Researchers from the University of Maryland and other institutions looked at records of alkalinity trends in 97 rivers from the U.S. state of Florida to the state of New Hampshire over the past 25 to 60 years and found that two-thirds of the region’s rivers are becoming “significantly more alkaline.”
Among the rivers impacted are those that provide water for big cities such as Washington D.C., Philadelphia, Baltimore, and Atlanta, the researchers said.
Increased alkalinity complicates drinking water and wastewater treatment, encourages algae growth, and can hasten the corrosion of metal pipe infrastructure. At high alkalinity levels, ammonia toxicity can also harm irrigated crops and fish in rivers, according to the researchers.
In what may seem like a paradox, human activities that create acid conditions, such as the burning of fossil fuels, are causing the problem, they said.
“This is because acid rain, acidic mining waste, and agricultural fertilizers speed the breakdown of limestone, other carbonate rocks, and even concrete and cement,” the researchers said in a statement. “The result: alkaline particles are washed off of the landscape and into streams and rivers.”
The airborne pollutants that cause acid rain are on the decline in the United States, as a result of tighter environmental regulations designed in the 1990s, but the legacy of acid rain remains.
“The acid rain problem is decreasing. But meanwhile, there are these lagging effects of river alkalinization showing up across a major region of the U.S.,” said lead author Sujay Kaushal, an associate professor and aquatic ecologist at the University of Maryland. “How many decades will river alkalinization persist? We really don’t know the answer.”
Co-author Gene Likens of the University of Connecticut and the Cary Institute of Ecosystem Studies described their study as “another example of the widespread impact humans are having on natural systems.”
“Policymakers and the public think that the acid rain problem has gone away,” said Likens, also a co-discoverer of acid rain in 1963. “But it has not.”