Two-thirds of rivers in the eastern US are becoming increasingly alkaline, making their waters more dangerous for crop irrigation and fish life.
Even though alkaline is the opposite of acid, the reason for the change is the legacy of acid rain, which eats away at rocks and footpaths high in alkaline minerals, researchers in the journal Environmental Science and Technology say.
"It's like rivers on Rolaids," said lead author Sujay Kaushal, a geologist at the University of Maryland.
Researchers examined 97 rivers from the north-eastern state of New Hampshire down to Florida over the past 25 to 60 years.
Each is important in providing drinking water to big cities such as Washington, Philadelphia, Baltimore, Atlanta, and other major metropolises.
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They found "significant increasing trends in alkalinity at 62 of the 97 sites," said the study.
None of the rivers studied had grown markedly more acidic over time.
Researchers said higher alkaline content in the water can complicate wastewater and drinking water treatment and lead to faster corrosion of metal pipes.
Water with higher alkaline levels can be harder, saltier and contains more minerals than soft water.
It can also lead to ammonia toxicity that can harm irrigated crops, as well as fish and other river life.
Alkalinity increased the fastest in places where carbonate or limestone rocks lay beneath bodies of water, at high elevations, and where acid rainfall or drainage was high, the researchers said.
A process known as chemical weathering - whereby acid eats away at limestone, other carbonate rocks and even footpaths - is blamed for dissolving alkaline particles that wash off into waterways.
Even though acid rain is on the decline in the United States, due in large part to tighter environmental restrictions put in place in the 1990s, its legacy is continuing.
