Uncovering the Secret in the Sand – Buried Oil Remains After Cleanup
Many of the oil-fouled beaches along the Gulf coast and barrier islands have been scoured, scraped, raked and filtered clean. Tourist beaches and beaches in front of high-rise condominiums along the Florida panhandle and eastern shores of Alabama are spectacularly white again.
But dig two feet below the pristine surface and a tarry, brown surprise awaits you — you’ve struck oil.
“Oil was coming ashore and was transported down into the sand by natural hydrodynamic processes, then trapped in the sand,” said Dr. Joel Kostka an environmental microbiologist and microbial ecologist at Florida State University (FSU).
Kostka, along with principal investigator Dr. Markus Huettel, also of FSU, has been investigating how quickly oil that flowed onto Gulf beaches is being degraded (“eaten” in the common vernacular) by the microbial communities that live naturally in the sand. They’re also trying to discover if the so called “oil-eating” bacteria and microalgae that normally lived in the Gulf’s water column and got washed ashore with oil helped or hindered the process.
Crude oil and, perhaps worse, dispersed oil (crude mixed with Corexit dispersant) was washed onto the sand by wave action. Then, as more and more sand was deposited on top of the oil, a horizontal layer of oil resulted about two to three feet below the beach surface.
So the restoration process is not just a matter of having clean, white sand beaches. Buried oil can contaminate ground water. Chemicals can infiltrate fresh water aquifers. Toxic compounds can be flushed back to the Gulf, killing fish and crustacean larvae. Lingering oil is a threat to wildlife and ecosystems because it can plug water flow that cleanses and enriches shoreline communities.
Kostka explained that crude oil is really a mix of thousands of organic (i.e., carbon-based) compounds. Over eons a variety of microbes — bacteria, microalgae and fungi — have evolved to metabolize specific compounds. Which is to say, different microbes in different places eat different parts of crude oil. Some microorganisms eat a variety of compounds but none eat all of them. Almost all stay away from asphaltenes — hydrocarbons that linger for years as asphalt-like tar.
Many oil-degrading microorganisms need oxygen and nutrients (nitrogen and phosphorous) to metabolize their preferred food.
“We can measure really high oxygen consumption rates in the sand where the oil layers are [a key indicator of microbial activity]. And we can also identify the oil-degrading microbes that are there,” Kostka said. “So from the oxygen consumption data and from the type of bacteria that we see there, there’s a thriving oil-degrading microbial community,” he added.
In June, Huettel, Kostka and a small contingent of grad students collected core samples of beach sands from around the Gulf. They braved the mosquitoes and no-see-ums on St. George Island, Fla. to collect control samples from a pristine beach. On Elmer’s Island, La., samples were collected from the opposite extreme: heavily oiled, extremely contaminated sands.
At Pensacola, Fla., the primary research site, they dug trenches and collected core samples.
What emerged was a listing of 14 microbes that metabolize specific compounds in oil. “People often think of these sands as dead. But they’re really not. They’re just covered with microbes. There are nearly as many microorganisms per gram of beach sand as there are in mud,” said Kostka.
Kostka and Huettel have hypothesized that these tiny organisms can be used as indicators of oil contamination in subsurface beach sand. So instead of sending an army of grad students out to the beaches with shovels and energy drinks to dig long, deep trenches in search of buried oil, smaller samples of beach sand could be collected and analyzed to identify microbes that are present in unusually large quantities.
Once an area of contamination has been located, the presence and activity of key microbial groups might provide an indication of what’s under there in terms of the remaining oil compounds and what needs to be done (or can be done) to clean the contamination or protect threatened wildlife.