Did nature clean up most of the Exxon Valdez oil spill?

June 18, 2010

Dear Cecil:

The current drama over the gulf oil spill reminds me of an article some years ago saying that, despite millions spent on massive coastline cleanup following the Exxon Valdez disaster, contaminated areas untouched by cleanup crews reverted to their pristine pre-spill condition just as quickly as those with human help. Is this a case of letting Mother Nature alone to do what she does best, or simply not true?

Cecil replies:

Apparently the good news hasn't gotten out to kiwi country. Rush Limbaugh has already assessed the situation in the Gulf of Mexico and announced, "The ocean will take care of this on its own if it was left alone and was left out there. It's natural. It's as natural as the ocean water is. Well, the turtles may take a hit for a while, but so what?" Still, maybe you won't mind getting a second opinion from me.

The basics: First, oil is mostly biodegradable. Some of it evaporates or breaks down with exposure to sunlight, and at least 20 types of marine bacteria plus several types of fungi can degrade what's left. Surprised to learn that bacteria eat oil? Don't be. Although oil spills from tankers and wells make the news, they account for less than 15 percent of the total petroleum entering the world's oceans, while 47 percent comes from natural oil seeps. (The rest largely comes from petroleum use.)

That doesn't mean oil biodegrades easily. Crude oil consists mainly of various types of hydrocarbons, some of which break down readily, others not at all. Light oils generally degrade faster than heavy ones. Very light crude might lose 60 percent of its volume due to biodegradation in four weeks, heavy crude just 10 percent. Temperature is important — warmer waters encourage bacteria growth, which is one bright spot for the gulf.

A big factor slowing oil breakdown is that oil doesn't contain much nitrogen or phosphorus, both of which are needed for good bacterial growth. Enter bioremediation, where fertilizer is added to encourage natural bacteria. First tried in the 1960s, it evidently works. One 2002 study showed that adding just 0.25 percent fertilizer to oil on a lab-simulated beach quintupled the natural biodegradation rate. Tests in 1994 in Delaware Bay, which is already rich with bacterial nutrients, showed fertilizer doubled the rate of oil degradation in shallow waters. The same year, scientists fighting a spill on a beach near Haifa, Israel, reported that bioremediation had reduced oil contamination 88 percent in just four weeks.

Sometimes nature doesn't need much assistance. Following the 1978 wreck of the Amoco Cadiz off the coast of Brittany, oil was broken down quickly by local microbes, which had grown accustomed to the stuff thanks to shipping leakage. Same for the 1980 Tanio wreck in the same area — biodegradation was detectable within 24 hours. The blowout of the Ixtoc well in the Gulf of Mexico in 1979 was a different story. Warm water and friendly bacteria raised hopes for speedy degradation, but in this case the oil formed an emulsion, or mousse, on the surface that proved resistant to breakdown.

To your question: It's true that human efforts didn't clean up most of the 1989 Exxon Valdez spill. Some hydrocarbons degraded rapidly without assistance, possibly because bacteria in Prince William Sound had acclimated to resin emitted by pine trees on shore.

But bioremediation seemed to help. Local bacteria were found to be starving for nutrients, and once fertilizers were added to a test area they got busy. Within a couple weeks a “white window” of clean rocks appeared among the gunk-covered ones. Eventually more than 70 miles of beach were treated this way.

Later researchers questioned how much oil the process actually got rid of, though. It's been calculated that all told, bioremediation, skimming, spraying, and scrubbing were responsible for removing less than a sixth of the spilled oil. Who or whatever deserves the credit, most of the Exxon Valdez spillage did eventually disappear.

Not all of it, though — biodegradation has its limits. Oxygen is key in much bacterial action, and once oil gets buried under sediment things really slow down. In 2001 researchers dug pits at 91 sites along the shore of Prince William Sound and found oil at roughly three in five. That may sound overly grim — another survey of 5,000 pits by U.S. government researchers in 2001 and 2003 found 98 percent had little or no oil.

Conclusion? Let’s break this down into more digestible bits. Do oil spills mostly go away on their own? Yes. Does that mean we’re better off leaving them alone? Of course not. Nobody doubts we need to plug leaks and contain spillage, and I’m persuaded bioremediation helps at least sometimes.

But other intervention may be wasteful or harmful. Excessive use of chemical dispersants may threaten wildlife. Animal rescue efforts may be expensive PR exercises. The chief lesson from past spills is how little we know about what works. We’d better find out.

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References

Atlas, Ronald and Bragg, James “Bioremediation of marine oil spills: when and when not – the Exxon Valdez experience” Microbial Biotechnology 2.2 (2009): 213–221.

Atlas, Ronald M. and Cerniglia, Carl E. “Bioremediation of Petroleum Pollutants” BioScience 45.5 (1995): 332-338.

Boehm, Paul D. et al. “Distribution and Weathering of Crude Oil Residues on Shorelines 18 Years After the Exxon Valdez Spill” Environ. Sci. Technol. 42 (2008): 9210–9216.

Bragg, James R. et al. “Effectiveness of bioremediation for the Exxon Valdez oil spill” Nature 368 (1994): 413-418.

Head, Ian M. and Swannell, Richard P.J. “Bioremediation of petroleum hydrocarbon contaminants in marine habitats” Current Opinion in Biotechnology 10 (1999): 234–239.

Paine, R.T. et al. “Trouble on Oiled Waters: Lessons from the Exxon Valdez Oil Spill” Annual Review of Ecology and Systematics 27 (1996): 197-235.

Roling, Wilfred F. M. “Robust Hydrocarbon Degradation and Dynamics of Bacterial Communities during Nutrient-Enhanced Oil Spill Bioremediation” Applied and Environmental Microbiology 68.11 (2002): 5537–5548.

Short, Jeff; Rice, Stanley; and Lindeberg, Mandy. “The Exxon Valdez Oil Spill: How Much Oil Remains?” Alaska Fisheries Science Center

Swannell, Richard P.J. et al. “Field Evaluations of Marine Oil Spill Bioremediation” Microbiological Reviews 60.2 (1996): 342–365.

Timmis, K. N. (ed.) Handbook of Hydrocarbon and Lipid Microbiology Berlin: Springer-Verlag, 2010 (Chapter 16: Bioremediation of Marine Oil Spills, R.C. Prince).

Venosa, Albert D. et al. “Bioremediation of an Experimental Oil Spill on the Shoreline of Delaware Bay” Environ. Sci. Technol. 30 (1996): 1764-1775.

Zoff, Rebecca Z. Responding to Oil Spills in Coastal Marshes: The Fine Line Between Help and Hinderance Hazardous Materials Response and Assessments Division, National Oceanic and Atmospheric Administration, Seattle, Washington USA, December, 1995.

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