Acid rain was the environmental cause celebre of the 1980s and 1990s, but lately its spotlight has been usurped by newer concerns, like climate change and biodiversity. But its effects still linger in rivers and streams throughout the United States, researchers say in a new study.

Ironically, acid rain is making waterways in the U.S. more alkaline – so actually they are less acidic than ever. But while alkalinity can be a good thing in moderation, in excess it can result in a range of effects, including hard water, ammonia toxicity and algal blooms. Increasing alkaline levels also contribute to processes that make fresh water more salty.

A team of researchers led by University of Maryland researcher Sujay Kaushal took a look at alkalinity trends in waterways up and down the U.S. East Coast. Of the 97 streams and rivers they studied, two-thirds have become significantly more alkaline, according to results published in the journal Environmental Science and Technology.

“The rise of alkalinity in many rivers throughout the Eastern U.S. suggests human-accelerated chemical weathering, in addition to previously documented impacts of mining and land use,” the authors wrote.

But how does acid rain result in more alkaline rivers? The acid – whether it comes from acid rain, agricultural fertilizers, or acidic mining waste -- is just the first step; when it falls on alkaline-containing materials and rocks, like limestone, concrete and cement, it erodes particles that are carried off downstream.

“What we are seeing may be a legacy effect of more than five decades of pollution. These systems haven't recovered,” Kaushal said in a statement. “Lagging effects of river alkalinization are showing up across a major region of the U.S. How many decades will it persist? We really don't know the answer."

Rivers with watersheds lined by limestone or other alkali-containing rocks were much more likely to have higher alkaline levels, but other factors contribute as well. The shape of the land around the river, as well as its proximity to pollution and urban centers, are also major factors. The rivers that alkalinized most rapidly were those at high elevations that also experienced chronic acid pollution exposure, according to the paper.

"This is another example of the widespread impact humans are having on natural systems,” coauthor Gene Likens, who pioneered the study of acid rain in the 1960s, said in a statement. “Policymakers and the public think that the acid rain problem has gone away, but it has not. Acid rain has led to increased outputs of alkalinity from watersheds and contributed to long-term increasing trends in our rivers. And this is 20 years after federal regulations were enacted to reduce the airborne pollutants that cause acid rain."

SOURCE: Kaushal et al. “Increased River Alkalinization in the Eastern U.S.” Environmental Science and Technology, 2013.