Acid rain commanded as much attention in the 1990s as global warming or the hole in the ozone does today.
But despite more than two decades of efforts to stop acid rain by wringing pollution out of the skies, millions of tons of toxins continue to rain down on the nation's watersheds, rivers and lakes.
And a new federal report suggests that the Environmental Protection Agency has hit a wall in its efforts to reduce the amount of acid rain that drifts down from the sky.
"Even with reduced emissions, NOx [nitrogen oxide], SO2 [sulfur dioxide] and mercury continue to pollute the nation's water bodies," the Government Accountability Office said in a report released last week.
Legal and scientific hurdles are blocking the EPA from making further advances in the battle against acid rain, the GAO wrote.
"It is unclear whether or when the agency will be able to address scientific uncertainties to enable adoption" of a tougher set of standards.
"According to agency officials, EPA has not identified alternative strategies to address acidification of the aquatic ecosystems if it cannot resolve the scientific uncertainties," the GAO concluded.
Acid rain is a combination of chemicals that consists primarily of nitrogen oxides (NOx) and sulfur dioxide (SO2). These chemicals react with the atmosphere to form nitric acid and sulfuric acids.
NOx at its highest concentrations can produce so much nitrogen in water it virtually eliminates the presence of oxygen, turning those bodies of water into "dead zones" where nothing can live.
A third key element in the airborne soup is mercury, which can be absorbed by fish and can be a serious health hazard when those fish are eaten by humans, especially children. Complicating U.S. efforts to halt the mercury bombardment is the fact that most of it originates outside the U.S. and is beyond control of American regulators.
Since the effort to curb acid rain began in the 1990s, the amount of NOx emissions into the atmosphere has declined from 26 million tons in 1990 to about 17 million tons in 2008. The SO2 reductions during that time went from 23 million tons to 10 million tons a year.
Much of the reductions in those airborne chemicals have come from the Clean Air Act mandating cleaner gasoline for cars and trucks and stricter rules on power plant emissions.
The amount of mercury emitted into the air has been greatly reduced from 246 tons in 1990 to 61 tons in 2008.
That still leaves tons of toxins wafting into the country's water each year and a sampling of the effect includes these stats from the GAO report:
- 53,000 square miles of the Great Lakes, or 88 percent of those lakes, are impaired by acid rain.
- 550 lakes in the Adirondack Mountains are affected by acid rain.
- 21,000 miles of streams in the central Appalachian Mountains are tainted.
- "Most of the Chesapeake Bay and its tidal waters are impaired" by an excess of nitrogen from acid rain and other sources.
- In the Northeast, the fish in more than 10,000 lakes, ponds and reservoirs and 46,000 miles of rivers are designated as unfit for human consumption because of high levels of mercury.
"EPA faces challenges in using air regulations to further address the effects of atmospheric deposition from NOx, SO2 and mercury," the report states.