Fresh water isn't what it used to be. New research has found that human activity has caused subtle but significant changes in the basic makeup of rivers in Maryland and elsewhere, with potential consequences for public water use and the health of aquatic ecosystems.
In a survey said to be the first of its kind, researchers say two-thirds of nearly 100 rivers and streams checked across the eastern United States, including the Patuxent and Potomac rivers, have become noticeably more alkaline over the past 25 to 60 years. It's as if they'd been fed a steady dose of antacid medicine, "like rivers on Rolaids," said Sujay S. Kaushal, a University of Maryland geologist and the study's lead author.
Several years ago Kaushal and other colleagues documented a gradual increase in salt levels in streams in the Northeast, which at the time they attributed to runoff of road salt. But, Kaushal said, a closer look at stream sampling from the U.S. Geological Survey and other sources found the rising salinity also tied to the rivers' increasing alkalinity.
"We weren't expecting it to be this widespread," he said. "These trends that we're seeing in the Schuylkill River and Potomac River, these trends are troubling."
Nor did the scientists expect to find that their leading suspect for changing river chemistry is an environmental menace thought vanquished long ago.
At first, they figured the rising alkalinity seen across the eastern United States came from landscape changes in a river's watershed, such as mining or urban and suburban development. But the changes in river chemistry were so extensive that they figured something more, and more widespread, was going on.
They concluded it had to be acid rain.
Blamed for killing forests and rendering New England lakes lifeless in the 1970s, acid rain has been reduced dramatically in the decades since by government regulation of coal-burning power plants and motor vehicle exhaust, two leading sources of pollution that turned precipitation acidic.
Rainfall today is much less acidic than it used to be, experts say, but all sources of acid rain haven't been muzzled. Rain in Maryland and elsewhere is still corrosive enough to dissolve limestone and other rocks.
"Despite the great progress we've made in controlling oxides of nitrogen and sulfur from smokestacks and tailpipes, the precipitation is still acidic, unhealthy for us and streams," said Russell R. Dickerson, an atmospheric chemist at the University of Maryland.
The minerals released when rocks dissolve under acid rainfall tend to be alkaline, Kaushal explained. And those minerals aren't just neutralizing the acidity in precipitation, but overwhelming it, he added.
The effect is pronounced in urban areas like Baltimore, he said, and even in streams like the Gwynns Falls, where there isn't much natural limestone in the watershed. But there, he pointed out, "you have pavement, sidewalks and concrete," with limestone often a major ingredient. They dissolve under acidic rain as well, in a process dubbed "chemical weathering."
Baltimore's streams also may get an extra dose of alkalinity antacid from its leaky sewers, Kaushal suggested, because wastewater can be alkaline.
"We think that acid rain is affecting all of them, coupled with mining and land use," he said. "Basically when you have dissolving either of the limestone or concrete, that's a kind of universal mechanism contributing to river alkalization."
Just as some mysteries remain about how rivers' chemistry is changing, the impacts also are uncertain, but could be problematic. Many streams in the Chesapeake Bay region are suffering from an overdose of nutrients. Increasing alkalinity coupled with an abundance of nitrogen, one of those nutrients, can generate ammonia in the water, which can kill fish. The changes also may accelerate algae growth, already a problem in bay waters.
With the rising salinity of rivers and streams, calcium concentrations are increasing as well, making the water "harder" for human usage, Kaushal said. That can complicate everything from showers to doing laundry, and can lead to "scaling" in water pipes, a buildup of calcium that constricts the flow.
William P. Stack, deputy program director of the Center for Watershed Protection, said he alerted Kaushal to the increasing chloride concentrations in Baltimore's raw water supply several years ago when he was with the city Department of Public Works. Stack said he also noticed the city's water was gradually getting harder. While the hardness is not a health threat, and can be treated to counter it, he said he still found it troubling.
"The broader concern is we're picking up a signature in the ambient concentration or quality of our waters," Stack said. "What else is going on that we're not picking up? And do we really fully know what the ramifications are?"