'Big dig' unearths clues to garbage decay.

'Big dig' unearths clues to garbage decay

Begun in 1948 atop a swamp on Staten Island, N.Y., the Fresh Kills landfill -- the world's largest -- covers 4,950 acres and holds 2.3 billion cubic feet of refuse. A year ago, this massive monument to a throwaway society hosted the first multidisciplinary excavation aimed at understanding why some landfill wastes decay more slowly than expected.

Five participating research teams unveiled their preliminary findings last week in Arlington, Va., at a meeting of the Society of Environmental Toxicology and Chemistry. Though most of their reports confirmed the suspected importance of moisture in fostering decay, several teams dug up surprises.

William L. Rathje and his archaeological team from the University of Arizona's Garbage Project joined in last year's "big dig" after unearthing wastes from six other landfills (SN: 10/6/90, p.218). At Fresh Kills, they used a bucket auger to chew out 47 boreholes, extracting 5,184 pounds of samples.

Near the surface, the Tucson researchers found that dry wastes showed little decay. But the deeper they dug, the wetter their finds. At 20 to 25 feet, they encountered heavily decayed gray slime. Because Fresh Kills has no clay barrier to separate initially interred wastes from the underground stream running another 5 to 10 feet down, the lower wastes suck up stream water "almost like a sponge," Rathje says.

Sampling revealed that paper accounts for 40 to 50 percent of the landfill's volume, plastics for up to 12 percent and metals for about 7 percent, he says. Construction and demolition debris held the only big surprise, representing 12 percent of Fresh Kills' volume. Because neither trade associations nor municipalities record contributions of such debris to the national waste stream, there were no formal data on its magnitude, Rathje says.

Yet construction debris may alter decay processes, asserts Joseph M. Suflita of the University of Oklahoma in Norman. Researchers have assumed that methane-producing bacteria complete the breakdown of cellulose-based wastes, such as paper, wood and yard clippings. However, the Oklahoma team discovered that when sulfates are present, competing "sulfate-reducing" bacteria may usurp that role.

Suflita's group detected sulfates, sometimes at extremely high levels, in most of the buried paper, textiles and other cellulosic materials at Fresh Kills. They also determined that fresh paper and textiles contain little or no sulfates. Suflita now suspects that gypsum board used in construction may be the sulfates' primary source.

While his findings point to a potential mechanism in cellulose decay, another team uncovered a potential obstacle to cellulose decay: the unexpected scarcity of microbes that initiate breakdown of this large polymeric molecule. Anna C. Palmisano of Procter & Gamble Co. in Cincinnati reports that only two of 28 soil samples tested by her group contained bacteria that can produce the enzymes needed to cleave cellulose into smaller units.