The Critical Zone

April in central Pennsylvania: Against the brilliant blue sky, the new leaves of oak, maple, and beech shine gold-green. Yellow flowers of cinquefoil and hot-pink blooms of gaywings dot the forest floor. Somewhere higher up in the watershed, a red-bellied woodpecker gives its rattling call. Susan Brantley, a Penn State geologist, walks along a path beside a step-across stream.

"This is a first-order watershed," Brantley tells me, "a small catchment carved into shale bedrock." Hills creased with swales hem in the narrow trickle of water. "The stream doesn't have a name," Brantley says. "It usually dries up by late summer. The entire watershed is about twenty acres."

Near the path, white plastic tubes poke up from the leaf duff like mushrooms. Trees have small numbered aluminum tags tacked to the bark near their bases. On the ground, gray PVC pipes carry electric lines. Small red and yellow and blue and green plastic flags attach to stiff wires poked into the earth.

We are in the Shale Hills Critical Zone Observatory (CZO), in Penn State's Stone Valley Forest, about ten miles south of University Park. Here Brantley and more than a dozen other scientists from a host of disciplines are studying the chemical, physical, biological, and geological processes operating in the Critical Zone, the region between the top of the vegetative canopy and the subterranean base of unweathered rock.

The Critical Zone exists all over the earth's landmass. It has been called "a constantly changing open system," "a living membrane," and "the fragile skin of the planet." Brantley's colleague and the principal investigator at the Shale Hills CZO, Penn State civil engineering professor Chris Duffy, has described the Critical Zone as "a single mechanism capable of nurturing life, supporting agriculture, cleansing water, buffering atmospheric gas levels."

How a Watershed Works

Brantley stops where a tree has fallen, its roots pushing up a hummock of dirt and rock. She picks up a grayish tan shard the size and thickness of a poker chip but with an irregular shape. "Rose Hill shale," she says. "It's Silurian, laid down several hundred million years ago, very common up and down the Appalachians. In some ways, it's a pretty boring rock—it doesn't have much organic matter, doesn't have a high sulphur or metal content other than iron.

"When geologists look at soil, we see rock that's evolving. We think about the parent rock: How long did it take to be weathered, transformed into the regolith?" The regolith is the layer of loose rock and soil sitting on top of the bedrock. At the Shale Hills CZO, Brantley and her research team of postdoctoral scholars and graduate students are collecting and analyzing soils and sediments, learning which chemical reactions affect soil production.

Susan Brantley
Scott Johnson

Susan Brantley examines oak seedlings.

"When soils scientists look at soil, they see an ecosystem," Brantley continues. "We all see different things. The way we look at the environment is influenced by our specialized fields—that's deep within the culture of science, which means it's fundamental to the way we think about scientific problems." She looks around at the wired-up watershed, at the colorful flags and the electrical outlets, the gray plastic boxes sitting on stainless-steel posts inside which instruments record data gleaned by probes in soil and trees and groundwater.

"There's so much going on here. Rain falls, and the water sinks into the ground. Trees and other plants take up some of the water; the vegetation absorbs CO2 from the atmosphere and releases oxygen and water vapor. In the regolith, the water dissolves minerals such as calcium and iron. When the water hits the bedrock-regolith interface, it moves downslope and ultimately gets picked up by the stream. It flows down to that weir"—she indicates a small dam made from a notched steel plate, with a red-painted plywood box built over it—"where we take samples. Then it leaves the watershed.

"We're trying to figure out why this watershed looks and works the way it does." She tosses the piece of shale onto the ground. "The time scales of these processes are vastly different. Rain events can be measured in hours or days. It can take months for water to move through the soil. For rock to weather and become soil requires tens of thousands of years.

"As scientists, we measure flows, record observations, and develop mathematical models to explain what's taking place within our areas of specialization. Doing research in a critical zone observatory like the Shale Hills encourages us to share our data, merge our models across disciplines so that they inform one another. Together we're trying to figure out the processes and multiple feedback loops operating within this system."

Laying the Groundwork

In his office on the second floor of Sackett Building, Chris Duffy tells me that Penn State forest hydrologists William Sopper and James Lynch—both now retired—conducted pioneering studies in watershed science in the Shale Hills during the 1960s and '70s.

in forest, female researcher points out something to male researcher
Scott Johnson

Chris Duffy (right) at Shale Hills with undergraduate student Shelly Pickett.

"Sopper and Lynch built an irrigation system that could deliver water all over the watershed—they drew the water out of nearby Lake Perez using a couple of 175-horsepower pumps. They installed piezometers to measure groundwater levels and neutron probes to measure the water content of the soil. Beneath the forest canopy, they spray-irrigated precise amounts of water to mimic rainfall events. Then they measured how the water in the soil and the stream responded to the added water. Sopper and Lynch published papers on water supply in the environment, flood control, drought behavior, forest health. Shale Hills became known as the best-monitored watershed study of its era."

In the 1990s the NSF and the National Aeronautics and Space Administration (NASA) funded Duffy to digitize Sopper's and Lynch's data, transfer it from IBM punch cards to contemporary computers. Duffy then built a mathematical model to describe the complex movement of water within the watershed.

"Sopper and Lynch did 'hydrology by experience,'" Duffy says. "They observed and recorded empirical relationships. Now, on the same site, we're deploying sensor networks that measure many more things: water chemistry, how the watershed interacts with the atmosphere above it, how trees affect the cycling of water, the weathering and soil formation that are taking place below the surface of the ground."

Duffy and research associate Kevin Dressler (who also coordinates all of the different research activities at the CZO) work toward building larger and more-inclusive models "that can better describe the passage of water through the landscape. In time, we want to scale up to models that can be moved and set up in different places and environments."

Using the Penn State Integrated Hydrologic Model, or PIHM, Duffy has determined the age of water within the watershed by measuring isotopes of oxygen and hydrogen at different points in the critical zone—in rain, in water suspended in the soil, in groundwater, in the stream.

"The water in the soil above the water table competes with the groundwater for space," he explains. "Rainfall drives the system, along with evaporation and transpiration, when trees release water vapor through their leaves.

"When it rains, the groundwater rises as water seeps down into the soil or moves downslope. The groundwater gobbles up the soil moisture, which is suspended in very small pores. The groundwater also picks up solutes—nutrients, metals, trace elements, inorganic chemicals.

"After a storm ends, the water table starts to drop. The larger pores in the soil drain first, and gradually smaller and smaller pores give up their water, either through draining or to the thirsty roots of plants.

"Different soils have different properties, depending on whether they evolved from porous or non-porous rock; shale, for instance, is less porous than some other rock types. Vegetation and land cover play a role—whether the landscape is dominated by wetlands, buildings, farm fields, trees, grass. These factors affect how water cycles through the landscape.

"Following an extended drought, it can take four or five years of normal rainfall to build up the water table and soil moisture to average levels that benefit plants and sustain rivers. What happens in a wet year, when a lot of rain gets dumped on the land? We're finding that water moves through the system much more rapidly during wet periods: It may take from a few days to a few weeks to enter the soil and exit the watershed. During dry periods, it may take months."

A Mecca for Scientists

In addition to Duffy and Brantley, the Shale Hills CZO has attracted other Penn State researchers. David Eissenstat, a forest ecologist and woody plant physiologist, has identified and tagged more than 200 canopy trees, charting the distribution of different species and exploring how water availability influences where they grow in the watershed. Sap-flow sensors embedded in tree trunks record how much water trees use and when they use it. By sampling water from small branches in the trees' crowns and then reading the water's isotopic signature, Eissenstat is learning whether the trees are getting their water from deep groundwater sources or from zones higher up in the soil.

Geoscientists Rudy Slingerland and Eric Kirby are exploring how the Shale Hills landscape reflects past climate regimes, especially the last ice age, some fifteen thousand years ago, when the area was locked in permafrost. Slingerland and Kirby study LIDAR images—high-resolution laser-based contour maps—to identify periglacial features, and are modeling how those features affect the erosion, transport, and deposition of sediments. They're also examining a more-modern ground-level feature at Shale Hills: its "mound-and-pit topography," caused by trees toppling during storms so that their roots pull up mounds of soil, a significant factor in erosion and the downslope movement of soils in the watershed.

Kamini Singha, a geophysicist, couples lab and field work in studying the shale-derived soils and regolith. Key to her research are four holes drilled down 17 meters into the bedrock. By analyzing solute samples taken from the boreholes, and using an optical televiewer to visually examine the boreholes' walls, Singha is exploring the roles that different layers, fractures, and pores play in conducting fluids and dissolved minerals below ground.

Kenneth Davis, a boundary-layer meteorologist, is responsible for a 110-foot-tall tower at the highest point in the watershed. There, instruments record precipitation, wind speed and direction, and the complicated turbulence that transports water vapor and CO2 out of the critical zone and into the atmosphere. Davis works closely with Duffy in developing the over-arching mathematical model that explains the interactions between atmosphere, land surface, and subterranean regions of the CZO.

Since 2004, soils scientist Henry Lin has been recording soil water content from the surface down to bedrock at more than 100 sites. He and his research team use x-ray tomography, ground-penetrating radar, and electromagnetic induction to determine the overall thickness of the soil and to detect and model subsurface networks through which water flows. They also study how soils store dissolved organic carbon.

As well as studying the weathering processes that produce soil, geologist Brantley is working with biogeochemist Jason Kaye to investigate how water movement and storage combine with the soil's texture to affect the rate of soil respiration. Soil respiration is the CO2 flux from soils to the atmosphere; driven by the activities of plant roots and soil microorganisms, it is one of the largest factors in the global carbon cycle. Brantley and Kaye use extensive soil mapping and moisture and temperature monitoring to build models that can predict the effects of climate change on this hidden process.

Brantley says that CZOs are "meant to be meccas that draw scientists together." Research groups from Princeton, Lehigh University, and several institutions in the United Kingdom have already applied for funding to do research at Shale Hills.

Where Rock Meets Life

Back in the Stone Valley Forest, on that halcyon April day. Susan Brantley leads the way up a steep path that climbs higher into the watershed. Crows call in the offing. A breeze rustles the new leaves overhead.

I ask Brantley about the depth of the soil on the site, which appears to be a fairly typical piece of terrain in the hills of Pennsylvania's Ridge and Valley region. "On the ridges, the soil is about twenty centimeters deep—eight inches. Down along the stream, it's anywhere from one to three meters deep.

"The forest on this site probably has been clearcut two or three times," she says. The first logging would have removed the large old-growth timber, the so-called virgin forest. In the mid- to late-1800s, the regrowing trees likely were cut to make charcoal that fueled local iron-smelting furnaces. Almost certainly, those activities contributed to the watershed's present topography.

"Many scientists believe that humans are now the biggest geological force acting on the planet. Through our activities, we're constantly moving soil and reshaping the landscape. Soils are crucial for agriculture. Are we in danger of losing or depleting our soils? At this point, we don't even know the rates at which soils are forming on different sites and in different environments.

"Up on the ridge, our research suggests that it takes about seven thousand years for rock to weather out of the bedrock, become part of the regolith, gradually move up through the soil—the particles becoming smaller all the time—and finally emerge at the surface.

"We're looking at acid rain. How does it affect the carbon in the soil? Globally, huge amounts of carbon are locked up in soil. Will changes such as soil warming, increased soil disturbance, or more-acidic precipitation cause a lot of that organic carbon to go up into the atmosphere, drastically raising CO2 levels?

"It is this sort of complex question that we can work toward answering by conducting interdisciplinary research in CZOs. As scientists in the twenty-first century, we have new and better ways of collecting and analyzing data. We have powerful computers to quickly run mathematical models that describe earth processes.

"These are the basic questions that we're hoping to answer: Why does a given landscape look the way it does? How does it function? How is it likely to change in the future in response to human activities? If we can answer those questions, perhaps we can determine how to protect this part of the earth where rock meets life."

Susan Brantley, Ph.D., is professor of geosciences in the College of Earth and Mineral Sciences, brantley@essc.psu.edu. Chris Duffy, Ph.D., is professor of civil engineering in the College of Engineering, cxd11@psu.edu. Brantley and Duffy are principal investigators for the Shale Hills Critical Zone Observatory. Other Penn State researchers currently working at Shale Hills include Kevin Dressler, Kenneth Davis, David Eissenstat, Jason Kaye, Eric Kirby, Henry Lin, Kamini Singha, Rudy Slingerland, and Tim White.

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Last Updated September 28, 2010