From: Marissa Weiss, Science AAAS
Published June 29, 2012 08:39 AM

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.

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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.

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