|Vol. 24, No. 17||March 10, 1999|
BY HANNAH FAIRFIELD
At the bottom of Central Park's picturesque rowboat lake lies the key to a New York City air pollution puzzle.
In sediment core samples taken from the lake, Columbia scientists have found evidence that incinerators-not leaded gasoline-were the main culprit in spewing lead into New York air.
Because the lake has never been dredged, it holds a 130-year history of the city's heavy-metal emissions deposited in thin layers. By correlating the sediment layers to individual years, the scientists saw that the peaks in sediment lead levels did not match the pattern of leaded gasoline use, which was widely expected.
"Urban lakes act like big bucket collectors for atmospheric fallout," said Steve Chillrud, geochemist at Columbia's Lamont-Doherty Earth Observatory and principal author on the paper, which appears in the March 1 issue of Environmental Science and Technology. "As soon as we saw the lead data, we got really excited," said Chillrud. "The sediment record showed that the highest atmospheric lead levels occurred before the maximum use of leaded gasoline."
Despite the excitement, the scientists had a mystery on their hands. The sediment history, which started from the 1860s, showed that the lead levels were highest in the 1930s to early 1960s, after which levels tapered off. In contrast, the maximum use of leaded gasoline was in the late 1960s and early 1970s, until health concerns about smog and childhood lead poisoning resulted in the phase-out of leaded gasoline. This phase-out had previously been linked to a known decrease in human blood lead levels, but the environmental monitoring had only begun in the 1970s. The Columbia scientists began to think that the strong correlation between leaded gasoline and airborne lead in New York City was erroneous.
The scientists, looking for clues that would guide them to an earlier source of lead, measured other metal concentrations in the sediment, collected samples of Central park soil cores and learned about the history of the lake from Central Park Conservancy records. "We had really large amounts of heavy metals, so we were looking for something that could put a huge amount of pollution into the air," said Chillrud.
Chillrud was reluctant to let go of the leaded gasoline explanation, but when he also found high levels of tin in the sediment, he knew that he had to look elsewhere. "It was a big clue; automobiles just don't produce tin in large amounts," he said.
Chillrud and Lamont colleague Jim Simpson collaborated with Rensselaer Polytechnic Institute geochemist Richard Bopp, who had been at Lamont from 1971-1990. They considered smelters, but the main metal refining plants were in New Jersey and the scientists' previous work had shown that New Jersey smelter emissions decreased after the 1930s. Several pieces of the puzzle pointed toward incinerators, both municipal and residential/commercial, as another possible source. The incinerator record to support their ideas was the missing link. Bopp suggested calling a former graduate student of his, Dan Walsh, who had an interest in New York City's history of waste management and was currently the Department of Environmental Conservation's chief scientist at the Fresh Kills landfill on Staten Island.
"Steve called me up and told me that his results were surprising," remembers Walsh. "But as soon as he explained the pattern, it made great sense to me. It fit the incineration pattern, hand in glove."
Walsh sent Chillrud the incineration data he had collected for his Ph.D. from incinerators in New York, which were progressively closed in the 1960s, and the Lamont scientists were amazed how well his data matched theirs.
Previous to this study, leaded gasoline was thought to be the predominant source of airborne lead in the United States. Many studies that looked at changes in atmospheric lead concentration since the 1970s, when most monitoring programs began, saw strong correlations between decreases in the combustion of leaded gasoline and decreases in atmospheric lead. However, the Columbia scientists think that the strong correlation between leaded gasoline and blood lead levels in some urban centers would not hold up if a longer time period for comparison was used.
In addition to short-term monitoring, previous studies on sediment cores, ice cores and corals, all collected from remote locations, had correlated leaded gasoline to air lead levels. Most scientists and policy makers assumed that the data could apply to cities as well, even though long term records of atmospheric pollution levels were not available for urban centers.
"It was such a surprise because away from cities, leaded gasoline was the greatest source of lead in the air, but we saw something completely different," said Simpson. "Our research indicates that urban pollution was most strongly tied to incinerators when they were in use."
The scientists propose that incinerators have been a previously unrealized primary source of atmospheric lead in other urban centers in the United States, the U.K. and Germany, where development and early use of incinerators was greatest.
Simpson, Bopp and Chillrud have spent over two decades recording and reconstructing the history of persistent contaminants in the Hudson River, its tributaries and the New York harbor. Their research, which quantifies the location and amount of toxic chemicals, allows them to evaluate how pollutants have behaved over time, and how they may be expected to behave in the future.
"Knowing the history allows us to evaluate how successful past regulatory actions were," said Bopp. "And that, in turn, allows us to predict how effective regulations in the future will be."