From Clouds Comes Rain -- and More

From Clouds Comes Rain — and More

By Randall Gentry

A glimpse of East Tennessee’s Great Smoky Mountains shrouded in low-slung clouds is an archetypical image and one that prompted the Cherokee Indians to term the region “the place of blue smoke.”

But while the daydreamers among us are inclined to study the cloud formations drifting overhead, the region’s scientists—UT researchers among them—are more interested in what’s happening on, and under, the ground. The cloud formations above and the moving water below are, in fact, closely linked.

Over a period of 5 months in spring and summer of 2007, Amanda McKenna and Candice Owen, former graduate students in UT Knoxville’s College of Engineering, tested the waters in a remote Smoky Mountain stream to explore the interaction between the cascades, seeps, and plunge pools on the surface and the catchments of water held within the earth.

The students were particularly attentive to the stream’s chemistry, including the water’s acidity as calculated on the pH scale. “pH” is the measure of the concentration of hydrogen ions in a solution, ranging from highly acidic (low pH) to highly basic (high pH), with a pH of 7 being neutral.

Schwartz, along with Steve Moore, supervisory fisheries biologist of Great Smoky Mountain National Park (GSMNP), is studying the relationship between episodes of high acidity in the park’s higher-elevation waterways and the disappearance of native brook trout in six headwater streams.

The clouds cloaking the Smokies’ higher peaks and the waters gurgling many feet beneath the mountains’ surface are in fact phases of the same system.

The hydrologic system is a continuing cycle through which evaporative water from oceans, rivers, and lakes, along with water vapors given off by plants and animals, enters the atmosphere and eventually forms clouds. Once airborne, this water returns to earth through rain and the columns of mist that drape the Great Smoky Mountains.

The Smokies, among the oldest mountains on earth, are known for their abundant rainfall, with some of the region’s peaks receiving 85 inches of rain each year. Some of the Smokies’ water is stored underground in aquifers that recharge the mountains’ surface streams and provide flow during dry periods between storms.

We know that the surface water cascading down the Smokies’ slopes is fed by tributaries and springs and that it interacts with the water stored underground, but exactly how these waters interact and the chemical changes that result from this interaction are not well understood.

With that challenge in mind, McKenna and Owen sampled the waters of Ramsey Prong, a small headwater stream that is part of the Little Pigeon watershed in GSMNP. The stream is accessible only by a steep foot trail.

Their research focused on water quality and flow, based in part on the study of ions and isotopes contained in the water at various monitoring points. Ions are atoms or molecules that have lost or gained electrons, and isotopes are differing forms of the same element, marked by varying numbers of neutrons. Both are reliable indicators of the chemical changes ­taking place in a water system.

On their forays into the park, the students were equipped with various tools for analyzing stream flow and the chemical composition of the water, along with a scholarly quest. Says McKenna: “We set out to study the hydrology of a remote mountain area and to learn more about groundwater’s role in neutralizing acids entering the streams.”

For the connection between acid and the Smokies’ seemingly pristine streams, we return to the clouds.

The Smoky Mountains’ remoteness and relatively unaltered state make them ideal for studying the interaction between surface and ground waters. Rivers and streams in other more developed areas are influenced extensively by activities that occur around them, including agriculture, construction, and pollutants entering storm drains from paved surfaces.

By contrast, the streams of the Smokies are largely protected from these impacts by their elevation and remoteness.

But they are not protected from airborne pollutants—chiefly the acids carried by the Smokies’ familiar cloud formations. Created through the combustion of fossil fuels, including coal and petroleum products, sulfates and nitrates enter the atmosphere and return to earth in the form of acidic rain and the moisture in clouds that’s transferred as the clouds make contact with the ground.

“Acid deposition from precipitation and fog or clouds has long been associated with low pH [high acidity], low acid-neutralizing capacity, and heightened levels of sulfates and nitrates in waters of GSMNP,” says Owen.

In fact, GSMNP experiences some of the highest acid rain levels in the nation, and nearly half of the total nitrogen in the park comes from dripping cloud water, reported Jim Renfro, chief of the GSMNP Air Quality Branch, at a 2005 ­conference on air quality hosted by UT.

Acid also comes from underground, as water moves through fractured rock—particularly highly acidic Anakeesta formations [mostly slate and metasiltstone]—disturbed through natural processes or mining.

The highest—and steepest—monitoring site in the watershed, as expected, displayed more prominent effects of acid deposition than the lower sites. The upper reaches of the stream are marked by poor soil cover, steep slopes, and a less-permeable rock underlayer, which means less interaction between groundwater and surface water. By contrast, the lower, more-moderately pitched sections of the stream were more likely to interact with groundwater, which dilutes and neutralizes some of the waterborne acids.

“The sensitivity of Ramsay Prong to acid deposition has notable ecological implications,” says McKenna. “Less-hardy fish species, such as native brook trout, may be eradicated from the stream, particularly in the high elevation reaches. Only the very bottom of the basin may prove suitable for these sensitive creatures.”

McKenna’s and Owen’s research helps us understand how the surface streams of the Great Smoky Mountains interact with water stored underground and the chemical changes that result. The study also demonstrates that limited sampling can be a useful tool in understanding the hydrology of sensitive and protected areas, like GSMNP, where more ­conventional field techniques would not be permitted.

The more clearly we understand the interaction of surface and ground water—as well as the complex constituents of the Smoky Mountains’ seemingly benign clouds—the better equipped we’ll be to protect our region’s valuable resources.

Dr. Randall Gentry is director of UT Knoxville’s Institute for a Secure and Sustainable Environment and an associate professor in the Department of Civil and Environmental Engineering.