What the Streams Say

The Clean Air Act has been a roaring success if you’re looking at how much pollution falls out of the sky. Emissions of sulfur dioxide from power plants—the pollutant that causes most acid rain—plummeted nationally from 17.5 million tons in 1980 to 3.3 million tons in 2012. That decline is astonishing in anybody’s book.

But if you’re interested in the health of America’s streams, the effect of the 1970 law isn’t nearly so dramatic. That’s because rain isn’t the only thing that determines what’s in the water; the soil and rock that streams flow through are also critical. Consequently, although the landmark legislation (and especially its 1990 amendments) has had a major impact in some places, in others, change is barely detectable.  

East of the Mississippi, where acid rain has always been the worst, the Adirondack Mountains have seen the greatest improvement in stream health. There, the amount of acid-producing sulfate declined in 90 percent of the streams being monitored from 1990 to 2008. To the south, in central Pennsylvania, sulfate concentrations also fell in a majority of streams. But in the Central Appalachians (including Virginia’s Blue Ridge Mountains), improvement was the exception: Sulfate declined in only 12 percent of monitored streams.

Something different is going on there. And Shenandoah National Park is helping reveal what it is.

Seventy-five miles southwest of the nation’s capital, Shenandoah is home to the longest study of water quality in any national park. The research is shedding light on “risk factors” for slow recovery from decades of acid rain.

The park is well placed for the task. The western slope of its mountains catches both dry pollution and acid rain from the industrial Midwest. Along with Great Smoky Mountains to the south, Shenandoah is more affected by acid rain than any other place in the National Park System. It’s also a good place to monitor the effects of acid rain because the water in these “headwater” streams comes only from rain, snowfall, and springs, not from lakes, ponds, or other watercourses.

“National parks are great places to do science because they let us control for things that we can’t in landscapes where there’s more human interference,” says Ami Riscassi, a scientist at the University of Virginia (UVA). “If you look at stream water in, say, Charlottesville, there are too many other things going on.”

UVA researchers began sampling Shenandoah water in 1979, visiting two streams at the southern end of the park each week to measure such variables as pH (acidity), temperature, and the concentration of key chemical compounds and elements. Two more streams were added to the study and devices were installed to grab samples during gully-washing storms.

Findings from the park’s program were added to data from the Virginia Trout Stream Sensitivity Study, which in 1987 tested water at 379 sites, about 60 inside the park. A subset of the streams was then regularly monitored, and the original sites were resampled in 2000 and 2010. At the last outing, 165 people in sneakers and hip boots spent 1,500 hours scooping water into plastic jars. 

The result is an unusually detailed record of Shenandoah’s water quality and how it’s changed (or hasn’t) over 35 years. 

A recent analysis of the two studies’ data found that whereas one-third of the park’s streams were classified as “acidic” in 1987, in 2010 only one-sixth were. Over the same period, the concentration of acid-causing sulfate declined and the streams’ chemical capacity to offset acidity rose on average. The trouble was that 16 percent of the streams showed little improvement. A few actually got worse.

“It’s a good news, bad news story,” says Rick Webb, the UVA scientist who coordinated the two studies before turning the task over to Riscassi this year. “We have reversed the damage for many systems. But some are so damaged they’re going to take a while to recover.”

The biggest risk factor for continued poor health appears to be geology. 

All of the still-acidic streams in Shenandoah flow through soil made from weathered sedimentary rock, which underlies about one-third of the park’s surface area.  (The rest of the park is equally divided between basaltic and granitic bedrock.) Grains of sand and clay in the sedimentary soil bind sulfate molecules, keeping some of the acid-producing chemical out of the water. But after a century of acid rain, the sedimentary soil is saturated with sulfate. It releases some of the load after each rainstorm—even though the rain itself now contains little sulfate.

As a consequence, streams in sandy watersheds are still acidic, and streams in rockier watersheds have seen acidity decline along with the fall in sulfate pollution.

“Even now, if we were somehow able to stop acid rain, those streams would be running acid for many decades because of that bankroll of sulfur,” says James P. Schaberl, chief of natural and cultural resources at the park. 

None of Shenandoah’s streams were acidic before modern times. Political action—specifically, implementing regulations that reduce power-plant emissions—has been necessary to move them back toward their primordial state. More stringent regulations may be necessary to hasten the process. 

Nevertheless, progress is undeniable.

“The people before me had the great foresight to say we need to understand whether these emission reductions are improving the environment,” says Richard Haeuber of the Environmental Protection Agency’s Clean Air Markets Division. “Is it working? Is it worth the effort being spent? Yes, the Clean Air amendments have worked.”

But Shenandoah National Park makes clear the work isn’t done.