Lakes may slow pollutant removal from air.

Lakes may slow pollutant removal from air

Surprisingly high levels of potentially toxic air pollutants return to the air from the waters into which they have fallen, a new study indicates. This finding helps solve the mystery of why polychlorinated biphenyl (PCB) levels over the Great Lakes haven't diminished since the 1977 ban on their production. It also suggests that lakes and other bodies of water may facilitate the migration of industrial air pollutants toward polar climes from the relatively warm areas where they're made and used.

Water-insoluble air pollutants such as PCBs and polycyclic aromatic hydrocarbons (PAHs) generally settle out of the atmosphere within days of their release. For years, scientists assumed these contaminants took a quick, one-way route into oblivion no matter where they fell. Those falling on land do tend to get buried in soil or washed into local streams with rain soon afterward. Similarly, those deposited in bodies of water were thought to attach to large particles, such as suspended bits of soil, and to settle out as sediment.

But the watery fate of these chemicals may prove more complicated, researchers now report.

In the 1950s, '60s and '70s, the atmosphere clearly served as the leading source of chemical contamination to the upper Great Lakes, notes Joel E. Baker, a chemist at the University of Maryland's Chesapeake Biological Laboratory in Solomons. That's not the case today, his new data suggest.

While at the University of Minnesota in Minneapolis, he and Steven J. Eisenreich analyzed air and water sampled from a number of sites in eastern and central Lake Superior for 35 different PCBs and 14 PAHs, including the carcinogens benz[a]-anthracene, phenanthrene, pyrene, chrysene and benzo[a]pyrene.

In the fall, winter and spring, they found, air appears to remain the net source of these pollutants, depositing them in the lake. In the summer months, however, the lake becomes the net source, recycling the contaminants back into the air through a process called revolatilizaton. The lake's summertime release appears to equal the pollutant levels deposited into the lake from the air during the entire rest of the year, the researchers say. Baker and Eisenreich report their findings in the March ENVIRONMENTAL SCIENCE AND TECHNOLOGY.

Though scientists had some hints that pollutants might return to the air from lakes, Baker says, "until we did this study, the magnitude of potential volatilization was not really appreciated."

Driven largely by temperature, the revolatilization of pollutants from the water into the air should occur more frequently in warmer climates than in cold ones, Baker explains. Each time the pollutants return to the air, they can waft farther from their initial source. But when they drift into colder regions, the less efficient revolatilization from surface waters should result in a proportionately higher rate of permanent pollutant fallout, Baker says. Indeed, this mechanism might help explain the high levels of PCBs and organochlorine pesticides found in the Arctic, far from their release sites.

If this pollutant recycling also occurs in oceans, "then the oceans could perhaps account for the level of contamination that we're seeing in Arctic," says Dennis J. Gregor, a geochemist with Environment Canada in Burlington, Ontario. However, Gregor says, the apparent lack of chemical transformation or "aging" he sees in the Arctic PCBs and pesticides suggests they were not recycled through water but instead were carried directly from their sources, probably in Eurasia. For this reason, he thinks the "most worrisome" aspect of the new findings is the implication that a disproportionate share of these contaminants will stay in the Arctic, where cold weather hinders natural pollutant degradation.