Landfills Make Mercury More Toxic

Mercury, a nerve poison, is a major ingredient in many products–from thermometers and fluorescent bulbs to batteries and old latex paint. A new study finds that landfill disposal of such products can chemically alter the mercury in them, not only rendering it more toxic but also fostering its release into the air.

While open landfills (above) may expose wildlife directly to poisonous mercury, closed landfills can vent tainted gases through pipes (below). Lindberg


Although even mercury in its elemental form is toxic, its most poisonous embodiment is methyl mercury, the result of a chemical modification by bacteria (SN: 3/9/91, p. 152). The finding of such a process in landfills underscores the importance of ensuring that mercury doesn’t enter the municipal-waste stream, says study leader Steve E. Lindberg of Oak Ridge (Tenn.) National Laboratory.

The decomposition of interred landfill wastes creates methane. Some landfill managers burn the gas in flares as it exits pipes atop the waste field. Most managers, however, merely vent the gas–and any contaminants it may carry–into the air.

Two years ago, Lindberg’s team found methyl mercury in the water vapor that condensed out of the gas emanating from a Florida landfill. Concentrations were at least 100 times those typically seen in water. The finding made sense, Lindberg recalls: In wetlands, researchers had previously identified certain bacteria that methylate natural, inorganic mercury derived from minerals. This same family of microbes resides in landfills.

However, methyl mercury comes in two forms–mono- and dimethyl-mercury–with the latter being the more toxic. To probe which form is made in landfills, Lindberg and his coworkers collected gases destined for flaring. In the August Atmospheric Environment, they report finding some 50 nanograms of dimethyl mercury per cubic meter of landfill gas.

That “is higher, by a factor of 30 or 40, than concentrations of total mercury in ambient air,” Lindberg notes, and it’s at least 1,000 times that of any dimethyl-mercury concentration ever recorded in open air. His team also detected lower concentrations of the less volatile mono-methyl mercury in the landfill gas.

Although chemists had detected methyl mercury in air and rain, “nobody had been able to demonstrate where it comes from,” notes John W.M. Rudd of the Winnipeg (Manitoba) Freshwater Institute, part of Canada’s Department of Fisheries and Oceans. The new study offers “the first real evidence that landfills might be a major source,” he says.

Some 60,000 U.S. children are born each year with developmental impairments triggered by fetal exposure to methyl mercury, usually as a result of their moms having eaten tainted fish (SN: 7/29/00, p. 77). “If it doesn’t get methylated, mercury doesn’t get into fish,” observes Edward Swain of the Minnesota Pollution Control Agency in St. Paul.

To limit the rain of mercury from human activities, regulators have focused on curbing emissions of inorganic mercury from coal burning. However, Lindberg notes, although chemists assumed that mercury could become methylated in the air, they couldn’t show it.

Now, Swain posits, a “shift in paradigms” may be in order. He says that sending mercury-containing wastes to landfills may essentially be spoon-feeding copious amounts of the toxicant to methylating bacteria, which then cough the injurious forms into air.

The new findings point to the need to inventory emissions by landfills–especially the older ones, which hold the richest stores of mercury-tainted wastes–says Frank D’Itri of Michigan State University’s Institute of Water Research in East Lansing.

Lindberg plans to embark on such an inventory. He says that the new data also suggest a need for technologies to capture methyl mercury from landfills before it can enter the atmosphere.

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

More Stories from Science News on Earth