As they walk the sands of Orange Beach, Alabama, T. Prabhakar Clement and Joel Hayworth have no difficulty finding traces of the Deepwater Horizon disaster—in fact, the Auburn University researchers have a harder time making sure those traces don't stick to their feet.

On a mid-February weekend, oil—in the form of hundreds of sticky tar balls—had washed up all over the beach following a storm the night before.

"We could have collected as many tar balls as we wanted to, from less than 1 centimeter up to 4 centimeters (.4 to 1.6 inches) in diameter," Clement said. "And these are really soft tar balls that are decaying, so there are probably also millions of tiny fragments that we can't even see. I collected over 1,000 tar balls within [an area of] about 10 miles (16 kilometers) in five hours. What does that mean? I don't know. What are the health ramifications? I don't know. But this clearly demonstrates the magnitude of the [ongoing] problem attributable to Deepwater Horizon."

Tar balls are soft clumps of weathered oil mingled with sand, shells, and other beach material. They can range from the size of a pinhead to larger chunks, about the size of a basketball. And nearly two years after the deadly explosion that destroyed the Transocean oil rig Deepwater Horizon, and unleashed nearly 5 million barrels of oil into the Gulf of Mexico from BP's Macondo well, the oil company's cleanup personnel are still gathering them up along some Gulf beaches. But many beaches exhibit no tar balls, and even on beaches where tar balls do appear periodically, they aren't present all the time. In fact, Clement and Hayworth stressed, those beaches often look just fine to the naked eye.

"It's a very dynamic system," Hayworth said. "After a run-of-the-mill storm you might come down, and the beach will look like it just got re-oiled. But then, there is so much movement of sand, the beach turns over quickly and the shoreline is constantly moving around. So you could show up as a tourist at any given moment, and it could look pretty pristine." That may be because omnipresent BP crews pick up the tar balls whenever they appear, he said.

Tracing the Tar Balls

Not all tar balls are the result of the Deepwater Horizon disaster. Routine leaking in offshore oil operations, discharges from marine vehicles, onshore runoff, and even natural seepage from the ocean floor also create some. In fact, each day enough oil to fill 1,300 barrels of oil seeps naturally into the Gulf of Mexico, according to a 2002 report by the U.S. National Research Council of the National Academy of Sciences. But the Auburn researchers say their studies show that the vast majority of the oily tar balls still washing up in the Gulf are directly attributable to the spill.

That is so, they say, even though the Shoreline Clean-up Completion Plan (SCCP) released in November by the Unified Command, the joint BP-federal-state entity coordinating the response to the Deepwater Horizon spill, noted that during six months of surveys across four states, cleanup teams found some 5,000 non-Deepwater Horizon tar balls.

"In light of this report, BP's subtle suggestion is that other sources of contamination are possibly of the same magnitude as Deepwater Horizon," Clement explained. "But we could have easily collected over 5,000 tar balls during our last survey day" on Orange Beach, he added. And it's easy to trace tar balls back to Deepwater Horizon. As Unified Command reports make clear, the BP tar balls are 80 percent sand and break up easily. Those physical properties make them unlike those of other Gulf tar balls, and easy to identify in the field, even without chemical "fingerprinting," according to the SCCP report.

"Also, the numbers tell the story, when you see several hundreds and thousands at a specific location [where tar balls have] never occurred before on our shores," Clement added.

What Happened to All That Oil?

Markus Huettel, a benthic ecologist at Florida State University, has been researching the status of Deepwater Horizon oil on Florida Panhandle beaches since June 2010. Huettel explained that while much of the BP well's oil was degraded or evaporated. A staggering amount—he suggests 60 percent is a conservative estimate—remains unaccounted for.

NOAA has developed an Oil Budget Calculator tool to compile research data and show scientists' best guesses about what happened to all the oil from the worst offshore spill in U.S. history. But Huettel cautioned that the fate of only one category, the 17 percent directly recovered from the wellhead, is actually known.

"All the other categories—like oil burned, skimmed, chemically dispersed, or evaporated—are guesses that could change by a factor or two or even more in some cases," Huettel said.

"It's still just a lot of arm-waving, unfortunately, at this point," he said. Ongoing research will help scientists learn what happened to more of the oil over time, Huettel stressed, but some categories, like how much oil was dispersed at depth, will never be accurately known. "That oil is somewhere, but nobody knows where, and nobody knows how much has settled on the seafloor. We really only know the oil we have in hand, that 17 percent. All the rest is very debatable."

One hypothesis suggests that much of the oil ended up in "tar mats."

"These mats would likely be covered under sand and sediment, then uncovered during storm conditions where they are subject to erosion and deposition on the shore," Huettel said. "There are pockets where many more of these tar balls come up on shore, and the idea is that there may be some underwater dips or depressions where this oil can accumulate. Then, the sand and waves grind them into pieces."

Jacqueline Michel, a geochemist who is the U.S. National Oceanic and Atmospheric Administration (NOAA) coordinator for the Shoreline Clean-up Assessment Program, is leading the survey of more than 4,373 miles (7,038 kilometers) of shoreline extending over four states. Her teams are familiar with near-shore tar mats, and have been removing them where possible.

"We've done quite a bit of work looking for them and we've found that most of the oil not buried on the beach is very close to shore, between the beach and that first sand bar," she said. "You can reach it in the surf zone with a long-arm excavator. That's where we see the balls coming ashore as they break off from those [deposits], and we've made a big effort to get those."

The Shoreline Cleanup and Assessment Teams (SCAT) led by NOAA and the U.S. Coast Guard include representatives from state agencies as well as BP. (Visit RestoreTheGulf.gov for more information.) They've been digging such mats out wherever possible, using excavators with screened buckets on the end, Michel said. Sometimes, she added, they get a considerable assist from Mother Nature.

"Hurricanes or tropical storms can play a very important role in this process," she said. "Tropical Storm Lee [in September 2011] really released a lot of this kind of oil to the beaches and we were able to get a lot of that oil out of the environment. Crews patrol the beaches and pick up what they see on the surface, but we can really depend on some help from these natural processes." In that sense, Michel reports, a small and totally non-destructive hurricane would actually be almost welcome in terms of accessing oil for cleanup.

Cleanup Quandary: How Much is Too Much?

Michel explained that her teams have tackled an enormous task over the past two years. "The current oiling, where you still see anything on the shoreline, is around 450 miles (724 kilometers) as of February 25," she said. That area is scattered across the eastern Gulf from Louisiana to Florida, she said.

Their efforts have removed a large amount of oil from the environment that once covered 1,096 miles of shoreline, though scientists like Clement and Hayworth bemoan the fact that hard data on just how much has been removed by the joint government-BP cleanup effort hasn't been made available to them. Today, the pressing question in many locations is just when cleanup efforts should wind down.

For beaches, Michel added, the team's goal is to clean up to the point at which there is no visible Deepwater Horizon oil at all. For marshes, which are both more difficult to clean, and more sensitive to damage from foot and vehicle traffic, it is tougher to evaluate how much cleanup should be attempted. In either case, it isn't always cut-and-dried exactly when cleanup efforts should stop, she emphasized.

"This oil had to go 50 to 150 miles (80 to 240 kilometers) before it came to shore and also rise through 5,000 feet (1,525 meters) of water, so we call it 'water washed,' and then it was buried," Michel said. "Federal and state agencies have determined that this really poses no human health risk, and often little risk to wildlife." In some cases, officials thought it was better to leave the buried oil be. "Sometimes, they didn't want to dig up a lot of their beaches," she said.

At Alabama's hard-hit Bon Secour National Wildlife Refuge, manager Jereme Phillips said the impacts of the spill are greatly reduced from a year ago, As a result, he said, so is the cleanup effort, which went on for seven days a week for the first year after the spill. Now, it is down to one day per week. Tar balls still do wash up here regularly, particularly after strong winds or rough seas.

"We want to remove as much as possible from our beaches," he said. "But we want to ensure that the cleanup operations, workers, ATVs, and other equipment aren't doing more harm than good to sensitive ecological areas like bird-nesting sites," he said. "We're approaching bird-nesting season, for example, and we want to make sure that cleanup wouldn't negatively impact the refuge even more than the oil."

One problem, Phillips said, is determining what threats the oil might pose. "We don't fully understand what all the impacts of the oil may be," he said. "It's a contaminant and we want to remove as much of it as possible from the refuge, but we want to do that in a way that minimizes ecological impacts."

Auburn's Joel Hayworth agreed that the oil's ecological impacts remain a puzzle. "How is this oil being used and broken down by the organisms that reside in these areas?" he asked. "Recent studies show that microbiological communities that reside in the sandy beach systems, in fact, have been affected by oil. How do changes at the microbiological level propagate up the ecosystem? The details are very complex and scientists are trying to unravel the reality of what happens to a north Gulf of Mexico sandy beach system when you hit it with a whole bunch of oil."

The problem has produced a long-running natural experiment with no known end date, because that buried oil isn't likely to go away anytime soon, Florida State's Markus Huettel said. "Unfortunately, when oil sits in sediments it's typically excluded from oxygen contact, so its degradation is slowed down dramatically."

Spokesman Tom Mueller, of BP America's office in Houston, Texas, said the bulk of the mechanical and manual cleanup effort is complete, but patrolling and maintenance will continue until data demonstrates that shorelines meet standardized specifications as determined by the federal on-scene coordinator, currently U.S. Coast Guard Capt. Duke Walker.

"So we continue focusing cleanup resources on those few areas where we see continuing incidences of tar balls or other impacts, while working with state and federal agencies to remove other areas from the response and into restoration phase," Mueller said.

"BP has committed up to $1 billion for early restoration projects," he added. "While the normal Natural Resource Damage assessment process takes years to complete—with restoration funding to follow—BP is making funding available today to help restore areas where we know there were coastal or wildlife impacts from the spill. State and federal trustees oversee the allocation of funding for these early restoration projects, the first group of which were announced earlier this year." (Details of those projects can be found here.)

For now, the scientists combing the beaches say it is important to continue studying the BP spill oil, its fate in the environment, and its impacts.

"The Gulf is full of wells and pipelines," said Auburn's Joel Hayworth. "We're in the age of hydrocarbon man, and this isn't only an interesting natural experiment, it's a harbinger of the future. We've got to understand what is going on here now, so we can prepare for the next time."

This story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.