Acid highs and lows in Adirondack lakes.

Acid highs and lows in Adirondack lakes

Ecologists studying 274 lakes in New York's Adirondack area report 80 percent have acidified over the past 50 years, probably due to acid rain. This study, which the researchers say is the first to profile regional lake acidification by elevation, also identified several factors apparently predisposing the area's high-altitude lakes to greater acidification.

The ecologists compared water-chemistry measurements made between 1929 and 1934 with data collected from the same lakes between 1975 and 1985. In the March ENVIRONMENTAL SCIENCE AND TECHNOLOGY, they describe the half-century changes in Adirondack alkalinity as "significant" and as a more "robust and accurate" gauge of regional acidification than a largely pH-based 1986 analysis by the National Academy of Sciences (NAS). The NAS study had concluded that possibly no change occurred.

Alkalinity is essentially a measure of water's acid-buffering capacity, whereas pH measures hydrogen ions. Because pH can vary with the amount of dissolved carbon dioxide in water, "it is not a reliable way to measure additions of strong acids -- like the sulfuric and nitric acids found in acid rain," says Mark D. Mattson, at the New York Botanical Garden's Institute of Ecosystem Studies, in Millbrook, a coauthor of the new study. Moreover, he says, pH is very difficult to measure with methods used during the 1920s and '30s. For these reasons, his group focused on alkalinity.

The researchers found a median alkalinity decrease of 50 microequivalents per liter ([mu]equiv/l) in Adirondack lakes--a value suggesting the annual addition of about 6 X 10.sup.17 hydrogen ions of strong acid per liter. For lakes measured at 250 [mu]equiv/l in the 1930s, a 50-unit drop would hardly change either their pH or their biology. But such a drop--the norm at around 582 meters elevation -- could easily prompt major changes, such as fish kills, in high-elevation lakes that measured only 50 to 100 [mu]equiv/l in the 1930s, Mattson says.

Geology and weather contribute to increased acid sensitivity at high altitude, the researchers found. Since rainfall and a lake's collection of rain runoff tend to increase with altitude in the Adirondacks, higher lakes generally receive more acid rain. Moreover, high-altitude lakes are more likely to sit near acid-fostering crystalline bedrock and thin acid soils.

The short stay of water in Adirondack lakes also fosters acidification, says Charles Driscoll, a civil engineer at Syracuse (N.Y.) University. Many researchers over the past several years have shown sediment-dwelling microbes can neutralize acid added to lakes. However, he notes, this process can take up to five years. Driscoll says his own research has shown that Adirondack lakes, unlike Midwestern ones, tend to flush water through their system far too quickly -- from once to 10 times per year -- to allow much in-lake buffering of acid.

Noting that the NAS couldn't decide if acidification occurred in Adirondack lakes, Mattson says, "We wanted to set the record straight" and show that the ambiguity is due to "unreasonable" assumptions about interpreting the older data.

"There are a lot of problems with those historical data [used by both groups]," Driscoll says. "But I think [Mattson and his co-workers] have done as good a job as could be done -- much better than the NAS."

James R. Kramer at McMaster University in Hamilton, Ontario, disagrees. Head of the NAS team, he charges that the new report contains serious errors -- such as its authors' claim to have used, for comparison, the same pH method as the NAS group. Kramer also challenges those researchers' understanding of historical pH-analytical methods and says they underestimated what the NAS team knew about limitations in the New York lake data. "The bottom line," he says, "is that there are too many unknowns to make a definitive statement" about Adirondack acidification.