July 30, 1996
Naturally occurring microorganisms gobbling toxic wastes at bottom of Eagle Harbor
Ferry passengers traveling to and from Bainbridge Island no longer see the remnants of the last creosote plant on the south shore of Eagle Harbor. The plant, defunct for about a decade, was dismantled this spring as part of an extensive cleanup mounted by the Environmental Protection Agency with input from half a dozen other state and federal agencies.
The EPA still faces a legacy that can’t be dismantled nearly as easily: On shore, oily wastes foul the ground water and the soil below it, in some spots going deeper than 70 feet. And in Eagle Harbor, a cap of clean fill entombs wide swaths of the harbor floor where sediments are most heavily contaminated.
Those marine sediments have polycyclic aromatic hydrocarbons (PAHs) in concentrations a hundred times greater than clean areas of Puget Sound. In some of the worst spots, concentrations are a thousand times greater. PAHs appear to be toxic to all life except for certain microorganisms that not only tolerate PAHs, they consume them. Some of these families of microorganisms have long made Puget Sound — including Eagle Harbor — their home.
University of Washington researchers are trying to determine which microorganisms recycle PAHs into harmless minerals and what actions might help or hinder their activity, according to Jody Deming, a professor of oceanography and director of the university’s marine bioremediation program.
Organisms that can help clean up pollutants and industrial wastes on land are well known, but the work at Eagle Harbor is some of the first in the nation dealing with organisms found in marine sediments. Of the half dozen institutions working with marine sediment remediation, UW has the largest program (12 faculty, nine graduate students and 15 undergraduates receive grant money at this time) and the only program that spans oceanography, microbiology, medical genetics, fisheries, forestry and civil and chemical engineering.
The basic research under way at Eagle Harbor could find ready application elsewhere in Puget Sound, the state and nation. The program is being supported in its first five years by a $4 million grant from the Office of Naval Research and by the UW provost’s reserve for research initiatives.
The principal marine organisms that can degrade organic pollutants are bacteria, with the possibility that fungi and protozoa also contribute to the process. Certain invertebrates help by burrowing in the mud, which brings oxygen and fresh nutrients to the bacteria on the sediment grains.
“One beauty of bioremediation is that it taps into the lower end of the food web,” Deming says. “The expectation is that you can keep PAHs from making their way higher in the food web if they are recycled into harmless minerals at the microbial level.”
Very little is known about these intriguing microorganisms even at clean sites, Deming said. Scientists have encountered surprises at Eagle Harbor:
Scientists thought that most of the microorganisms surviving in such heavily polluted sediments would be specialists adapted to living off contaminants. Instead, sediment samples show a wide diversity of organisms.
Given the level of contamination, researchers also speculated that there might be fewer organisms in the sediments than in clean sediments. Results thus far show that the sediments are teeming with life: There are about 10 times as many bacteria per ounce of sediment as there are humans on Earth. The numbers found in some of the contaminated sediments have even surpassed those found in samples taken from some clean sites.
It also appears that microorganisms needing oxygen to fuel their activities (aerobic) and those operating in oxygen-starved environments (anaerobic) are both recycling PAHs in the marine sediments of Eagle Harbor. This has not been true of microorganisms being studied at land-based sites. There, only the oxygen-dependent microorganisms are known to recycle contaminants.
It’s important to learn more about the conditions under which these organisms function in marine sediments, Deming says. For example, it’s encouraging to know that an oxygen-starved environment does not halt the natural cleaning-activities because humans create such environments when they put layers of clean fill over contaminated sediments.
Right now, spreading clean fill appears to be a reasonable way of holding contaminated marine sediments in place, and the Puget Sound region is at the forefront in using this approach, Deming says. At Eagle Harbor, a cap covers sediments contaminated with PAHs from the creosote plants as well as heavy metals, such as mercury, left behind from ship-building activities. (Microbial action on the harbor floor may not help with heavy metals because they can’t be broken down into less harmful substances.) The EPA used fill dredged from the Snohomish River by the Army Corps of Engineers, a cooperative enterprise that saved the agencies several million dollars. The cap is approximately a yard thick across the central portion of the harbor floor.
“Knowing what features of the sediment allow it to carry the maximum number of PAH-degrading bacteria would help agencies design the best caps possible in the future,” Deming says. For example, the porosity and permeability of the cap is important because there needs to be a certain amount of water between grains for the microorganisms to function optimally.
While caps can confine contaminants they cannot destroy them. Then, too, pollutants may continue to accumulate on top of the caps and storms and other turbulence can disturb a cap. In Eagle Harbor, for instance, the EPA is monitoring for chemical contamination at the cap surface and checking the impact of propeller wash near the ferry terminal.
“By exploring degradative processes in contaminated sediments and their caps, the UW marine bioredmediation program hopes to bring us closer to an understanding of longer-term solutions based on the work of naturally occurring marine microorganisms,” Deming says.