The Arctic holds a telltale record of how humans have used chemicals globally during the past several decades. These cold corners of the earth act as a sink of sorts - chemicals used in industry and agriculture worldwide slowly migrate to and settle there - in sizeable quantities - in water, snow, ice, soil and vegetation.
In a new study, researchers found that the pollutant breakdown process depends largely on the type of dissolved organic matter residing in a body of water, as well as the presence of sunlight. They reported their findings on September 11 at the meeting of the American Chemical Society in New York City.
"Once pollutants enter the water column, their behavior is poorly understood - particularly the processes that govern their lifetime and concentrations," said Amanda Grannas, a postdoctoral researcher in chemistry at Ohio State University. "Such pollutants are now being found in wildlife, from fish to seals to whales, and even in people living in the Arctic."
She and her colleagues analyzed the behavior of two pesticides - lindane and hexachlorobenzene (HCB). Both are prominent in Arctic waters, and both are part of a group of chemicals known as persistent organic pollutants. HCB was banned from use in the United States in 1984, but is still used as a pesticide in many developing countries. Farmers in the United States use lindane to treat seeds prior to planting.
In this study, HCB rapidly broke down into at least two detectable compounds, while lindane remained nonreactive. Aside from their ubiquity, the researchers chose these substances because of their water solubility - lindane's solubility in water is higher than that of HCB.
"A pollutant's water solubility may play a role in its interaction with dissolved organic matter," Grannas said. "Chemicals that are more water soluble are less likely to interact with organic material."
"This interaction may play an important role in the ability of organic matter to react with a pollutant in the presence of light," said Yu-Ping Chin, a study co-author and an associate professor of geological sciences at Ohio State.
"In addition to solubility, a pollutant's chemical properties will play a role in its potential to interact with organic material," Grannas said. "While lindane and HCB are very similar in terms of their chemical formulas, the molecular bonding in each is different. Because of this, we expected a stronger interaction between HCB and organic matter."
Grannas and Chin conducted the study with Penney Miller, an assistant professor of chemistry at the Rose-Hulman Institute of Technology in Terre Haute, Ind. The research was funded by the National Science Foundation.
The fact that some pollutants do degrade may lead to a false sense of security, Grannas said.
"There's a belief that if a pollutant degrades via natural processes, then it's okay to still emit it and let nature take care of the mess," she said. "Other studies analyzing different pollutants have found that their breakdown products in some cases are more toxic than the original pollutant."
Grannas and her colleagues collected surface water samples from several waterways in the Alaskan Arctic. The organic matter detected in each waterway was primarily from plant material that washed into the water. (Algae and bacteria comprise the other primary source of organic matter in water.)
Although the source of organic matter was similar in each water sample, the researchers found that HCB degraded at the highest rate in the presence of organic matter containing the highest nitrogen levels.
"Nitrogen levels can vary even within a body of water, and may be an explanation for why pollutant degradation is higher in some areas," Grannas said. "Other researchers have found that pollutant levels can spike in otherwise similar areas." v In a related experiment, the researchers found that about 30 percent of the original HCB concentration in a lake water sample had degraded in about six hours in the presence of organic matter and sunlight. They conducted another experiment, this time mimicking the amount of sunlight that would penetrate to a depth of four inches. The degradation of HCB was three times slower.
Sunlight penetration drops off quite significantly with depth - much of it reflects off the surface, is scattered in the water column or is absorbed by organic matter in the water.
"There's a significant loss of light even in the first few centimeters of the water column," Chin said. Because of this, the most active area of photochemical decay will be within the top few centimeters of the lake.
Lindane showed no signs of degrading in the presence of organic matter and sunlight.
"Lindane is one of the most persistent of pollutants," Grannas. "This could be because it's photochemically inert, whereas pollutants like HCB degrade relatively quickly.
"The main message is that pollutants can behave quite differently," Grannas said. "These pollutants already affect local ecosystems, and could have repercussions for human health."
[Embargoed for release until 8:30 A.M. ET Thursday, September 11, 2003, to coincide with presentation and the annual meeting of the American Chemical Society.]