Acid rain may seem, like, so 1980s, but the problem has not gone away.
Researchers reported this week that soils throughout the Northeast are continuing to acidify, despite a 50 percent decrease in acid rain since the peak in 1973.
This may be contributing to declines in sugar maples and red spruce in the region, the researchers said.
"The quality of water is improving, but the soils are continuing to get worse," said study lead author Richard Warby, now at Environ International Corporation in Princeton, N.J.
Warby, who conducted the study while at Syracuse University in N.Y., presented the findings this week at a meeting of the American Geophysical Union in Fort Lauderdale, Fla.
Acid rain in the United States is caused primarily by emissions from coal power plants, especially sulfur dioxide. Acid rain has decreased since restrictions on sulfur dioxide emissions were enacted under the Clean Air Act in 1970 and 1990.
In 2001, Warby repeated surveys done in 1984 by the Environmental Protection Agency of 145 watersheds throughout the Northeast region. He gathered soil and water samples and compared the change over 17 years.
"What we found is rather alarming," Warby said. The levels of calcium ions in the soil had halved throughout the region while aluminum ions had doubled.
Calcium ions are basic, and provide the soil with a way to neutralize acid it is exposed to. They also provide essential nutrition to trees like red spruce and sugar maple.
Aluminum ions, on the other hand, are acidic, and soil aluminum shifts from an inert form into an available form under acidic conditions. The available form is toxic to plants at high concentrations.
"You're replacing a nutrient by a toxic substance," said Charles Driscoll of Syracuse University, who was a part of the study.
"The soils appear to be more sensitive than surface waters," he added. The amount of acid rain seems to have dropped enough that lakes and streams can recover, perhaps with the help of shoreline wetlands and lake sediments, Driscoll said.
But it is not sufficient for soils.
"The level of acidity is still too high to eliminate the stripping of calcium and magnesium from soil," he said.
The effect was strongest in central New England and Maine, Warby said, which was surprising because this region received less acid rain than other regions like the Adirondacks, Catskills and Poconos. This may be because the soils in these other regions were already largely depleted of calcium by the time the first samples were taken in 1984, he said.
Even though surface waters have recovered somewhat, if the neutralizing capacity of the soils continues to decline, lakes and streams may take another hit.
"We've done model simulations that suggest that there will still be recovery for the next couple of decades," Driscoll said. "But unless the base status of the soils is allowed to recover, that may not continue and you may see declines in surface water quality over 70, 80, 100 years."
The Clean Air Interstate Rule, which sets tighter limits for the release of sulfur dioxide and nitrogen oxides from power-plant smokestacks in 28 states, will take effect over the next seven years and should lead to additional reductions in acid rain.
"It will still be elevated," Driscoll said. "It's still a long way from what we think it was 200 years ago."Discovery News
