Legacy of acid rain threatens water supplies

The ongoing effects of river alkalinization – paradoxically caused by acid rain – are showing up across the East Coast of the U.S. and scientists are unsure how long they will persist.

In the first survey of its kind, researchers looked at long-term alkalinity trends in 97 streams and rivers from Florida to New Hampshire. The sites ranged from small headwater streams to some of the nation’s largest rivers. The findings, published in the journal Environmental Science and Technology, indicate that over the past 25 to 60 years, two-thirds of the waterways have become significantly more alkaline.

Excess alkalinity can cause ammonia toxicity and algal blooms, as well as altering water quality and harming aquatic life. It also hardens drinking water, causing pipe scaling and costly infrastructure problems. And, perhaps most alarming, it exacerbates the salinization of fresh water.

Counter-intuitively, human activities that create acid conditions are driving the problem. 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 result: alkaline particles are washed off of the landscape and into waterways.

The survey found watershed geology was the strongest predictor of river alkalinization, with rivers receiving water from porous, limestone, and other carbonate rocks being more alkaline. Topography and pollution were also triggers. The most rapid rates of alkalinization were at high elevation sites that were chronically exposed to acid pollution.

The researchers report that the rivers impacted by higher alkalinity are those that provide water for Washington, D.C., Philadelphia, Baltimore, Atlanta, and other major cities. This is due, in part, to acid rain exposure, urbanization, and the extent of land covered by cement and concrete. Also affected are rivers that flow into water bodies already harmed by excess algae, such as the Chesapeake Bay, where managers are struggling to contain algal blooms that are toxic to marine life.

“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,” noted the study’s lead author Sujay Kaushal, from the University of Maryland. “What we are seeing may be a legacy effect of more than five decades of pollution. These systems haven’t recovered. How many decades will it persist? We really don’t know the answer.”

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Source: Cary Institute of Ecosystem Studies

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