Rivers and Streams in U.S. Change Their Chemistry Due to Acid Rain

Past acid rain could be having a major impact on the streams and rivers in the United States. In the first survey of its kind, scientists looked at long-term alkalinity in these freshwater sources. Now, they've announced that human activities are drastically changing the water chemistry of these riparian ecosystems.

Alkalinity is a measure of water's ability to neutralize acid. In excess, it can cause ammonia toxicity and algal blooms. This can alter water quality and harm aquatic life. In addition, increasing alkalinity can harden drinking water, causing pipe scaling and infrastructure problems; it can also result in the salinization of fresh water.

In order to test the health of the streams and rivers in the eastern United States, the scientists looked at long-term alkalinity trends in 97 of these freshwater sources, stretching from Florida to New Hampshire. Sites ranged from small headwater streams to some of the nation's largest rivers.

Surprisingly, it turns out that human activities that create acid conditions are driving the problem. Acid rain, acidic mining waste and agricultural fertilizers all filter into rivers and streams. Yet these acidic components speed the breakdown of limestone, other carbonate rocks and even concrete and cement. This means that alkaline particles from this breakdown are washed off of the landscape and into water sources.

"This is 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, but it has not," said Gene Likens, one of the study authors, in a news release. "Acid rain has led to increased outputs of alkalinity from watersheds and contributed to long-term, increasing trends in our rivers. And this is twenty years after federal regulations were enacted to reduce the airborne pollutants that cause acid rain."

In fact, the researchers discovered that the rivers that were most impacted by higher alkalinity were those that provide water for Washington, D.C., Philadelphia, Baltimore, Atlanta and other major cities. This is probably due to acid rain exposure, urbanization and the extent of land covered by cement and concrete.

"What we are seeing may be a legacy effect of more than five decades of pollution," said Suja Kaushal, lead author of the research, in a news release. "These systems haven't recovered. 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.

The findings are published in the journal Environmental Science and Technology.