the oil looked like when it came out of the pipe," he says. "Now I want to know what it looks like a year later."

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