Human activities are changing the basic chemistry of rivers in ways that could have major consequences for urban water supplies and aquatic ecosystems.
In the first survey of its kind, researchers looked at long-term records of alkalinity trends in 97 rivers from Florida to New Hampshire. They found that over time spans of 25 to 60 years, two-thirds of the rivers had become significantly more alkaline.
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.
“The change across so many larger rivers of the eastern U.S. is a big surprise,” says study co-author Michael Pace, an environmental scientist in the University of Virginia.
Among the rivers affected are those that provide water for Washington, DC, Philadelphia, Baltimore, Atlanta, and other major cities, the researchers report. Also affected are rivers that flow into water bodies already harmed by excess algae growth, such as the Chesapeake Bay.
None of the rivers became more acidic. But paradoxically, notes lead author Sujay Kaushal, a University of Maryland geologist, higher acid levels in rain, soil, and water, caused by human activity, are major triggers for rising alkalinity.
The researchers hypothesize that acid rain, a byproduct of fossil fuel combustion, and acid runoff from mining are hastening the dissolving of rocks and human-made surfaces that are naturally high in alkaline minerals.
In a process known as chemical weathering, the acid eats away at limestone, other carbonate rocks, and even concrete sidewalks, dissolving alkaline particles that wash off into streams and rivers.
“It’s like rivers on Rolaids,” Kaushal says. “We have some natural antacid in watersheds. In headwater streams, that can be a good thing. But we’re also seeing antacid compounds increasing downriver. And those sites are not acidic, and algae and fish can be sensitive to alkalinity changes.”
Long-term studies, like those the University of Virginia scientists are conducting on streams in the Shenandoah National Park, have documented the effects of increased chemical weathering in small mountain streams tainted by acid runoff.
Many streams have acidified, except in areas where the underlying rocks provide buffering that helps rebalance streams’ pH levels. But researchers had not previously looked at the accumulating levels of alkalinity in downstream reaches of numerous major rivers and evaluated potential causes.
“Large rivers are among the most modified ecosystems on the planet and this study provides evidence of widespread change in river chemistry and further reason to reduce the input of pollutants,” Pace says.
The researchers link the river changes to acid rain. Beginning in the mid-1990s after Congress amended the Clean Air Act, new federal regulations have reduced the airborne pollutants that cause acid rain.
“It may be that these are legacy impacts of acid rain in addition to mining and land use,” Kaushal says. “The acid rain problem is decreasing. But meanwhile there are these lagging effects of river alkalinization showing up across a major region of the US
“How many decades will river alkalinization persist? We really don’t know the answer.”
On carbonate rocks
The team focused on Eastern rivers, which are often important drinking water sources for densely populated areas and have decades’ worth of water quality records.
Much of the eastern US is also underlain by porous, alkaline limestone and other carbonate rocks, making the region more prone to the types of water chemistry changes that the researchers found. This is especially true in the Appalachian Mountains where soils are thin, steep slopes cause erosion, and acid rain from smokestack industries have had a major impact on forests and streams.
Water alkalinity has increased the fastest in areas underlain by carbonate rocks, at high elevations, and where acid rainfall or drainage was high. The researchers also found that the chemical weathering of these carbonate rocks adds to the carbon burden in rivers and streams, in a trend that parallels rising carbon dioxide levels in the atmosphere.
NASA Carbon Cycle & Ecosystems, the National Science Foundation’s Long Term Ecological Research Program, and the Andrew W. Mellon Foundation supported the research. The study is published in the online edition of Environmental Science and Technology.
Source: University of Virginia