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Lasting effects. Studies of soils in New Hampshire and Maine (inset) suggest acid rain is still impacting regions of the U.S. northeast.

Greg Lawrence/USGS; (inset) Marissa Weiss

Is Acid Rain a Thing of the Past?

The story of acid rain from the 1970s is preserved in newspaper headlines, textbooks, and, it turns out, the soils of the northeastern United States. Forty years after humans first began tackling the problem, the impact of acid rain still lingers in New York, Vermont, New Hampshire, and Maine, according to a new study. But the research also shows the first signs of recovery.

At the height of the acid rain problem, sulfur dioxide from burning coal drifted into the atmosphere and lowered the pH of rainwater. When this acidic rain fell to the ground, it leached calcium from the soil, depriving plants of a key nutrient. Acid rain also dissolved aluminum-rich minerals, freeing the metal to further poison plants.

To combat the problem, the U.S. Congress imposed strict emission regulations on industry in 1970 through the Clean Air Act, which was strengthened in 1990. By 2003, sulfur dioxide raining down on the northeastern United States had decreased by as much as 40%. But were soils improving, too?

To find out, Gregory Lawrence, a biogeochemist at the U.S. Geological Survey in Troy, New York, and colleagues tested soils in six spruce forests. The sites included the Adirondack Park in New York, the White Mountain National Forest in New Hampshire, Groton State Park in Vermont, and two research reserves in Maine. Buried below the forest floor, soil mixes with rocks that, as they weather, slowly leak calcium. The researchers reasoned that if they dug beneath the surface, they might find one early indicator of recovery: rising calcium concentrations in soil. They had first tested the soils in the region in 1992 and 1993. Eleven years later, they went back and tested again.

There were modest signs of improvement, the team will report online next month in the Soil Science Society of America Journal. Calcium levels in the soil were still low, but aluminum in surface soils had begun to disappear—at least in Vermont, New Hampshire, and Maine; New York soils still sported high levels of the metal. “The way the soils were recovering was not really the way we expected,” Lawrence says.

Lawrence suggests a two-step explanation. First, less acid rain means less aluminum dissolving from minerals and circulating in the soil. Second, surface soils are being replenished by decaying plant matter, which has low levels of aluminum and is essentially diluting the concentration of the metal in soil. “This is a response to the declining acid rain levels," Lawrence says. "It's just being driven more by the plants than it is the geology.”

Calcium is not rebounding in the soil because the rocks at these sites, which are typical of the region, are not rich in the nutrient and weather very slowly, says Lawrence. That’s one reason the soils take so long to recover. In fact, calcium can buffer soils against some of the worst consequences of acid rain, but now—because there is so little calcium left to stand in the way of harmful chemical reactions such as the ones that mobilize aluminum— these soils “are actually more sensitive to acid rain today than they were 25 years ago,” he says. On their way to recovery these soils are hanging by a precarious thread.

The study “is the first to hint that the deterioration of northeastern U.S. soils from acidic deposition has finally bottomed out,” says Brenden McNeil, a biogeochemist at West Virginia University in Morgantown, who was not involved in the work. He points out, however, that the impacts of acid rain extend beyond northeastern spruce forests to areas where the extent of the damage and the status of the recovery remain unknown. A 2012 global acidification assessment reports, for example, that in Canada and Western Europe, sulfur dioxide emissions have declined at about the same rate as in the United States, but in places like China, sulfur dioxide emissions are now reaching levels that haven’t been seen in the U.S. since 1970. Even in the region studied, McNeil says, the subtle improvements in soil are “not near as dramatic as the reductions in emissions”—a sign that clearing the air of sulfur dioxide is just the first milestone on a long road to recovery.